Appendix G to the
NASA
RESEARCH ANNOUNCEMENT
for the
COMPTON GAMMA RAY OBSERVATORY
GUEST INVESTIGATOR PROGRAM
TABLE OF CONTENTS
II. OSSE GUEST INVESTIGATOR PROGRAM G-2
A. Introduction and Scientific Objectives G-2
B. The OSSE Instrument G-2
C. Instrument Parameters and Capabilities G-8
D. Data Reduction and Analysis G-10
E. OSSE Guest Investigator Opportunities G-18
F. OSSE Observation Definition Form G-20
G. OSSE Data Product Availability G-21
III. COMPTEL GUEST
INVESTIGATOR PROGRAM G-22
A. Introduction and Scientific Objectives G-22
B. The COMPTEL Instrument G-23
C. Instrument Parameters and Capabilities G-28
D. Data Reduction and Analysis G-30
E. COMPTEL Guest Investigator Opportunities G-34
F. COMPTEL Facilities for Guest Investigators G-43
IV. EGRET GUEST INVESTIGATOR
PROGRAM G-47
A. Introduction and Scientific Objectives G-47
B. The EGRET Instrument G-48
C. Instrument Parameters and Capabilities G-51
D. Data Reduction and Analysis G-51
E. EGRET Guest Investigator Opportunities G-54
F. Feasibility Evaluation for EGRET Observations G-55
G. Specification of EGRET Observations G-57
H. Data Products to be Provided to EGRET Guest Investigators G-60
I. Use of EGRET During Cycle 6 and Beyond G-60
V. BATSE GUEST INVESTIGATOR PROGRAM G-63
A. Introduction and Scientific Objectives G-63
B. The BATSE Instrument G-64
C. Data Reduction and Analysis G-67
D. Data Analysis Environment G-71
E. BATSE Guest Investigator Opportunities G-71
F. BATSE Flight Performance and Observation Sensitivity G-72
G. Defining an Observation G-78
H. Data Products G-78
VI. CGRO SCIENCE SUPPORT CENTER G-83
A. General G-83
B. Guest Investigator Support G-83
C. Data Catalog and Archives G-85
D. Other User Support Activities G-88
THE COMPTON GAMMA RAY OBSERVATORY AS A GUEST INVESTIGATOR FACILITY
I. INTRODUCTION
The Compton Gamma Ray Observatory (Compton), was conceived, designed,
and developed as a Principal Investigator class mission but has subsequently
become an observatory class facility accessible to the international astrophysics
community. Its four distinct instruments are optimized to perform simultaneous
observations of specific targets or regions. These instruments combine
to cover over five decades of energy with more than a factor of 10 improvement
in sensitivity throughout the band, and with improvements in both spectral
and spatial resolution in selected energy regions over previously flown
instruments. The purpose of this Appendix is to describe Compton's
capabilities and the opportunities available to Guest Investigators.
The approach to the Compton Guest Investigator Program takes into
account the complicated nature of the data reduction and analysis processes
for each of the four instruments. These instruments are described below.
Details of the Program are instrument dependent and these are described
in separate sections. However, some aspects of the Program are common to
all instruments. There are various ways in which Guest Investigators can
participate in Compton observations and data analysis. The descriptions
listed below are not intended to limit or otherwise constrain the nature
of proposed investigations, but only to provide examples of the primary
forms of scientific investigations and collaborations presently anticipated.
At this stage of the mission, most Investigators propose to work more or
less independently of the Instrument Team, especially if existing data
is being used. It may be advisable to contact the Science Support Center
(SSC) and/or Instrument Teams should be consulted regarding any technical
feasibility issues or policy issues. Alternatively, an Investigator can
propose to work directly and closely with one or more of the Compton
Instrument Team scientists or CGRO-SSC in the analysis of data received
from the Observatory. Guest investigators wishing to be present at the
instrument team's prime site (UNH, MSFC, GSFC, or NRL) during the observation
of a target assigned to the Guest are welcome to be present and to participate
in the data analysis from the start if they so choose. Generally speaking,
flux estimates for targets are available in about 7-10 days. Full processing
of target data takes 1-4 months.
The OSSE instrument has been designed primarily to investigate localized
sources and to a lesser extent to map Galactic diffuse emissions with its
3.8 x 11.4 (FWHM) field-of-view. During the normal two week observing period,
OSSE will observe two sources using its two-source per orbit observing
capability. This will result in approximately 50 source observations per
year.
B. The OSSE Instrument
The Oriented Scintillation Spectrometer Experiment (OSSE) has been designed
to undertake comprehensive observations of astrophysical sources in the
0.05-10 MeV energy range. Secondary capabilities for gamma-ray and neutron
observations above 10 MeV have also been included, principally for solar
flare studies. The use of NaI(Tl) scintillation techniques provides large
area and high sensitivity in the 0.05-10 MeV region.
The highest sensitivity is maintained by implementation of an observational
technique which minimizes susceptibility to background variations due to
orbital geomagnetic latitude effects and spallation produced radioactivity.
This technique uses an offset pointing capability to modulate a celestial
source's contribution to the observed gamma-ray flux on a time scale short
compared to the background variations, so that it can be reliably extracted
from the background. Normally, OSSE performs two-minute viewing on and
off the targeted source. Background variations are modulated by the spacecraft's
orbital period of about 90 minutes.
The offset pointing capability also enables the experiment to undertake
observations of selected sources (e.g., transient objects) with a significant
independence of their location on the sky or on pointing restrictions which
may be imposed by the spacecraft. For example, this capability will be
used to monitor solar flare activity without impacting the planned observational
program of the other Compton experiments. It will also be used to
achieve a two-source per orbit capability wherein a secondary source will
be studied during orbital periods of earth occultation of the primary source.
In addition, this capability is used to allow OSSE to respond to gamma-ray
bursts on time scales of a few seconds if they occur near the OSSE scan
plane.
Four identical detector systems are used to obtain the operational flexibility
to meet the variety of scientific objectives previously outlined. Each
of OSSE's four detectors operates, to a large degree, as an independent
experiment. They have independent electronic systems and pointing systems.
The synchronization of the four detectors is provided by the central electronics,
which provides the data acquisition timing and coordination of the pointing
directions. As shown in Figure II-1, each of the detectors is mounted in
a single axis orientation control system which provides offset pointing
over a range of 192 in the spacecraft X-Z plane. The detectors are generally
operated in co-axial pairs. While one detector of a pair is observing the
source, the other detector can be offset to monitor the background. After
a programmable time interval, typically two minutes, the detectors will
interchange observation directions by opposite rotations. Each detector's
maximum rotation (or slew) rate is 2o per second.
Figure II-2 displays the major components of one of the OSSE detectors.
The primary element of each detector system is the NaI(Tl) portion of a
330-mm diameter NaI(Tl)-CsI(Na) phoswich consisting of a 102-mm thick NaI(Tl)
crystal optically coupled to a 76-mm thick CsI(Na) crystal. Each phoswich
is viewed from the CsI face by seven 89-mm diameter photo-multiplier tubes
(PMTs), providing an energy resolution of 8% at 0.661 MeV. Active
gain stabilization is used to maintain this energy resolution by individually
adjusting the gain of each of the seven PMTs (the absolute gain of the
OSSE phoswich remains stable to within 0.1% during a typical two week observing
period. Utilizing the differing scintillation decay time constants of NaI(Tl)
and CsI(Na), the detector event processing electronics incorporates pulse-shape
analysis for the discrimination of events occurring in the NaI crystal
from those occurring in the CsI, allowing the CsI portion of the phoswich
to act as anticoincidence shielding for the NaI portion. A tungsten alloy
passive slat collimator, located directly above the NaI portion of each
phoswich, defines the gamma-ray aperture of the phoswich detector, providing
a 3.8x 11.4 FWHM rectangular field-of-view throughout the 0.1-10 MeV energy
range. Each phoswich aperture is covered by a charged particle detector
(CPD) consisting of a 508-mm square by 6-mm thick plastic scintillator
which is viewed by four 51-mm diameter photo-multiplier tubes and is used
for charged particle background rejection (energy losses greater than a
nominal threshold of 0.20 MeV in the CPD trigger rejection of coincident
energy losses in the phoswich). Both the phoswich and tungsten collimator
are enclosed in a 349-mm long by 85-mm thick annular shield of NaI(Tl)
scintillation crystals which, together with the CsI(Na) portion of the
phoswich and the CPD, forms the active anticoincidence shield for background
rejection. The NaI(Tl) annular shield is divided into four optically-isolated
quadrants or segments, each viewed by three 51-mm diameter photo-multiplier
tubes.
The tungsten collimators are installed in the detectors so that OSSE detector
motion scans the source with the collimator's 3.8 FWHM field-of-view. A
background offset of 4.5 provides optimum modulation of the source contribution.
This background offset, however, is programmable so that alternate background
offsets can be selected in source-confused regions or for other scientific
reasons. the nominal observing sequence observes the background on both
sides of the source target. the average of these two backgrounds will generally
be used in the data analysis as the "background" for the source
observation. This technique minimizes the systematic effects of modulation
of gamma-ray background local to the spacecraft. A typical detector observing
sequence might consist of continuous repetition of the following sequence:
1. observe the source direction for 2 minutes,
2. move to a background position 4.5 counter-clockwise from the source
and observe for 2 minutes,
3. return to the source direction and observe for 2 minutes, and
4. move to a background position 4.5 clockwise from the source and observe
for 2 minutes.
The motion occurs at a fixed rate of 2 per second. These observing programs
can define sequences of up to 3 source and background offsets which can
be used for source positioning or more complicated background modeling.
In the 90 between the spacecraft Z and X axes, all four detectors can be
used for observations. Outside this range, the upper pair and lower pair
of detectors begin to intrude into each other's fields of view. In that
situation, the upper pair of detectors can be programmed to view one source
and the lower pair can view another, independent target.
The ability to select viewing directions which contain interesting targets
well away from the spacecraft Z-axis allows OSSE to make good use of the
Z-axis earth occultation periods; by selection of the spacecraft orientation
at the start of a two week observation, OSSE can move its detectors between
the primary Z-axis target and the secondary target each orbit without reorientation
of the spacecraft.
The experiment's data acquisition and control system incorporates varied
modes of operation depending on the type of information desired during
a particular observation. Diagnostic capabilities and redundancy are important
features of the system; the ability to re-configure the experiment in the
event of a failure has also been included. This versatility is achieved
through the use of redundant programmable microprocessors.
Energy losses (spectra) in the phoswich are processed by three separate
pulse-height and pulse-shape analysis systems for each detector. A low
range pulse-height analyzer (PHA) covers the energy range 0.05-1.5 MeV;
a medium range PHA covers the energy range 1-10 MeV; and a high range PHA
covers energies greater than 10 MeV, nominally 10-250 MeV. Pulse-shape
discrimination in the highest range is also used to separate neutron and
gamma-ray energy losses in the NaI portion of the phoswich by utilizing
the differing time characteristics of the secondaries produced by these
interactions.
Figure II-1
Figure II-2
Energy losses above 0.10 MeV in each of the four annular shield segments
are detected and utilized for rejection of coincident energy losses in
the phoswich. The energy losses in the 0.10-8 MeV range are also pulse
height analyzed as part of a diagnostic calibration analysis system. The
good spectral resolution of the shield segments (10.5% at 0.66 MeV) permit
the use of shield spectra in the study of solar flares and possibly gamma-ray
bursts. In addition, the low level discriminator event rates from the shields
(set at 0.10 MeV, but programmable from 0.03 to 0.47 MeV) are processed
by the OSSE central electronics for the detection and capture of gamma
ray bursts. This gamma ray burst trigger operates independently from the
trigger provided by the BATSE instrument.
The primary data from the OSSE instrument consist of time-averaged energy
loss spectra from the individual detectors (spectral memory data). Each
detector accumulates separate energy loss spectra from the validated gamma-ray
events in each of the three phoswich energy ranges. The time interval for
these accumulations, called the spectrum acquisition cycle time (TSAC),
is a function of the overall operating mode of the OSSE experiment and
is typically 16.384 or 32.768 seconds. The acquisition cycle time is controlled
by the OSSE central electronics which selects the TSAC interval based on
the OSSE telemetry format. Acquisition times in the range between 2.048
seconds and 32.768 seconds are available. The detectors' accumulation memories
are double-buffered so that accumulation occurs in one memory while the
other memory is being transmitted. The accumulation memories can support
a maximum of 4096 events per spectral channel per TSAC.
Gamma-ray events for the two lowest phoswich energy ranges (0.05-1.5 MeV
and 1-10 MeV) are separately accumulated into 256 channel spectra. These
energy loss spectra have uniform channel widths of approximately 6 keV
and 40 keV, respectively. the highest phoswich energy range (greater than
10 MeV) is processed to discriminate gamma-ray and neutron events. These
gamma-ray and neutron events are then separately accumulated into 16 channel
spectra. These energy loss spectra have uniform channel widths of approximately
16 MeV.
If high time resolution is required, e.g. for the analysis of fast pulsars, an alternate data mode is provided. In this mode (pulsar mode), spectral resolution or bandwidth is sacrificed in order to obtain time resolutions of up to 0.125 milliseconds. The pulsar mode processing permits the definition of up to eight energy bands to be included in the transmitted pulsar data. Gamma-ray events qualified as being in one of these eight energy bands are then processed in one of two pulsar modes:
1. event-by-event mode, where selected events are time-tagged and both
energy loss and arrival time of the event are transmitted in the telemetry,
or
2. rate mode, where high time resolution rate samples are taken in each
of the eight energy bands.
The event-by-event mode of pulsar data provides the highest time resolution
for the study of fast pulsars. This mode time-tags events accurate to 0.125
milliseconds at its highest resolution. Depending on the OSSE telemetry
format, a maximum of approximately 290 events/second is supported in the
event-by-event pulsar mode.
The pulsar rate mode can accommodate a much higher event rate but at the
sacrifice of spectral resolution. This mode records the number of events
in each of the defined energy bands at a specified sample frequency. The
highest sample rate in this mode provides a resolution of 4 milliseconds.
Sample times from 4 msec to 512 msec can be selected.
OSSE includes a gamma ray burst capture capability (burst mode) which is
based on the measurement of the gamma-ray event rates in the NaI annular
shields. These event rates are sampled by the OSSE central electronics
on a selectable time scale of from 4 msec to 32 msec. A gamma ray burst
trigger signal, either from BATSE or from internal processing, activates
the storage of the next 4096 samples in a burst memory. A selectable fraction
of this memory preserves the rate samples prior to the trigger. With coarser
time resolution (approximately 1 second), rate samples from the individual
NaI shield quadrants are available which can be used to identify the direction
of the gamma ray burst. Selected shield spectra can also be accumulated
using an on-board calibration and diagnostic PHA (CALPHA) in each detector
subsystem.
The OSSE internal burst trigger monitors the annular shield rate samples
for successive samples above a specified event rate as the indication of
a gamma ray burst. The BATSE Burst Trigger Signal will be the primary burst
trigger signal for OSSE since it incorporates more sophisticated trigger
detection and a burst direction measurement which will be able to identify
possible solar flares. The ability to react to solar flares will significantly
enhance the OSSE science.
As an alternative to solar flare detection, the BATSE Burst Trigger Signal
can be set to encode the results of an on-board calculation of the approximate
position of a cosmic gamma ray burst. If the burst strength and position
meet a set of criteria on rate, scan and azimuth angles (typically these
are: rate greater than 40 counts per 64 msec, azimuth from the OSSE scan
plane less than 11 degrees, and OSSE scan angle within the range of the
instrument), the signal is sent to OSSE, which then enters a special burst
mapping mode, typically mapping the region within 10 degrees of the reported
position. This mode can be used to search for delayed emission from gamma
ray bursts for a pre-determined period, typically 12 hours. If, in the
meantime, an improved position becomes available via the BACODINE network,
the OSSE mapping strategy can be updated appropriately, usually within
one hour.
The OSSE instrument also carries a separate charged particle monitor (CPM)
detector (see Figure II-1). This 19-mm diameter plastic scintillation detector
is enclosed in a passive shield of approximately 3 g/cm2 of
aluminum and is viewed by a 19-mm PMT. The event rates in this detector
provide a monitor of the high energy charged particle environment for OSSE.
Additionally, it provides a detection of the spacecraft entry into the
South Atlantic Anomaly (SAA). The spacecraft is designed to turn off the
OSSE detectors during traversals of the SAA; this charged particle monitor,
however, remains on to provide integral charged particle dose monitoring
for background modeling.
C. Instrument Parameters and Capabilities
Table II-1 provides a summary of the instrument parameters and capabilities
of the experiment.
TABLE II-1
OSSE PARAMETERS AND CAPABILITIES
Detectors
1. Type: 4 identical NaI(Tl)-CsI(Na) phoswich, actively-shielded, passively
collimated
2. Aperture Area (total): 2620 cm2 Photopeak Effective Area:
see Figure II-3.
3. Field-of-View: 3.8 x 11.4 FWHM (0.05 - 10 MeV).
4. Energy Range: 0.05-10 MeV gamma rays (primary objectives), 10-250 MeV
gamma rays (secondary objectives), 10 MeV solar neutrons (secondary objectives)
5. Energy Resolution: see Figure II-4.
6. Time Resolution: 16.384 or 32.768 sec in normal mode 0.125 msec in pulsar
mode, 4 msec in burst mode
Source Exposure per Observation Period
An exposure time of 5 x 105 seconds can be achieved
for a priority OSSE target viewed during a 14-day observation period with
full telemetry coverage (this includes source and background observation)
Experiment 3- Sensitivity
1. Line Sensitivity (5 x 105 sec exposure; point source):
see Figure II-5
2. Continuum Sensitivity (5 x 105 sec exposure): see Figure
II-6 and Section VI of this Appendix
3. Gamma-Ray Burst Sensitivity: 1 x 10-7 erg cm-2
4. Solar Flare Line Sensitivity (103 s flare): (5-10) x
10-4 ph cm-2 s-1
5. Solar Flare Neutrons Sensitivity: 5 x 10-3 n cm-2
s-1
Pointing System
1. Type: Independent Single Axis
2. Drive System: Redundant Stepper Motor
3. Maximum Drive Speed: 2/sec
4. Range: 192 about the spacecraft Y-axis
(see figure II-1)
Compton Spacecraft-Experiment Interface Data
1. Weight: 1810 kg
2. Power Average: 192 Watts
3. Telemetry: 6492 Bits/sec
D. Data Reduction and Analysis
1. OSSE Data Types
The procedures required for analysis of OSSE data are ultimately determined
by the instrument operating characteristics and the types of data available.
As described above, data from OSSE consist of the following categories:
a. Time-integrated Spectra ("Spectral Memory Spectra")
OSSE internally accumulates spectra in the approximate energy ranges
0.05-1.5 MeV (low range), 1-10 MeV (medium range) and 10-250 MeV (high
range), integrated over periods as short as 2.048 seconds or in multiples
of 2.048 seconds. The low and medium energy-range spectra are sampled into
256 channels per detector, while the high-range spectrum is routinely sampled
into 16 channels per detector (256 channels are available on longer time
scales). In addition, the high energy range contains a 16-channel spectrum
of >10 MeV neutrons. These spectra represent a large dynamic range and
the best energy resolution attainable at the cost of moderate temporal
resolution.
During ground data processing, spectral memory spectra are further summed
into two-minute intervals corresponding to the OSSE source/background pointing
periods. A two-week observation of a given celestial source could consist
of as many as 10,000 of these basic two-minute intervals, or, after background
subtraction, as many as 5000 separate source observations. For a typical
source observation, the two-minute pointing interval will be the shortest
time scale for which spectral memory spectra will be examined.
b. High Temporal Resolution Data ("Pulsar and Burst Data")
OSSE may also transmit data in one of several modes with temporal resolution
finer than 2.048 seconds. The trade off is either reduced dynamic range
(i.e., energy range), reduced energy resolution, or both. High time resolution
data may be mixed with spectral memory data in the telemetry stream, resulting
in further compromises in range, energy resolution and temporal resolution.
Figure II-3. OSSE photopeak effective area for a single OSSE
detector (approximate). The effective area is plotted for (I) a 0 source
offset exposure (source is at the center of OSSE's field of view), (ii)
a 4.5 source offset exposure in scan angle (typically used for measuring
the background), and (iii) the difference between the "source on"
and "source off" exposure plots, which is the modulated effective
area after background subtraction.
Figure II-4. OSSE energy resolution (approximate)
Figure II-5. OSSE 3- line sensitivity for a point source. The
exposure time of 500,000 seconds includes half time on source and half
time on background. This exposure time is typical of what can be achieved
for a priority OSSE target viewed during a 21-day observing period with
the current telemetry coverage efficiency.
Figure II-6. OSSE 3- continuum sensitivity for a point source. The exposure 500,000 seconds includes half time on source and half time on background. This exposure time is typical of what can be achieved for a priority OSSE target viewed during a 21-day observing period with the current telemetry coverage efficiency.
Pulsar data may consist of either "event-by-event" or "rate"
modes. For "event-by-event" data, a time and energy are transmitted
for each gamma-ray event and temporal resolutions of 0.125 milliseconds
and 1 millisecond are available, with the lower temporal resolution yielding
a larger energy range. For either temporal resolution, the energy range
must be adjusted to limit the count rate to approximately 80 counts/second/detector
maximum. Pulsar "rate" mode data consists of counts integrated
into consecutive time intervals as short as 4 milliseconds but with limited
energy resolution (maximum of eight energy bands).
Burst data, used for gamma-ray burst measurements, consist of 4096 samples
of NaI(Tl) anticoincidence shield rates, with each sample representing
a time interval as short as 4 milliseconds (yielding a total interval of
16 seconds) or as long as 32 milliseconds (yielding a total interval of
132 seconds). Shield burst data contain no energy information, with the
exception of using the roving PHA (RPHA) to acquire limited spectra.
2. OSSE Data Product Characteristics
(See also Table V-2 in the Project Data Management Plan) The accessible
data products are categorized as being Low Level or High Level depending
on the degree of processing and the volume of the resultant product. The
breakdown below includes the volume of data involved for Low Level products;
the volume of High Level data is small relative to Low Level data.
a. Low Level Data Products
2-minute detector count spectra:
Approximately 10,000 intervals per 2-3 week observation period. Includes
extensive header with instrument configuration, rates, and orbital environment.
Primary spectrum data, calibration data, no diagnostic data. Volume: approximately
150 Mbyte per 2-3 weeks of observations.
Background-subtracted 2-minute count spectra:
Like the 2-minute spectra, noted above, but with a standard background
subtraction algorithm applied, including propagated uncertainties. Volume:
approximately 150 Mbyte per 2-3 weeks of observations (backgrounds and
calibrations removed but includes uncertainties).
Pulsar data:
Event-by-event with resolution down to 0.125 milliseconds; limited band
coverage. Rate with resolution down to 4 milliseconds; limited energy resolution.
Volume: approximately 900 Mbyte per 2-3 weeks of observations.
Burst data:
4096 time bins from shields, Time resolution adjustable. Volume: approximately
20 Kbyte per burst.
b. High Level Data Products:
In general, all High-Level products will be presented for each of the four
OSSE detectors.
Integrated Count Spectra:
These are background subtracted spectra summed over some or all of a
given two week observation period (or multiple observation periods). They
include header information listing instrument parameters, exposure, and
integrated rates. Propagated uncertainties are included, but background
spectra are not. Background subtraction is via the standard algorithm with
no special corrections. Typically, data selection criteria will have been
applied to the spectra to be summed. These criteria may include orbital
environment parameters such as magnetic field values, time from SAA passage,
etc., and/or pointing parameters such as earth horizon angle. Multiple
spectrum sets with different selection criteria applied shall be available.
These spectra will not have been corrected for instrument response. However,
pulse height (PHA) calibrations (i.e., representative energies for each
pulse height channel) will be supplied.
Integrated Incident Photon Spectra:
These spectra are similar to the integrated count spectra except that
they have been corrected for instrument response. It should be emphasized
that these are estimated incident photon spectra. The quality of the estimate
(systematic uncertainty, resolution, compliance to models) depends on which
of the several available instrument deconvolution algorithms has been used
on the spectrum. Uncertainties included will reflect deconvolution errors,
but will not necessarily indicate that some errors are correlated over
large parts of the spectrum. Accompanying documentation will indicate algorithms
used and point out possible problems.
Integrated Pulsar Light Curves:
Epoch-folded pulsar light curves integrated over selected portions of
an observation shall be available in user-specified energy bands. These
may be viewed as two-parameter spectra, as a function of energy and pulsar
phase. Descriptive headers, exposure information per phase, and uncertainties
will be included. Light curves as a function of trial period may also be
supplied.
Energy Band Time Histories:
Time histories of the rates (corrected for exposure) in any energy band
will be available. The time resolutions available depend on the instrument
mode, and, at high time resolution, the energy band coverage may be limited.
Multiple-correlated time series will be available as well (e.g., multiple
energy bands, or energy bands and orbit environment data, correlated in
time).
3. OSSE Data Analysis
The OSSE data analysis systems may be divided up into the following three
areas for discussion:
a. OSSE Databases
The OSSE data analysis systems must be able to deal with diverse types
of data, each of which is described by a large number of parameters and
each of which can come from a wide variety of instrument operating modes.
In addition, the volume of data of all types will be large. Spectral memory
data alone will amount to approximately 100 Mbytes for a typical two-week
source observation.
OSSE data will be stored in three database formats. All data will be available
in an archived telemetry format. This format has no redundancy but also
no compression and requires substantial analysis to generate usable forms.
Production analysis procedures will create three more refined databases.
The spectral database will be the primary repository for spectra. Each
spectrum, regardless of time scale, will include an extensive header containing
most quantities required for further analysis (e.g., housekeeping, live
times, etc.) integrated over the spectrum collection interval. This database
may be viewed as the central interface for all OSSE processing. In order
to facilitate access to individual spectra and groups of spectra, a separate
catalog of spectral database headers will be maintained and managed by
a formal database management system with an efficient query language and
pointers to the spectral data itself.
In general, each scientist will have his/her own spectral database of spectra
from the observation periods with which he/she is working. This database
will not be sharable and, hence, the individual scientist will be free
to modify it at will.
Production analysis also produces Pulsar Data Files (PDF's) containing
high time resolution data together with some housekeeping information.
These files are generally large and may reside in secondary storage.
In addition, calibration and instrument status data and results will
be stored in an instrument model database. A certain degree of redundancy
exists between this database and the spectral database header information,
but the instrument model database maintains finer time scales and more
information. In particular, each spectral database header will include
a pointer to the instrument model database such that spectral data and
calibration results may be associated. The instrument model database will
include time variable information, such as energy calibrations, and fixed
information such as simulation and preflight calibration results. The latter
will be sharable among all users, while each user will generate his own
copy of the former relevant to the observation period of interest.
b. OSSE Production Analysis
Production analysis consists of routine procedures performed on all
data as it is received. These include reception, quality analysis, archiving,
decommutation into separate data streams for various types of data, automatic
summation into pointing intervals, database loading, automatic calibration
and searching algorithm execution and hard copy record production. Production
analysis will produce spectral database and instrument model database files
on magnetic disk, optical disk and/or magnetic tape. These may then be
accessed by the individual scientific user. A tape (or optical disk) library
system will allow the user to select the volumes for the interval, observed
source, etc., of interest. Production analysis functions will routinely
be performed at the Naval Research Laboratory (NRL) within 24 hours of
data reception. Archive and derived media will be shipped to Northwestern
University (NU) on a regular, frequent basis.
Production analysis procedures will be largely transparent to the scientific
user. A limited number of applications may require either special production
runs, in particular if spectra on time scales faster than the pointing
interval are desired, or specialized telemetry archive analysis routines.
These routines, beyond a basic set, will be supplied on an as-needed basis.
c. OSSE Scientific Analysis
Scientific analysis consists of processes required for the scientific user
to interact with the data with the goal of producing a scientifically usable
result (e.g., estimates of production spectra from a source, models of
source emission, etc.). Scientific analysis starts with semi-routine operations
to remove instrumental effects. Foremost of these is background subtraction.
Since all subsequent results are sensitive to errors in this procedure,
it is included in the scientific rather than production analysis in order
to give the scientific user more direct control over the process and to
allow the user a feel for the compromises involved. As experience with
OSSE data grows, background subtraction will become progressively more
routine. However, certain situations (e.g., data near the limb of the earth),
may always require special care.
Other scientific analysis procedures include unfolding the instrument response
and examination of results for systematic effects and artifacts. The latter
will be aided by the ability to select data sets from the spectral database
while applying a wide variety of instrument state and environmental criteria.
These procedures will include specialized algorithms for dealing with such
situations as source confusion and extended source analysis.
As experience with the OSSE instrument has grown, revisions have been
made to the energy calibration routines and response matrix generator.
In some cases, usually bright and/or hard sources, this can have a significant
impact on the analysis results.
Final results are achieved by a variety of processes such as data selection,
combination and summation of spectra in a statistically meaningful manner,
combining results from different pointing positions into sky maps, and
modeling spectra and spatial distributions. Presentation and publication
quality graphics utilities will be available to display results. The diverse
processes that comprise scientific analysis will be tied together by a
common interface known as the "Integrated GRO/OSSE Reduction Environment"
(IGORE). This system will supply a common parameter and data interface
to be used by all application programs. Based on the IDL data analysis
language, IGORE has all of IDL's data manipulation and programming capabilities.
Database, statistics, graphics and instrument specific applications will
all be able to communicate with the user and with each other through IGORE.
If the user wishes to write a specialized application, he only needs to
know the IGORE interface rather than individual interfaces for each application
and utility with which he must communicate.
Of course, this additional layer of software exacts a performance penalty,
so in some compute-intensive applications, special purpose software may
be written that by-passes IGORE and communicates directly with databases,
graphics utilities, etc.
4. OSSE Data Analysis Facilities
Production analysis will take place at NRL only, but full scientific analysis
facilities will be available at NRL, and the CGRO-SSC
locations. IGORE accounts can be obtained by contacting the
CGRO-SSC.
5. OSSE User Documentation
Detailed design documents for both hardware and software are currently
available if required. Additional general documentation for new users of
OSSE is available or in preparation; including an introduction to OSSE
operating modes, data analysis procedures, database formats, production
analysis procedures, and data analysis software.
a. GRO/OSSE Guest Investigator Guide - A basic introduction to OSSE -functions,
data and terminology. Designed for use by persons totally unfamiliar with
OSSE and with only minimal knowledge of gamma-ray observations. Numerous
examples on using OSSE high-level data products. Available in PostScript
format from CGRO World Wide Web homepage under OSSE and High-Level Products.
b. Mission Operations Guide - A description of OSSE operations and options,
including OSSE operating modes, pointing capabilities, mission operations
utilities and overall observing plan.
c. IGORE User Reference Guide - An encyclopedic reference to IGORE functions
and procedures.
d. OSSE Instrument Paper - (Johnson, W.N, et al., 1993, ApJ Supp. 86,
693). A description of the calibration and operational characteristics
of the OSSE instrument.
e. OSSE World Wide Web Server and Home Page - Brief OSSE description and
OSSE papers available for downloading
<http://osse-www.nrl.navy.mil/osse/osse.html>.
E. OSSE GUEST INVESTIGATOR OPPORTUNITIES
Because of the different operational and data analysis requirements
of the Compton instruments, the Compton Guest Investigator
program is tailored to the individual instruments rather than Compton
as a whole. The complexity of the OSSE data analysis and the extreme care
which must be exercised in interpreting results in a very low signal/background
environment may require that the Guest Investigator initially work very
closely with members of the OSSE Instrument Team or the OSSE Instrument
Specialist. New Guest Investigators may find it useful to spend time at
either NRL, NU or the CGRO-SSC for investigations involving extensive data
analysis.
In accord with the original OSSE proposal, the OSSE team is retaining proprietary
data rights and responsibilities to analyze and publish data on several
selected topics or sources; these are:
a. New Galactic or local group supernovae (proprietary for data obtained
up to two years after the event)
b. Comprehensive Galactic plane survey (not including proposed discrete
source observations)
c. Comprehensive sky survey during Phases 3 and 4 (not including proposed
discrete source observations)
Note that retention of exclusive data rights does not mean that Guest
Investigator participation is excluded -- collaboration with the team is
encouraged.
OSSE also offers a significant capability for observations of the sun.
Solar array articulation and spacecraft thermal constraints require the
Sun to be in the +X hemisphere of the spacecraft, OSSE's orientation capability
often permits direct viewing of the Sun even when the other instruments
are observing in some other direction. Thus, the Sun is often a candidate
secondary target for OSSE viewing. Additionally, OSSE has a rather unique
capability to quickly respond to solar flares based on the BATSE solar
flare trigger signal and the offset pointing capability of the OSSE instrument.
BATSE nominally takes 8 seconds to identify the transient event as coming
from the solar direction. The OSSE response time to a solar flare would
be about 60 seconds to re-orient the detectors to solar viewing. OSSE can
also react to a BATSE solar trigger by successively accumulating spectra
from the OSSE annular shield segments with a time resolution of up to approximately
4 seconds. This capability can provide modest spectral and temporal resolution
data on solar flares without re-orientation of the OSSE detectors.
3. OSSE Data Archiving
OSSE will provide the Compton Science Support Center with reduced
data sets within one year of processing the data to a usable format. This
archival data, largely in FITS format, will be accessible by the community
at large following NASA policy. Independent Investigators wishing to use
archival data from OSSE will be assisted by the Compton Science
Support Center.
F. OSSE Observation Definition Form
1. Defining an OSSE Observation.
Guest Investigators who are requesting data from an OSSE observation should
fill out an Observation Definition Form. Hard copy of this form is included
at the end of Appendix D of this NRA, however, the form must be included
as part of the mandatory electronic proposal submission. The purpose of
this form is for us to obtain from the proposer a definition of the data
set needed for the planned investigation. If the form is inadequate for
this purpose, the proposer should not feel constrained by its structure
- text can be appended to the form under the special requirements box at
the end. The instructions for filling out this form are as follows:
1. Use as many pages as you need to provide your target information. If
the target is an extended source (> 1), the radius should be indicated.
2. In the "constraints" column, indicate any expected variability
of the target, any spectral features to be studied, and the time resolution
required for the observation (if less than 32.768 seconds). If the target
is a pulsar, indicate the period. For spectral line observations, indicate
the energy resolution required; refer to Figure II-4 for OSSE's limiting
energy resolution.
3. Using the sensitivity plots provided in Figures II-5 and II-6, and
the Guest Investigator's knowledge or model of the target, indicate the
desired duration of the observation in weeks. The sensitivity plots are
based on the typical exposure of 5x105 seconds possible
during a normal 2 week observation of a priority OSSE target (includes
source and background observation).
Note that with the expected 500-km orbit altitude during the 1997-2001 timeframe,
a larger background count rate is expected. Predictions suggest a factor
of ~1.5-2X current (November 1996) levels. To obtain the same detection
sensitivity, OSSE observing time requests should be increased proportionately.
4. Indicate if there are special collimator field-of-view constraints
that should be considered in planning the observation (for example, if
the target is less than 5o from a potentially contaminating
source).
5. Indicate any other special requirements (for example, increasing
the normal gain of the phoswich; in this case indicate 2X gain in the mode
column). Depending on the nature of these requirements, the Guest Investigator
may wish to contact the OSSE team prior to proposal submission to discuss
the feasibility of any special needs.
G. OSSE Data Product Availability
A summary of OSSE data formats, volume, and availability may be found in
the GRO Project Data Management Plan, which is included as Appendix H of
this NRA. See Tables V-1 and V-2 (p. 39, 45).
III. COMPTEL
GUEST INVESTIGATOR PROGRAM
A. Introduction and Scientific Objectives
The Imaging Compton Telescope (COMPTEL) occupies the middle position
(in both physical location and energy coverage) on the Compton spacecraft
and surveys the gamma-ray sky in the energy range from 0.8 to 30 MeV. Measurements
by COMPTEL effectively bridge the gap in observations previously existent
between the hard X-ray and high-energy gamma-ray bands. COMPTEL combines
a wide field of view (about 1 steradian) with good angular resolution.
Its imaging properties have made it an ideal instrument for the first comprehensive
survey of the sky at MeV-energies. With an energy resolution of 5% to 10%
FWHM COMPTEL is also capable of conducting gamma-ray line spectroscopy.
Measurements at MeV gamma-ray energies provide information critical to
the detailed understanding of a wide range of high-energy astrophysical
phenomena. The scientific objectives of the COMPTEL experiment can be grouped
under the following headings:
Principal objectives:
Secondary objectives:
During Phase 1 of the Compton mission, COMPTEL conducted the first complete survey of the sky at MeV energies. From Phase 2 onwards selected celestial objects and regions are being studied in greater detail.
The Principal Investigator of COMPTEL is Dr. Volker Schönfelder of the Max-Planck-Institute for Extraterrestrial Physics (MPE), Germany. Co-Principal Investigators are located at the Laboratory for Space Research (SRON-Utrecht) (Dr. Wim Hermsen), Utrecht, The Netherlands, the Space Science Center of the University of New Hampshire (UNH)
(Dr. James Ryan), USA, and the Space Science Department, Astrophysics
Division, of ESA/ESTEC (Dr. Kevin Bennett), Noordwijk, The Netherlands.
B. The COMPTEL Instrument
A detailed description of the COMPTEL experiment and its operating characteristics
can be found in the report "Instrument Description and Performance
of the Imaging Gamma-Ray Telescope COMPTEL aboard NASA's Compton Gamma
Ray Observatory" by V. Schönfelder et al., 1993, ApJ Supp.,
86, 657. Further general information on the COMPTEL instrument,
as well as an electronic publications archive, can be found on the World
Wide Web pages of the COMPTEL collaboration at
<http://wwwgro.unh.edu/comptel/>.
We provide in the sections below a summary description of the COMPTEL instrument
and its properties.
1. Description
COMPTEL consists of two detector arrays. In the upper one, a liquid
scintillator, NE 213A, is used and, in the lower, NaI crystals. Gamma rays
are detected by two successive interactions: an incident cosmic gamma-ray
is first Compton scattered in the upper detector, then totally absorbed
in the lower. The locations of the interactions and energy losses in both
detectors are measured. The accuracy in the measurement of these parameters
determines the overall energy and angular resolution of the telescope.
In some ways, COMPTEL is similar to an optical camera: the upper detector,
analogous to a camera's lens, directs the light onto the second detector,
comparable to the film, in which the scattered photon is absorbed. Although
the photons are not focused, as in the case of an optical camera, data
obtained by COMPTEL can be used to reconstruct sky images over a wide field
of view with a resolution of a few degrees.
A schematic drawing of the telescope is shown in Figure III-1. The two
detectors are separated by a distance of 1.5 m. Each detector is entirely
surrounded by a thin anticoincidence shield of plastic scintillator which
rejects charged particles. On the sides of the telescope structure, between
both detectors, are two small plastic scintillator detectors containing
weak 60Co sources, used for in-flight calibration of the instrument.
Each calibration source consists of a cylindrical piece of 60Co-doped
plastic scintillator of 3 mm thickness and 1.2 cm diameter viewed by two
1.9 cm-diameter photomultiplier tubes (PMTs).
The Upper Detector (D1) consists of 7 cylindrical modules of liquid
scintillator NE 213A. Each module is 27.6 cm in diameter, 8.5 cm thick,
and viewed by eight photomultiplier tubes. The total area of the upper
detector is approximately 4188 cm2. The Lower Detector (D2)
consists of 14 cylindrical NaI (Tl) blocks of 7.5 cm thickness and 28 cm
diameter, which are mounted on a supporting base plate. Each block of NaI
is viewed from below by seven photomultiplier tubes. The total geometrical
area of the lower detector is 8620 cm2. Each anticoincidence
shield consists of two 1.5 cm-thick domes of plastic scintillator NE 110,
with each dome viewed by 24 photomultiplier tubes. With the exception of
the front-end electronics, all detector electronics are mounted on a platform
outside the detector assembly.
2. Instrument Modes for Data Acquisition
The Instrument Ground Support Equipment (IGSE) for COMPTEL is located at
the University of New Hampshire, USA. All commands sent to the instrument
originate at the University of New Hampshire, which also receives daily
"quick-look" data during real-time passes of the spacecraft.
These data are inspected immediately upon receipt in order to verify the
satisfactory health, safety, and performance of the instrument.
a. Primary Modes for the Acquisition of Scientific Data
1) Double-Scatter Telescope Mode
COMPTEL operates primarily as a double-scatter gamma-ray telescope. In
this mode an incident cosmic gamma ray is electronically identified by
a delayed coincidence satisfying time-of-flight criteria between the upper
and the lower detectors, combined with the absence of a signal from all
charged particle shields and from the calibration detectors. Gamma-ray
events are initially screened on board the spacecraft according to coarse
pulse-height, time-of-flight, and charged-particle veto limits, which are
applied to reject background events. The data are transmitted in telemetry
packets where events satisfying all selection criteria (referred to as
"Gamma 1" events) have first priority for transmission in the
telemetry stream, followed by events selected according to a secondary
set of onboard criteria ("Gamma 2" events). The telemetry rate
of 6125 bits per second permits the transmission of approximately 20 events
per second from the spacecraft.
The quantities measured for each selected gamma-ray event are:
(1) The energy of the recoil electron of the Compton scattered gamma
ray in the upper detector Ee' (Ee' > 50 keV),
determined from the summed pulse heights of the eight PMTs associated with
the triggered D1 module;
(2) The location of the interaction in the upper detector, determined from
the relative pulse heights of the eight PMTs associated with the triggered
D1 module;
(3) The pulse shape of the scintillation in the upper detector, provided
by the output of a pulse-shape discriminator circuit;
(4) The energy loss Ee" in the lower detector ( Ee"
> 500 keV), determined from the summed pulse heights of the seven PMTs
associated with the triggered D2 module;
(5) The location of the interaction in the lower detector, determined from
the relative pulse heights of the seven PMTs associated with the triggered
D2 module;
(6) The time-of-flight of the scattered gamma ray from the upper to the
lower detector; and
(7) The absolute time of the event.
The energy of an incident cosmic gamma ray is estimated by summing the
energies deposited in the upper and lower detectors, assuming total absorption
of the scattered photon. From the energy losses and the interaction locations
recorded for both the upper and lower detectors the arrival direction of
an incident gamma ray is calculated by application of the Compton scattering
formula. For a true Compton scatter event the arrival direction of a cosmic
gamma ray is known to lie on a so-called "event circle" on the
sky. The center of the circle is the direction of the scattered gamma-ray
within the telescope, and its angular radius is derived from the energy
losses in both detectors. In the ideal case of totally-absorbed Compton-scattered
photons, the intersection of many such event circles on the sky yields
the location of the gamma-ray source. In actual practice a detailed understanding
of the properties of the scintillation detectors, and of the response of
the telescope to background, multiple interactions, and partial absorption
events, must be exploited to determine accurate source locations.
2) Burst Mode
In addition to the normal double-scatter telescope mode of operation, two
of the NaI crystals in the lower D2 detector assembly of COMPTEL are also
operated simultaneously as burst detectors. These two modules are used
to measure the time history and energy spectra of cosmic gamma-ray bursts
and solar flares. The burst detector modules are equipped with a dedicated
electronic subsystem, the Burst Spectrum Analyzer (BSA). One of the burst
modules provides energy coverage over a low range (~100 keV to ~1 MeV),
and the other over a high range (~1 MeV to ~10 MeV). The burst detectors
are sensitive to incident radiation over 4 steradians, though the field
of view not obstructed by other portions of the spacecraft totals approximately
2.5 sr.
The burst system operates in three different internal modes. The background
mode is the usual operating mode: spectra from both burst detectors are
accumulated over a tele-commandable period of time (from 2 s to 512 s)
and read out continuously or at a reduced rate (also tele-commandable,
e.g., every 5 min). These data are used to establish the celestial and
instrumental background before and after a burst event. The BSA switches
into burst mode upon receipt of a trigger signal from the BATSE experiment.
A total of six spectra per energy range are accumulated with an integration
time that can be set from 0.1 s to 25.6 s. After completion of the
sixth spectrum, the BSA enters a tail mode where up to 256 spectra per
energy range with tele-commandable integration times from 2 s to 512 s
are accumulated and stored for transmission. The tail mode is of value
for longer-lasting
Figure III-1
burst events (e.g., solar flares). After completion of the tail mode,
the BSA resumes the background mode and is ready to receive the next burst
trigger.
3) Solar Modes
i. Solar Gamma-Rays
In the event of a solar flare, COMPTEL can observe solar gamma rays in
both the double-scatter telescope mode, provided the Sun is within the
field of view of the instrument, and in the burst mode. Telescope observations
of solar gamma rays have the advantage of a known source position, thus
eliminating the need in later analysis to consider events that do not originate
from the direction of the Sun. Upon receipt of a burst trigger from BATSE
solar gamma rays can also be recorded by the two burst detectors aboard
COMPTEL. The two burst modules, due to their rapid accumulation rates,
can provide information on the initial fast burst of gamma rays from a
solar flare.
ii. Solar Neutrons
In addition to observing gamma rays from a solar flare, COMPTEL is also
capable of detecting solar neutrons. Neutron interactions within the instrument
occur when an incident solar neutron elastically scatters off a hydrogen
nucleus in the liquid scintillator of an upper D1 module. The scattered
neutron may then interact and deposit all or a portion of its energy in
one of the lower D2 modules, providing the internal trigger signal necessary
for a double scatter event. The energy of the scattered neutron is deduced
from its time of flight from the upper to lower detector, which is summed
with the energy measured for the recoil proton in the upper D1 module to
obtain the energy of the incident solar neutron. The computed scatter angle
of the neutron, as with gamma rays, yields an event circle on the sky,
which can be further constrained since the true source of the detected
neutrons is assumed to be the Sun.
In practice, neutron observations are conducted in the following manner:
within two minutes of an initial burst trigger, BATSE sends a second signal
to COMPTEL indicating that the burst originates from the general direction
of the Sun. COMPTEL can then automatically be commanded to enter an alternate
event selection mode to measure solar neutrons for a period of 90 minutes
(or approximately one orbit of the spacecraft). This is achieved by shifting
the acceptance window for the time of flight of particles from the upper
to the lower detector to allow for the slower-moving neutrons, compared
to the speed-of-light gamma rays. Gamma ray events continue to be accumulated
simultaneously with the neutrons; the two types of particles are later
distinguished by their respective time-of-flight and pulse-shape signatures.
b. Secondary Modes for Data Acquisition
In addition to those instrumental modes intended for the acquisition of
scientific data, the following modes of operation can also be specified.
These secondary modes are intended for the collection of data relevant
to in-flight calibration, the assessment of instrument performance, and
for the study of background effects due to instrumental activation and
atmospheric albedo.
1) SAA Mode During passages through the region of the South Atlantic
Anomaly (SAA) the high voltages are switched off to prevent damage to the
photo-multiplier tubes. The period of SAA transits can be used to verify
the pedestals of the on-board pulse height analysis system and to test
the burst analysis system.
2) Proton Calibration Mode In this mode data can be obtained to investigate
the instrument response to charged particles, and to evaluate the performance
of the anticoincidence veto domes in the suppression of the background
due to charged particle interactions.
3) Upper Detector (D1) Single Event Calibration Mode Energy spectra
of the interactions occurring in the D1 modules are obtained, in addition
to normal gamma-ray telescope events. Spectral lines of background interactions
can be used to verify the calibration of individual D1 modules. This calibration
mode is typically invoked during earth occultation of the primary scientific
viewing target.
4) Lower Detector (D2) Single Event Calibration Mode Similar to
(3) above, but for the D2 modules.
5) Atmospheric Neutron Mode Similar to the Solar Neutron Mode, described in the previous section, but on command only; intended to investigate instrumental background due to neutrons from the Earth's atmosphere.
C. Instrument Parameters and Capabilities
A summary of the principal operating characteristics of COMPTEL is given
below in Table III-1 (see also section III.E.2 of this Appendix).
TABLE III-1
COMPTEL CAPABILITIES AND OPERATING CHARACTERISTICS
FOR GAMMA-RAY OBSERVATIONS
a. Detector type:
1) upper detector D1: liquid scintillator NE 213A
2) lower detector D2: NaI (Tl)
b. Geometric arrangement:
1) D1: 7 cylindrical modules 27.6 cm in diameter and 8.5 cm deep; total geometric area 4188 cm2
2) D2: 14 cylindrical modules 28 cm diameter and 7.5 cm deep; total
geometric area 8620 cm2
c. Effective area for gamma-rays: 20 - 50 cm2 (no event selections
applied to the data)
d. Energy range: 0.8 - 30 MeV
e. Energy resolution: 5 - 8% (FWHM)
f. Angular resolution: 1.7 - 4.4 (FWHM)
g. Geometric factor: 5 - 30 cm2 sr
h. Field-of-view: ~ 1 sr
I. Accuracy of Source Position Determination: 5 - 30 arcmin
2. Experiment Sensitivities:
a. Telescope Observations of a Point Source
1) Minimum source detectability at 3s, 1-30 MeV (2-week observation) = 1.6 x 10-4 cm-2 s-1
(See E.2. and Section VI for additional detail)
2) Line sensitivity (3, 2-week observation) = 6 x 10-5
cm-2 s-1 at 1 MeV = 1.5 x 10-5 cm-2
s-1 at 7 MeV
b. Burst Observations
1) Telescope mode lower limit: S(>1 MeV) = 2 x 10-6 erg cm-2 ,upper limit: S(>1 MeV) = 1 x 10-4 erg cm-2 (45 s duration)
2) Single detector burst mode variable, depending on duration (see section
III.E.2.b, Tables III-3, III-4).
c. Solar Flare Observations
1) Telescope mode = 1.2 ph cm-2 (1-3 MeV, 100 s duration) ,= 0.5 ph cm-2 (3-10 MeV, 100 s duration)
2) Single detector burst mode as for burst observations, above
3. Miscellaneous Instrument Specifications:
a. Weight: 1460 kg
b. Dimensions: 2.61 m x 1.76 m diameter
c. Power: 206 W
d. Telemetry Rate: 6125 bit/s (equivalent time-average)
e. Timing accuracy: ±1/8 msec with respect to UTC.
D. Data Reduction and Analysis
COMPTEL is a multi-detector instrument consisting of 21 main detector elements
(7 upper and 14 lower modules) and their 154 photo-multiplier tubes; each
possible pairing of an upper and lower detector can be treated as a separate
telescope system, yielding (7 x 14 =) 98 "mini-telescopes." Auxiliary
to the main detectors are the four veto domes and two small scintillator
detectors containing radioactive sources used for in-flight calibration
of the instrument. The raw data consist of individual "event messages"
containing approximately 20 parameters which characterize a selected event
once the telescope is triggered by an incident gamma photon. The COMPTEL
database consists primarily of large sets of such event messages, in addition
to a variety of calibration and in-flight orbital and "housekeeping"
datasets which are maintained to monitor instrument performance and to
correct for gain shifts and drifts in the operation of various detector
elements over the course of the mission.
To process and analyze the enormous quantities of data received from COMPTEL,
the collaboration has developed a customized and modular data analysis
package, COMPASS ("COMPTEL Processing and Analysis Software System"),
which uses the ORACLE database management system for configuration control
and to monitor user access to the data. Each of the four collaborating
institutions is responsible for the development and maintenance of particular
software subsystems which make up the COMPASS package. COMPASS is designed
to run at all four collaborating institutions with approximately equal
capabilities for data processing and scientific analysis, though particular
processing tasks have been assigned within the collaboration to specific
sites.
The routine processing and analysis of data received from COMPTEL proceeds
in three major steps: (1) the processing of raw data, (2) instrument response,
background, and exposure determination, and (3) scientific analysis. For
a further introduction to COMPTEL data analysis procedures, refer to the
reports by Diehl et al. (1992) and den Herder et al. (1992),
in the proceedings of the Compton Observatory Science Workshop (NASA CP-3137).
1. Routine Processing of Raw Data
The routine, "production" processing of raw data from COMPTEL
is carried out at one central site, the Max-Planck-Institute in Germany.
Once data tapes are received from the Packet Processor (PACOR) facility
at NASA Goddard Flight Center, the first priority is the decoding and sorting
of the telemetry data into various data categories (event messages, burst
system spectra, instrument status data, in-flight calibration data, and
housekeeping data). The next major task is to establish the correction
factors for the gain fluctuations of the 154 photo-multiplier tubes (PMTs)
associated with the upper and lower detector modules, using in-flight calibration
data. Energy loss values are then assigned to each detected interaction,
and the locations of gamma-ray interactions within the individual detector
modules are determined, using pre-existing reference tables derived from
the analysis of pre-launch calibration datasets. From these values the
direction of the scattered photon is computed in celestial coordinates,
and the angle of scattering calculated. The resultant values for each calibrated
event, along with its absolute time of occurrence and associated time-of-flight
and pulse-shape information, is written to a database of processed events,
and constitutes "low-level" processed data.
2. Determination of the Instrument Response, Background, and Effective
Exposure
Scientific analysis requires an accurate knowledge of the instrument response
(i.e., COMPTEL's efficiency in detecting incident radiation), of the background
contained in the dataset of accumulated events, and of the effective exposure
of the telescope during a given observation period. Detailed knowledge
of the background level is obtained from data taken in special instrument
modes, by comparison to pre-launch calibration data, and by comparison
to data produced in Monte Carlo simulations based on a model of the COMPTEL
instrument. The fraction of background events can be significantly reduced
by applying certain selection criteria to individual events within the
dataset (e.g., time-of-flight and pulse-shape windows). The optimum setting
of these event acceptance limits, and the choice of "standard"
selection criteria, requires an exhaustive and detailed analysis of all
recorded events.
The exposure factor of the telescope for a given observation period is
the product of the observation time and the telescope response. The telescope
response is an extremely complex function, which depends both on the energy
and arrival direction of the incident gamma-rays, and on the range of accepted
scatter directions and derived scatter angles. The response of the telescope
changes with time since, at any given moment in orbit, the subset of COMPTEL's
(7 x 14 =) 98 mini-telescopes exposed to the high radiation of the Earth's
horizon will vary. Similarly, if the energy thresholds of the detectors
or the acceptance criteria for events are altered, then the overall response
of the telescope will change. A software module has been developed within
COMPASS to calculate the instrument response for each possible configuration
of the telescope, based on the results of pre-launch calibration, Monte
Carlo simulations and an analytic understanding of the telescope. Finally,
the observation time must be corrected for instrument dead-time effects
in order to obtain the true live time for a given observation period; again,
it will be different for each mini-telescope. The final product of this
analysis effort is a set of matrices whose contents characterize the response,
background, exposure, and overall sensitivity of the instrument for a given
observation period and field of view. This collection of datasets will
be used in further scientific analysis, and constitutes the first stage
of "high-level" processed data.
3. Routine Scientific Analysis
Routine scientific analysis of processed COMPTEL data begins with the production
of images of the sky over the field of view of the instrument for a given
observation period. These skymaps are generated using a "standard"
set of selection criteria for accepted events, and the instrument response
and exposure datasets appropriate for that particular observation period.
Model distributions of source and background counts are convolved with
the instrument response, and a map which best fits the observed data is
produced. A number of deconvolution algorithms can be employed within the
COMPASS environment to produce images of the sky; these are based primarily
on maximum entropy techniques, or on one of several event projection methods.
Each skymap is searched for gamma rays using, for example, maximum likelihood
and cross-correlation techniques, and the parameters of an identified source
are determined. Physically extended sources (e.g., due to molecular clouds)
and regions of diffuse gamma-ray emission are also identified by means
of analysis techniques developed within COMPASS for that purpose. If known
or suspected pulsars, or other sources with expected temporal signatures,
lie within the field of view of the instrument, a search for modulated
emission can be performed. Similarly, the analysis of burst events, and
of gamma rays and neutrons of solar origin, is carried out by specialized
tasks within COMPASS. The scientific data which result from these analyses
are considered "high-level" processed data.
Detailed examination of these "standard" scientific data products
may indicate that further data processing for particular regions or sources
is warranted. Selection criteria for accepted events may be altered to
optimize the signal-to-noise ratio, or it may be deemed necessary to produce
a new set of response, exposure, or background matrices more appropriate
to the circumstances of a given observation. Several iterations of the
analysis procedure described in the preceding sections may be required
before consistent and satisfactory results are obtained. In the course
of periodic team meetings the COMPTEL collaboration reviews all routine
analysis procedures, and certifies the scientific integrity of "low-level"
and "high-level" data products obtained to date.
4. Categories of COMPTEL Data Products
The general characteristics of COMPTEL data products are outlined below
with recommendations regarding their use by Guest Investigators. A more
detailed description of COMPTEL data products can be found in the document
"The Gamma Ray Observatory Project Data Management Plan" (PDMP=Appendix
H of this Research Announcement).
COMPTEL data products are grouped into three broad categories reflecting
the level of data processing up to that stage of the analysis:
a. "Raw data" include all instrumental effects, uncorrected
for drifts in gain, etc. These data are essentially reblocked telemetry
data.
b. "Low-level data" are "calibrated" datasets
in which instrumental effects have been removed, and a conversion from
"engineering" to "physical" units has been accomplished
(e.g., pulse-height channel number to energy). These data are primarily
processed event lists, along with associated instrument status and housekeeping
files.
c. "High-level data" result from the scientific analysis
of low-level data; a wide range of data products is possible, as outlined
in the next section. Two general sub-catagories of high-level data are
possible: one involving data where "standard" event selection
criteria have been employed (for initial scientific interpretation, and
assessment of data quality for further scientific analysis), and another
where modifications to the standard sets of selection criteria may be used
to obtain a final, fully-optimized scientific data product.
Typical COMPTEL data products available to Guest Investigators will be:
Low Level: Event data files (calibrated), burst raw spectra (calibrated),
orbit and aspect data tables, instrument mode tables, and instrument performance
summaries. These data permit some timing analysis (e.g., for known pulsars),
the study of instrumental background, and possibly some limited analysis
of strong sources. Detailed study of individual sources is problematic
at best using low-level data, since exact source locations cannot be unambiguously
assigned at this stage of data reduction; events from sources as far apart
as 30 on the sky may populate the same regions in COMPTEL dataspace (i.e.,
photons incident on the telescope from different parts of the sky may produce
nearly identical measured event parameters). Only after proper deconvolution
with the instrument response function is it possible to accurately separate
the multiple sources which may be present within COMPTEL's wide field of
view.
High Level: The high-level data products immediately available to Guest
Investigators will normally be those where "standard" event selection
criteria have been applied in the data processing and analysis. Such data
products would include: maximum entropy skymaps, source parameter tables
(e.g., source positions and fluxes with confidence intervals), the binned
event, background and response matrices used in the skymap and source analyses,
pulsar light curves, and deconvolved energy spectra. These data are appropriate
for direct scientific interpretation. The event, background, and response
matrices may be used to perform an independent analysis of COMPTEL data,
and to undertake comparisons with data taken at other wavelengths. However,
this is not a simple task due to the multi-dimensional event signature
and the highly non-diagonal response matrix of COMPTEL. Standard analysis
techniques and software packages appropriate for other spectral regimes
may require extensive modification before they can be applied to COMPTEL
data.
It is the recommendation of the COMPTEL team that:
Guest Investigators intending to use low-level data products should be
prepared to invest an extended period of time learning the details of instrument
operation and data processing procedures. This can most efficiently be
done at one of the COMPTEL institutions.
Guest Investigators requesting high-level data products must decide if
"standard" COMPTEL images, spectra, etc. are sufficient for their
purposes, or whether the required degree of sensitivity demands an independent
re-analysis using the event data, background, and response matrices that
would be provided. The latter approach would require a minimum of several
weeks of in-depth study of COMPTEL data processing procedures and related
instrumental subtleties.
E. COMPTEL Guest Investigator Opportunities
1. Overview
Research proposals for the COMPTEL Guest Investigator program re solicited from outside scientists through this NASA Research Announcement, and a similar one released by the German Ministry for Research and Technology (BMFT). Selected investigators may wish to spend time at one of the collaborating COMPTEL Institutions or the Compton Observatory Science Support Center.
Some scientific objectives are not suited for a Guest Investigator program.
These include, for example, the production of an all-sky gamma-ray map,
and the establishment of a source catalogue. Such items require the analysis
of the entire COMPTEL data set and are reserved for the instrument team.
Refer to the section on "Proprietary Data Rights" in Appendix
C of this Research Announcement.
2. Sensitivity Estimates
To assist potential Guest Investigators in assessing the feasibility of
a proposed observation preliminary sensitivity estimates are provided below
for each of the main scientific observing modes (see also Table III-1).
a. Telescope Observations
1) Point Sources of Continuum Emission
An expression for the minimum detectable flux, Fs min, from
a point source at n standard deviations above some background level, over
a specified energy range, is given by
Fs min = n (NB)½ / (A D)
(T T) ph cm-2 s-1 MeV-1
= n (NB)½ / Aeff Teff ph cm-2 s-1 MeV-1
where
n = "confidence" level
(NB)½ = uncertainty in the total background (ph MeV-1)
A = detector area (cm2) = area of D1 modules for COMPTEL
D = detector efficiency (a function of photon energy and incidence angles)
T = total time of observation (sec)
T = observing efficiency over the time T and
NB = total background counts
= FB (A D) (T T) (ph MeV-1)
FB = background flux (ph cm-2 s-1 sr-1 MeV-1)
= solid angle for the detection of background (sr)
The product of detector area and detector efficiency is defined to be the "effective area," Aeff, and the product of observing time and observing efficiency the "effective observing time," Teff. The product (Aeff Teff; units: cm2 s) is termed the "effective exposure" of the instrument for a given observation.
In Table III-2 below we provide estimates of the source flux detectable by COMPTEL in double-scatter telescope mode over four energy ranges; these values correspond to a 3 detection of an on-axis source. An observing efficiency of T = 0.3 was assumed (allowing for earth occultation of the source, SAA passages, etc.), yielding Teff = 4 x 105 seconds for a typical two-week observing period. The number of background events was estimated by assuming a 3-wide (±1.5, = radius of the angular resolution over a specified energy range) acceptance interval around a celestial gamma-ray source positioned exactly on the telescope axis.
The following estimates of instrument sensitivity are based on pre-launch
calibration data, Monte Carlo simulations, and actual measurements of the
instrumental background observed in flight by COMPTEL. The background rate
can be influenced in a very sensitive way by event selection criteria.
The most critical selection parameters are the energy thresholds in D1
and D2, the time-of-flight window for accepted events, and the rejection
criteria for earth albedo events. Based on experience acquired during the
Phase 1 Sky Survey of the Compton mission, an initial set of selection
criteria has been defined to optimize the scientific content of the data
received from COMPTEL. These selections may be further modified in the
future.
TABLE III-2
COMPTEL 3 POINT SOURCE CONTINUUM SENSITIVITY
(FOR A TWO WEEK OBSERVATION)
Energy Angular Acceptance Effective NB Fs min(3)
Resolution Radius for Exposure
Background Aeff Teff
Events
(Mev) (1s - deg) (deg) (cm2 s) (ph) (cm-2 s-1) 0.75 - 1 2.0 3.0 24.4 x 105 9570 1.2 x 10-4 1 - 3 1.67 2.5 35.6 x 105 50920 1.9 x 10-4 3 - 10 1.2 1.8 49.2 x 105 16870 7.9 x 10-5 10 - 30 1.0 1.5 63.6 x 105 1150 1.6 x 10-5
1 - 30 1.6 x 10-4
Note that these values reflect the response of COMPTEL to an on-axis source. Observing efficiency declines with increased zenith angle. Figure III-2 illustrates the dependence of detection efficiency on the zenith angle of a source at a representative energy.
2) Point Sources of Line Emission
An analysis similar to that outlined above can be performed for sources of line emission. The 3-sigma sensitivity of COMPTEL to such mono-energetic emission ranges from approximately 6x10-5 cm-2s-1 at 1 MeV, to approximately 1.5x10-5 cm-2 s-1 at 7 MeV. COMPTEL is slightly less sensitive to cosmic line emission in the vicinity of two prominent instrumental background lines at 2.2 and 4.4 MeV.
b. Burst Observations
We provide below a summary of sensitivity estimates for the detection of cosmic gamma-ray bursts by COMPTEL. Refer to the COMPTEL calibration paper of Schoenfelder et al. (1994, ApJ Supp., 86, 657) for additional details and references.
1) Double-Scatter Telescope Mode
In general, the angular resolution of a point source observed with the double-scatter telescope mode of COMPTEL is determined by the spatial and energy resolution of the upper and lower detectors. If S(>E) is the burst fluence (erg cm 2), then the lower detection threshhold is given by
S(>1 MeV)low = E x N(>1 MeV)/Aeff
where E is the mean photon energy (5 MeV) over the energy range 1 to 30 MeV, assuming a power-law spectrum, and Aeff is the effective detection area for totally absorbed on-axis events. If we require N(>1 MeV) = 30 events and Aeff = 30 cm2, we obtain S(>1 MeV) = 8 x 10 6 erg cm 2. The number of background evenets within a 4 x 4 degree resolution element of COMPTEL is about 0.2 counts s 1 (see Table III-2). Therefore the detection of gamma-ray bursts is photon limited, not background limited.
An upper limit to the burst-detection threshhold is defined by the rate of coincidence-by-chance events between the COMPTEL upper and lower detectors. This chance-coincidence rate has been estimated to be approximately 2000 counts s-1, for a gamma-ray burst of typical duration (45 s) and spectral shape (S(>20 keV) = 3 x 10 3 erg cm 2). Therefore, for such typical burst parameters, COMPTEL is sensitive to those burst events with S 10 4 erg cm 2.
For the 4-year period April 1991 to April 1995, COMPTEL has detected 28 cosmic gamma-ray bursts within its field of view of one steradian, in reasonable agreement with pre-launch estimates. Depending on the burst fluence, burst locations can be determined to an accuracy of approximately one degree at MeV gamma-ray energies. Further summary results on the detection of gamma-ray bursts within the field of view of COMPTEL can be found in Kippen et al. (1995, Proc. 24th ICRC (Rome), 2, 61), and Hanlon et al. (1994, A&A, 285, 161).
2) Single Detector Burst Mode - Continuum Sensitivities
Using a background spectrum estimated by the BATSE team, 5- continuum sensitivities of burst fluence S (erg cm 2) for 4 energy ranges and 4 typical burst durations have been calculated for the single-detector burst modules of COMPTEL. Mean photon energies per energy range were derived using typical continuum spectral shapes for burst events: a thermal bremsstrahlung model (exp{-E/E0}; E0 = 250 keV) for energies below 300 keV, and a power law model (E 2) for energies above 300 keV. The number of burst photons were calculated following Li and Ma (1983 Ap. J. 272, 317): S = (Non - a x Noff)/[a x (Non + Noff)]1/2
where the significance S = 5 sigma; number of burst photons Ns = Non - a x Noff; Non = source counts plus background counts accumulated during ton; Noff = background counts accumulated during toff; a = ton/toff with toff = 100 s and ton = 0.1 s, 1 s, 10 s, and 100 s of burst duration. The 5- threshholds on burst fluence are shown in Table III-3.
3) Single Detector Burst Mode - Line Sensitivities
The sensitivity of the COMPTEL burst modules to possible line features in the spectra of cosmic gamma-ray bursts has been estimated, based on pre-CGRO reports, for a range of potential line strengths and burst durations. Refer to the COMPTEL calibration paper of Schoenfelder et al. (1994, ApJ Supp., 86, 657) for a description of these sensitivity estimates and their derivation.
TABLE III-3 CONTINUUM SENSITIVITIES (5 ) TO BURST FLUENCE S (x 10-6 erg cm-2) FOR VARIOUS ENERGY RANGES AND BURST DURATIONS (ton) Energy < E > S Range (x 10-6 erg cm-2)
(MeV) (MeV) ton = 0.1 s ton = 1.0 s ton = 10 s ton = 100 s
0.1 - 0.3 0.19 0.113 0.360 1.19 5.08
0.3 - 1.0 0.55 0.158 0.502 1.66 7.11
1 - 3 1.70 0.264 0.839 2.78 11.9
3 - 10 5.40 0.748 2.38 7.90 34.1
Figure III-2. A plot of COMPTEL detection efficiency, in double-scatter telescope mode, as a function of the zenith angle of the incident gamma-ray photon, at a representative energy of 1275 keV. The upper curve is without any software selections on the data; the lower curve with "standard" event selection criteria applied.
c. Solar Observations
1) Gamma-Ray Observations
Observations of gamma-rays from solar flares can be undertaken in both the double-scatter telescope mode and the burst mode of COMPTEL . Sensitivity limits in general will be comparable to those outlined above for gamma-ray bursts. Preliminary analysis of COMPTEL flight data indicates that, for a solar flare of 100 s duration, the COMPTEL sensitivity in telescope mode to solar gamma radiation is approximately 1.2 ph cm-2 (1 - 3 MeV) and 0.5 ph cm-2 (3 - 10 MeV). An added complication for solar observations is that elevated fluxes, from even moderately strong flares, can result in severe instrumental dead times. Absolute flux values can therefore be extremely difficult to determine.
2) Neutron Observations
Initial analysis of COMPTEL flight data for solar flare observations indicates a 5 sensitivity to solar flare neutrons, above the quiescent solar neutron background, of approximately 1.0 x 10-3 n cm-2 s-1 over a neutron energy range of 10 - 150 MeV. For a duration of 1800 s, this translates to an integrated neutron fluence of 1.8 n cm-2. At present, an unknown factor in the neutron sensitivity is the dead time experienced by the telescope in the later phases of a solar flare. Initial observations by COMPTEL of solar flare neutrons have been affected significantly by the pulse pile-up of thermal x-rays in the forward charged particle shields of the telescope. Even though the first possible detection of neutrons from a flare occurs more than 8 minutes after the onset of the gamma-ray "flash," thermal x-rays persist for considerably longer periods, negatively influencing the telescope's ability to register solar neutrons; elevated and extended fluxes of solar gamma rays can also contribute to the instrumental dead time. Modifications to the solar neutron observation mode of COMPTEL have been implemented to minimize the effects of thermal x-rays.
3. Parameters Required for the Specification of an Observing Program
A potential Guest Investigator who wishes to propose for observing time with COMPTEL should provide the following information on the observation definition form(s) included with this Research Announcement. The primary requirements are outlined below. Note that there are relatively few instrumental configurations that can be selected; detector energy thresholds and the settings for time-of-flight and pulse-shape windows, for example, will, in general, already be optimized for each possible observing mode.
a. Target Name and Instrument Pointing. Provide the name and type (pulsar, AGN, etc.) of the desired target of the observation, along with its position in right ascension and declination epoch 2000.0). For multiple targets within a given field of view, specify the desired center of the field.
b. Energy Range (MeV). Specify the energy range of interest for this observation.
c. Duration of the Observation. An estimate of the number of days of on-axis observation required.
d. Observing Mode. Specify one of the standard scientific observing modes of COMPTEL: double-scatter telescope mode, burst mode, solar gamma or neutron mode.
e. Special Operational Requirements. Indicate "non-standard" observational requirements, if any, for the observing mode specified (e.g., threshold settings, accumulation rates for bursts)*. f. Special Scheduling or other Time Constraints. Indicate if special scheduling constraints are required (e.g., time of year, simultaneous with observations by other instruments, etc.).
g. Standard Data Products. Indicate the standard data products desired for this observation.
h. Special Data Products. Indicate if any "non-standard" data products are required for this observation. Note that special data processing requirements may delay the delivery of data products to the Guest Investigator.*
*N.B. Subject to technical review/feasibility study by the instrument team.
F. COMPTEL Facilities for Guest Investigators
Any of the four collaborating COMPTEL institutions may serve as a facility for the pursuit of a guest investigation: the Max-Planck-Institute for Extraterrestrial Physics (MPE), Garching, Germany, the Laboratory for Space Research (SRON-Utrecht), Utrecht, The Netherlands, the Space Science Center of the University of New Hampshire (UNH), Durham, New Hampshire, USA, or the Space Science Department of ESA/ESTEC (SSD), Noordwijk, The Netherlands. Two of these sites, the Max-Planck-Institute in Germany, and the University of New Hampshire in the USA, have received funding from national sponsoring agencies to support Guest Investigators at their respective institutions. It is expected that most Guest Investigator activities will take place at these two sites. This does not preclude the possibility, however, of pursuing guest investigations at either the Laboratory for Space Research in Utrecht, or the Space Science Department of ESA/ESTEC in Noordwijk, if appropriate for a particular scientific program.
1. COMPTEL Team Facilities for Data Processing and Analysis by Guest Investigators:
All four sites of the COMPTEL collaboration maintain extensive computing facilities for the analysis of COMPTEL data. The COMPASS data analysis software package has been ported to UNIX platforms at all sites, and is usually run on a network of workstations linked to the master ORACLE database and a mass-storage data archive. A variety of graphical-display and higher-level analysis tools are typically also available. Further, all COMPTEL sites maintain high-speed connections to the Internet and the World Wide Web. The various COMPTEL sites can normally accommodate 2-3 short-term visitors for guest investigations.
2. Data Processing and Analysis by COMPTEL Guest Investigators at a COMPTEL Institution
As described in previous sections, the processing and analysis of COMPTEL data is undertaken with a customized modular software package, COMPASS ("COMPTEL Processing and Analysis Software System"), based on the ORACLE database management system. Specialized tasks within COMPASS perform all routine COMPTEL data processing and analysis. COMPASS is currently installed at each of the four collaborating institutions and is the primary means of access to COMPTEL data. The "production" processing of raw data is carried out at the Max-Planck- Institute in Germany. The low-level processed data is then distributed to other COMPTEL sites for further scientific analysis. Processing runs are monitored automatically by COMPASS at each site, and all sites receive periodic updates of processing activities elsewhere by the routine exchange of a database of "dataset descriptors." Bulk data are transferred between the four sites as requested.
COMPTEL Guest Investigators resident at a COMPTEL institution would be encouraged to become conversant in the use of the COMPASS analysis package, and associated software tools. Given the modular nature of the COMPASS system, it is in principle possible to link special- purpose analysis software provided by Guest Investigators to run within COMPASS; analysis could then be carried out experimentally by the Guest Investigator in a special "domain" available within COMPASS for that purpose. In practice, however, this is a difficult and time-consuming endeavor; the COMPASS manager at the affected site would have to assess the feasibility of such an action on a case-by-case basis. The highest computational priority at any site will always be reserved for the routine processing and analysis of incoming COMPTEL flight data.
At the highest level of processed data (e.g., skymaps) COMPTEL datasets conform as much as possible to standard FITS formats. Such images can be displayed on workstations supporting one of the standard astronomical image processing packages (e.g., IRAF, AIPS, MIDAS) where they can be compared to other data of interest to the Guest Investigator. All COMPTEL sites also share a library of customized IDL-based image-processing software.
COMPTEL data processed by or for a Guest Investigator would most likely be delivered on magnetic tape or, if of sufficiently small size, transferred electronically to the Investigator's home institution. Data delivery will be monitored by the Compton Science Support Center.
Standard data products requested for an approved guest investigation are expected to be available for delivery to the Guest Investigator within 2-4 months of the arrival of the relevant flight data tapes for routine processing at MPE in Germany.
3. Data Processing and Analysis by COMPTEL Guest Investigators at a "Remote" Site
Guest Investigators who have previously visited a COMPTEL institution, or who only wish to receive the highest level of processed data from COMPTEL, may choose to pursue their guest investigation independently at their home institution. It is expected that any use of low-level COMPTEL data at a remote site would require first an extended visit to a COMPTEL institution for an in-depth introduction to the intricacies of COMPTEL data processing.
Processed COMPTEL data for an approved guest investigation would be delivered to the Guest Investigator's home institution on magnetic tape, or sent via electronic transfer. It is not intended at present to export routinely any part of the COMPASS analysis software to individual users. The COMPASS software package is however, installed at the CGRO-SSC where it is available for general GI use via guest accounts
Standard data products requested for an approved guest investigation are expected to be available for delivery to the Guest Investigator within 2-4 months of the arrival of the relevant flight data tapes for routine processing at MPE in Germany.
4. Points of Contact for Further Information
For further information respondents to this Research Announcement may contact the CGRO-SSC. Additional information is available through the World Wide Web pages of the COMPTEL collaboration at <http://wwwgro.unh.edu/comptel/>.
IV. EGRET GUEST
INVESTIGATOR PROGRAM
A. Introduction and Scientific Objectives
The broad objective of the Energetic Gamma Ray Experiment Telescope (EGRET)
is to make a major advance in high energy (20 MeV to about 30 GeV) gamma-ray
astrophysics using a gamma-ray telescope with more than an order of magnitude
greater sensitivity and better angular and energy resolution than instruments
previously flown. The study of the gamma-ray sky reveals the sites of the
most energetic interactions occurring in astrophysics. Because these interactions
are generally associated with the dynamic, non-thermal processes in nature,
gamma-ray astrophysics provides an excellent opportunity to learn about
the forces of change occurring throughout the universe. In addition, since
high energy gamma-rays have a very low interaction cross section, they
have a very high penetrating power and can reach the Earth from essentially
any part of the Galaxy or universe.
Beginning with compact objects and proceeding to the Galaxy, other galaxies,
and the universe, the specific objectives of this experiment are:
(1) To search for localized gamma-ray sources in the energy range from
20 MeV to 30 GeV and to measure the intensity, energy spectrum, position,
and possible time variations of each;
(2) To improve the knowledge of the locations and nature of known high
energy gamma-ray sources not identified with objects seen at other frequencies;
(3) To examine supernova remnants and search for evidence of cosmic-ray particle acceleration revealed by high energy gamma-rays, and, if there are cosmic rays at detectable levels, to study their expansion;
(4) To search for high energy gamma-ray bursts, to study the spectra of low energy gamma-ray bursts, and to contribute to the observation of gamma-rays from solar flares;
(5) To obtain a detailed picture of the diffuse high energy gamma-ray emission from the Galaxy, which in turn will provide information of fundamental importance in the study of the dynamics of the Galaxy (This study should provide a high contrast picture of Galactic structure, including spiral arm segments, clouds, the Galactic center, and the extended disk);
(6) To determine the relative importance of cosmic ray electrons and cosmic ray nucleons throughout the Galaxy by studying the gamma-ray spectrum above 20 MeV and, by combining the results with the continuum radio data, to obtain a better understanding of the Galactic magnetic fields;
(7) To detect and examine the high energy gamma-ray emission from other galaxies, including both normal galaxies and active galaxies; and
(8) To study the diffuse celestial radiation in the energy range above
20 MeV including its energy spectrum and the degree of uniformity, both
on small and broad scales (These measurements are of particular importance
in relation to cosmological models.).
B. The EGRET Instrument
EGRET is designed to cover the energy range from 20 MeV to about 30 GeV.
The instrument uses a multilevel thin-plate spark chamber system to detect
gamma rays by the electron-positron pair production process. A calorimeter
using NaI(Tl) is placed beneath the instrument to provide good energy resolution
over a wide dynamic range. The instrument is covered by a plastic scintillator
anticoincidence dome to prevent triggering on events not associated with
gamma-rays. The combination of high energies and good spatial resolution
in this instrument provides the best source positions of any Compton
GRO instrument.
The telescope is shown schematically in Figure IV-1. A gamma-ray entering
the telescope within the acceptance angle has a reasonable probability
(about 35% above 200 MeV) of converting into an electron-positron pair
in one of the thin plates between the spark chambers in the upper portion
of the telescope. If at least one particle of the pair is detected by the
directional time-of-flight coincidence system as a downward moving particle,
and if there is no signal in the large anticoincidence scintillator surrounding
the upper portion of the telescope, the track imaging system is triggered,
providing a digital picture of the gamma-ray event, and the analysis of
the energy signal from the NaI(Tl) detector is initiated. Incident charged
particles are rejected by the anticoincidence dome. Low energy backward-moving
charged particles which do not reach the anticoincidence dome are rejected
by the time-of-flight measurement. Events other than the desired gamma-rays,
such as those gamma-rays interacting in the thin pressure vessel inside
the anticoincidence scintillator, are rejected in the subsequent data analysis.
The directional telescope consists of two levels of a four by four scintillator
array with selected elements of each array in a time-of-flight coincidence.
The upper spark chamber assembly consists of 28 spark chamber modules interleaved
with twenty-seven 0.02 radiation length plates in which the gamma ray may
convert into an electron pair. The initial direction is usually determined
from the upper spark chamber data. The lower spark chamber assembly, between
the two time-of-flight scintillator planes, allows the electron trajectories
to be followed, provides further information on the division of energy
between the electrons, permits seeing the separation of the two electrons
for very high energy gamma-rays, and shows the entry points of the electrons
into the NaI detector.
The energy of the gamma-ray is determined in large part from measurements
made in an eight radiation-length thick, 76 cm x 76 cm square NaI(Tl) scintillator
crystal below the lower time-of-flight scintillator plane. Spark chamber
measurements of Coulomb scattering in the thin plates and position information
in the spark chamber system also aid in the energy determination. The energy
resolution of the experiment is about 20% (FWHM) over the central part
of the energy range. The resolution is degraded to about 25% above several
GeV due to incomplete absorption in the NaI calorimeter, and at energies
below about 100 MeV where ionization losses in the spark chamber plates
comprise an appreciable portion of the total energy. In addition, some
particles may completely miss the calorimeter. These losses can be partially
recovered through an analysis of the scattering characteristics.
The anticoincidence dome event rate is determined every 1/4 second. These
data may be used in the study of gamma-ray bursts and solar flares. Also,
the large NaI(Tl) crystal may be used for spectroscopy between 0.6 and
140 MeV independently of the spark chamber system. If a BATSE trigger signal
is received, spectra are recorded in four commandable time intervals (normally
0 to 1 sec, 1 to 3 seconds, 3 to 7 seconds, and 7 to 23 seconds) and then
sent in the EGRET telemetry stream over the next 35 minutes. In addition,
a spectrum between 0.6 and 140 MeV is accumulated every 33 seconds. These
spectra are useful for the study of gamma-ray bursts with no associated
BATSE trigger, (e.g. when BATSE is reading out a previous event) and for
the study of the high-energy portion of the solar flare gamma-ray spectrum.
A model of the spacecraft mass and its effect on the gamma-ray spectrum
for a specific direction will be available for these studies.
The EGRET operating mode may be altered in two ways. In the common mode,
an energy deposition of 7 MeV in the NaI(Tl) calorimeter is required for
an event. This mode increases instrument live time by reducing the rate
of triggers. It does reduce the sensitivity somewhat (see Table IV-1).
For an observation where low energy sensitivity is desired in spite of
the poor angular accuracy, it would be reasonable to remove the calorimeter
coincidence requirement. The event rate may be reduced by limiting the
time-of-flight telescope tile combinations which cause a trigger. The 16
tiles in each of two planes form 96 recognized sub-telescopes which point
in 9 different directions, each approximately 0.1 steradians in size. These
9 direction groupings are utilized to limit triggers due to earth albedo
gamma-rays, but simultaneously maximize celestial exposure. It would be
possible to observe with a subset of these direction modes. Vertical only
would give the best efficiency. The field of view would be greatly reduced
and not available to others.
The principal characteristics of the EGRET instrument are summarized in
Section C. The Co-Principal Investigators of EGRET are Dr. Carl E. Fichtel
of the NASA/Goddard Space Flight Center (GSFC) and Dr. Klaus Pinkau of
the Max Planck Institute for Plasma Physics. Co-Investigators are at the
Goddard Space Flight Center, Stanford University, the Max Planck Institute
for Extraterrestrial Physics, Grumman Aerospace Corporation and Hampden-Sydney
College.
Figure IV-1. The EGRET Instrument
C. Instrument Parameters and Capabilities
1. Type: spark chambers, NaI(Tl) crystals, and plastic scintillators.
2. Energy Range: 20 MeV to about 30 GeV
3. Energy Resolution: approximately twenty percent over the central part
of the energy range.
4. Total Detector Area: approximately 6400 cm2
5. Effective Are : approximately 1500 cm2 between 200 MeV and
1000 MeV, falling at higher and lower energies.
6. Point Source Sensitivity: varies with the spectrum and location of the
source and the observing time. Under optimum conditions, well off the galactic
plane, it should be approximately 6 x 10-8 cm-2s-1
for E > 100 MeV for a full two week exposure. See Section VI of this
Appendix for more detailed sensitivity information.
7. Source Position Location: Varies with the nature of the source intensity,
location, and energy spectrum from 5 - 30 arcmin.
8. Field of View: approximately a gaussian shape with a half width at half
maximum of about 20. Note that the full field of view will not generally
be used.
9. Timing Accuracy: 0.1 ms absolute
10. Weight: about 1830 kg (4035 lbs)
11. Size: 2.25 m x 1.65 m diameter
12. Power: 190 W (including heater power)
D. Data Reduction and Analysis
The EGRET instrument is a wide field telescope even when only one of its
subtelescope directional modes is used. Each observed gamma ray is detected
individually. Even though the instrument is designed to detect high energy
gamma rays, much less than half of the events resulting from spark chamber
triggers satisfy the rigid rules for finally accepted celestial gamma ray
events. It is the EGRET Science Team's responsibility, together with their
EGRET team of data analysts and programmers, to separate the gamma rays
from other triggers and determine within errors the direction and energy
of each gamma ray based on carefully quality controlled procedures and
the calibration data. An entire observation is analyzed as a set since
there is no way to select gamma rays or subsets prior to analysis. The
direction will be given in terms of galactic, celestial and Earth coordinates.
It is the responsibility of the EGRET Science Team to make a determination
of the diffuse celestial radiation, galactic and extragalactic, to the
best of their ability for all regions observed. They are also responsible
for determining source strengths and spectra in a consistent, carefully
reviewed fashion and maintaining a catalog. They will also provide upper
limits, when it seems appropriate, for a potential source. When a Guest
Observer source has been named and the data from the appropriate observing
period analyzed, he or she will be given the information on the relevant
photons and the estimated source strength and spectrum or upper limit.
These data will be withheld from the public domain including the catalog
for the duration of the proprietary period after it is given to the Guest
Observer.
The telemetered data from EGRET are organized into packets of 1,786
bytes that span a time interval of 2.048 seconds. Each packet contains
a header with the time of day to the nearest second, and spacecraft position,
pointing, and earth-center coordinate information. Following the header,
housekeeping data consisting, for example, of voltages, currents, pressures,
temperatures, command verification, counter rates, and instrument operating
modes are included. Spectral data from the Total Absorption Shower (~ 0.6
to ~140 MeV) Counter (TASC) are routinely sent with each set of 16 packets.
Likewise, burst data, when present, are encoded into a series of 1,024
packets. The remainder of the packet data consists of individual event
information. Events are associated with a triggering of the spark chamber
and consist primarily of coordinate readouts of chamber tracks, although
other information, including a precise time vernier, the set of sub-telescopes
that were triggered, and the TASC energy measurement are part of each event
data. Since gamma rays can trigger the detector at any time, the event
data are asynchronous to the packet formation. Events are therefore stored
in a buffer on-board and are moved from there to the packets during their
assembly by the on-board firmware. Up to 12 events of variable length may
occur within any packet, and events may be continued from one packet to
the next.
Production processing of the telemetry information consists of decoding
the packet data, scaling all housekeeping data to engineering units, assembling
the TASC spectra from the appropriate set of packets, separating and assembling
events along with event related information, and, finally, analyzing each
event. During the course of one day, approximately 7.5 Megabytes of housekeeping
are accumulated to monitor instrument performance and to determine the
sky exposure to gamma-rays. The TASC will generate 1,760 spectra, and the
spark chamber is triggered approximately 5 x 104 times.
The event analysis is the most critical part of the entire ground analysis
system. Each event contains spark coordinate information in two orthogonal
views. First, it is necessary to structure the event. This process involves
determining which spark coordinates to associate with tracks from the gamma-ray
interaction, and then to correlate the multiple track projections in the
two views of the read-out. The usual signature of a gamma-ray interaction
is an inverted "V" formed by the electron and positron that issue
from the point where the gamma-ray conversion occurs.
Once the tracks are properly structured, the event is screened to assure
that the appropriate signature has been found and that the interaction
occurred within the spark chamber volume. A multiple-scattering analysis
is then performed for each track to estimate the energy of each secondary
particle. Analysis of the total energy of the event is made using the data
from the TASC and correcting for energy loss in the plates between the
spark chambers, and for leakage of particles out the side of the detector,
using the track information and the scattering results. When both the scattering
and energy calculations have been made, a procedure is followed to select
the best energy values for the two principal tracks that form the vertex.
A least-squares fit to the points of each track in each view is made where
the number of points used is based on track straightness. A weighted bisector
of the angle between the tracks is constructed for each orthogonal view.
This direction, together with the pointing direction of the instrument,
is used to determine the arrival direction of the gamma-ray. In summary,
the event processing provides arrival time, energy, and direction for each
gamma-ray.
The computer analysis that generates the initial structuring must be
closely monitored by trained personnel and knowledgeable scientists who
study the two orthogonal views of analyzed events in light of the developed
rules and comparisons to calibration data. If improvements are to be made
in the identification of the sparks associated with tracks, an analyst
can make these changes using an interactive graphics workstation. The production
processing system assigns categories of potential structuring problems
which are then selectively reviewed manually. In addition, a random selection
of events are also reviewed to insure that the system is meeting standards.
The most time consuming task, requiring trained scientists and data analysts,
is the individual gamma-ray event studies on the graphics units for those
events which have been flagged by the automatic analysis as needing more
study for any of several specific reasons. Some of these include: uncertainty
in the automatic program regarding whether the event comes from the wall,
inability to satisfactorily match the two orthogonal views for any of several
reasons, or starting point difficulties. This tedious process must be accomplished
for an entire field of view before any of the results from that field of
view can be determined since not only gamma-ray identification, but also
direction and energy may be affected. The series of steps outlined above
must be performed for all photons in the field of view for a given exposure
before the sky map can be generated and the point source analysis be done.
The determination of the exposure factors is especially complex for EGRET.
The telescope is divided into 96 sub-telescopes that are organized into
groups that point in 9 different directions. During any orbit, the directional
groups are dynamically commanded "off" and "on" by
the Compton GRO computer depending on whether or not the Earth is
in the field of view for that direction. The 9 sets of direction groupings
are not independent. That is, an event may be able to trigger more than
one mode. The direction modes therefore must be considered in combinations.
A separate set of instrument performance properties of efficiency, angular
resolution, and energy resolution as a function of energy must be referenced
for each of 74 combinations in order to construct the exposure. If the
TASC threshold or instrument triggering requirements are changed, another
set of tables must be generated for use. Typically the determination of
exposure will involve integrating the product of live-time, detection efficiency,
and solid angle for each bin on the sky, taking into account the time varying
mode changes. Any hardware anomalies that require turning off part of the
telescope system will require that complete new performance tables be generated
from the calibration event data by eliminating events that could not have
been detected if the disabled portion of the instrument had not been active.
A software system will be in place to generate exposures from the housekeeping
data of the instrument, but it must be carefully monitored, especially
if trigger requirements or thresholds are changed, or if any of the coincidence
tubes have to be turned off.
The combination of overlapping fields of view is desirable in order
to increase the statistical significance and, in some cases, to reduce
the uncertainties produced by the low sensitivity for an object far from
the axis. The analysis of a point source must also take into account the
energy dependence of the angular resolution and the background spatial
distribution, including its energy dependence.
The result of the standard production processing is a comprehensive
EGRET primary database (PDB). From the PDB, a secondary set of databases
are constructed which serve as the input for the scientific analysis. The
calibration database contains information needed to produce spectra and
skymaps and build the event databases. Two other crucial databases are
a summary database which is a condensed form of the gamma ray event database
and will be the primary input for many scientific analyses and an exposure
history database which contains pointing and instrument mode information
which is needed for determining the exposure maps.
An extensive library of programs exist for manipulating and analyzing science
data. For instance, skymaps can be developed with the programs MAPGEN,
INTMAP, and SKYMAP. MAPGEN uses the summary database to create or update
gamma-ray skymaps for a user specified range of arrival directions, energy
and time ranges in a variety of coordinate systems.. The output of MAPGEN
is a FITS format file. The INTMAP module takes the maps created by MAPGEN
and utilizes the timeline and exposure history files to create corresponding
exposure and intensity maps, again in FITS format. These files can also
serve as the input to SKYMAP which is useful for interactively displaying
and plotting skymaps. SKYMAP capabilities include rescaling the map, adding
contours, and displaying multiple maps. The program SPECTRAL is used for
the analysis of the energy spectrum of the detected source.
If the temporal properties of a source are of interest, PULSAR and SEARCH
are available to analyze the time series for an observation. PULSAR is
an interactive program which will fold the data at an assumed pulsar period
and generate the light curve. Proper barycentering, period derivative,
and binary orbit corrections are made if needed. For searches over a range
of trial frequencies, SEARCH will allow the user to apply statistical tests
to the set of arrival times over a user specified range of periods. The
significance as a function of frequency is the most important output of
SEARCH.
Other programs and tables will be available in the near future. Theses
include the numerous tables in the EGRET Phase 1 catalog, a program to
give source positions and strengths, a table of estimates of the gamma-ray
diffuse radiation, and a program to search for pulsed radiation within
a limited range of parameters.
E. EGRET Guest Investigator Opportunities
