WARNINGS
SYSTEMATIC ERRORS IN EARTH OCCULTATON MEASUREMENTS WITH BATSEWe have identified several sources of systematic error which are present in the BATSE earth occultation measurements. Even though care was exercized in the preparation of these results, the user is strongly urged to examine the data carefully before drawing conclusions.
The flux histories for various sources which are being delivered to the COSSC at this time report only statistical errors. However we have identified several possible sources of systematic error which affect the measured count rates for a particular source and the determination of absolute fluxes. Potential users of such data obtained should be aware of these sources of error, some of which are under study at this time:
(1) calibration and response uncertainties
These effects, are to first order, unrelated to the occultation method itself, but affect the final absolute values of flux and spectra. We are comparing Crab Nebula data between the eight BATSE large area detectors in order to understand and quantify various effects, and to possibly remove some of the inconsistencies between detectors.
(2) Source occultation step measurements
a) Outliers due to transient structures, spikes, or pulses in the background data. These are flagged in the data (See User's Guide for flag setting information) where possible, although the measurements at the flagged times are not removed from the source history files.
b) Shortcomings in the atmospheric attenuation/occultation model used to fit the occultation features, atmospheric scattering. These have not yet been investigated.
(3) Interfering sources
a) known sources which are fit simultaneously. Residual improper sharing of flux in poor Earth-source limb geometries.
b) unknown sources during the times in which the flux history was obtained (unavoidable, but as more careful searches are peformed most emissions should be found and accounted for on subsequent passes through the data)
c) background components, the occultation range, dependence on sky location. Most of these effects, except the occultation range have not been investigated rigorously. Most tests we have performed show that, when the data sampling range has been increased, the sensitivity has not been improved.
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