Description and Use of TTE Data
Time-tagged event (TTE) data is derived from the on-board data stream of BATSE's eight Large Area Detectors (LADs). For up to 32768 photons from a triggered event, this data contains 1) the arrival time (2-microsecond resolution), and 2) the discriminator energy channel into which the photon was pulse-height analyzed. The channel boundaries are the same as those for the DISCLA, DISCSC, PREB, and TTS data types: approximately 25-55 keV, 55-110 keV, 110-320 keV, and >320 keV. The TTE time resolution is commandable from the ground, but in fact has been set at 2 microseconds throughout the Compton GRO mission. Thus, when decommutated from the native TTE_IBDB format, the resulting data stream consists of ones and (many) zeroes in evenly spaced 2-microsecond bins and an associated energy channel (1=>4) per detected photon.The high temporal resolution of TTE data is particularly well-suited for studies of very short high-energy transient phenomena, including Gamma-Ray Bursts (GRBs) shorter than ~2 seconds (those bursts comprising the short mode of the gamma-ray duration distribution), Terrestrial Gamma Flashes (TGFs, thought to be associated with thunderstorm cells), and Soft Gamma Repeaters (SGRs). All three phenomena have been detected by BATSE. Time profiles of the latter two types are almost always contained within the 32K event window; portions of "short" GRBs, however, are sometimes missed, for reasons elucidated below.
Except for the minimum in the GRB duration distribution near 2 seconds, there is no a priori reason to truncate the "short" GRBs at that duration; however, the 32K event buffer would be used up after ~3 seconds, even in the absence of any GRB signal, since the summed LAD background counting rate is of the order 10 counts per millisecond.
Correspondence between times in TTE and, e.g., the PREB+DISCSC burst record is, in principle, established by the fact that one-fourth of TTE's 32K event ring buffer is frozen at trigger time (junction between PREB and DISCSC); thence, three-fourths of the TTE events are logged after trigger time. However, three effects can complicate this simple picture, so that correspondence with PREB+DISCSC is not always straightforwardly had:
- There is a ~30 ms offset between trigger time and TTE data load time.
- Data gaps (incomplete 32K buffer) can occur (invariably) at buffer end.
- Before trigger, all eight LADs contribute to populating the TTE 32K buffer, whereas after trigger, only triggered detectors (2 or more) contribute (therefore, even when the event buffer is completely captured, one should not estimate trigger time by dividing the photons in ONLY triggered detectors into the 1/4 : 3/4 ratio).
A thorough simultaneous visual inspection of PREB+DISCSC and TTE time profiles for short bursts was performed in order to determine which short GRBs are completely or partially captured by the TTE data type. A list was then generated that grades (A=>E) TTE "completeness" for each short burst with TTE data.
A slightly more detailed description of the TTE data type can be found in the BATSE Flight Software User's Manual (MSFC-MNL-1405), page 32, available in hardcopy from the Compton Observatory Science Support Center.
TTE data is available in ASCII format for "short" GRBs with T90 durations shorter than ~2 seconds.
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