The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

Chapter 3
BAT Operating Modes and Data
Types
3.1 Introduction
The BAT flight software produces several data products. As mentioned in the previous chapter,
the BAT is a photon counting instrument. That is, each photon interaction is recorded separately
by the flight electronics and flight computer. However, like other missions there is not enough
on-board storage or downlink capacity to send all of these events to the ground. Instead, the BAT
sends full event data for special triggers (i.e. for gamma-ray bursts), and otherwise makes binned
products.
Normally, the BAT is searching for a new GRB trigger while in survey mode. In this mode,
an energy spectrum is accumulated for each detector in the array on a time scale of approximately
every 5 minutes. While this data allows an observer to recover the fluxes of sources over long
time scales there are significant systematic errors that must be addressed, and have not yet been
accounted for in the BAT software.
After a successful trigger, several products are produced in response to a gamma-ray burst.
The most significant of these is a dump of event data which brackets the trigger time, with a total
duration of about 10-15 minutes.
The BAT also produces several products all the time, regardless of whether there is a GRB
or not. These are typically various array rates, housekeeping values, and trigger diagnostics.
A more complete description of these products is presented in the rest of this chapter.
3.2 GRB Products and Response
Most of the time the BAT is in survey mode. Trigger criteria are evaluated on a continuous basis.
When the BAT detects a rate increase (in one of several energy bands or spatial regions), then it
indicates a “rate trigger,” which in turn initiates on-board image processing to confirm a new point
source. Typically, there are many rate triggers per hour, each given its own trigger number, but
few are confirmed by imaging. On a periodic basis, the BAT flight software also performs image
analysis independent of the rate triggering system, to search for slow rising GRBs and transients.
When the BAT receives a GRB trigger the BAT and the spacecraft have a preset set of responses.
The typical response process is as follows, roughly in order of the time of production:
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