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

78 APPENDIX A. BAT DATA FORMATS
will slew. In some cases, while the new BAT position may have a high merit, spacecraft pointing
constraints might be such that it is impossible to slew immediately.
A.7.4 BAT Light Curves
The purpose of the BAT TDRSS light curve messages is to provide a quick indication of what
the burst looks like and to allow it to be classified as long or short. For the lightcurve messages,
all counts detected by the entire BAT are telemetered. Note that BAT TDRSS light curves
are not background subtracted. This is made especially complicated during a slew, as other
astrophysical sources enter and leave the BAT field of view. More complete information to do a
faithful background-subtraction on BAT light curves is available from the event data.
A set of BAT light curve messages is sent if there is a position for the GRB (i.e., the BAT
position message is ACK). All light curves are produced in four (adjustable) energy ranges so that
some information about the hardness of the GRB may be obtained quickly; the current defaults
are 15–25 keV, 25–50 keV, 50–100 keV, and 100–350 keV.
To insure the very important early-time energy and time evolution information from the GRB
is received on the ground, Swift transmits three light curve messages containing overlapping data
via TDRSS, each containing the four band-limited light curves. These are:
First lightcurve message Up to 32 seconds long, spanning T - 24 seconds through T + 8 seconds,
where T is the BAT rate trigger time. This message is typically cut off by the slew and might
also be truncated by the data available when it is produced (i.e. if there are only 5 seconds
of data past the trigger at production time, the last three seconds the first light curve will
contain “UNKNOWN” values). Typically sent within 20 seconds of the BAT rate trigger.
Updated first lightcurve message This is a complete 32-second light curve message. To generate
it, the BAT software waits about 40 seconds, then continues to wait until Swift is not
slewing; it then computes the 32-second light curve spanning the T - 24 seconds through T
+ 8 seconds, then transmits the TDRSS message. This is essentially a copy of the first light
curve message, but it is guaranteed to contain the entire 32 seconds around the GRB trigger
time. This message will be useful for identifying bursts that re-brighten within a couple of
minutes after the main trigger.
Second lightcurve message This is 128 seconds long, spanning T - 24 seconds through T + 104
seconds. This message is sent within a second of the end of the 128-second light curves, and
about 200 seconds after the BAT rate trigger.
UPDATE (as of Mar 1, 2006) the BAT produces an additional set of light curve information, which
covers more time, pre- and post-trigger. Also, finer time sampling is available around the
trigger time (to enhance the sensititivity to short bursts). The original light curve is preserved
unchanged; the new light curve segments are placed in another extension (EXTNAME =
‘TDRSS LC2’).
The final BAT TDRSS message, the scaled map, is for verification of sources foundand/or
refined analysis of sources not found.