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

A.7. TDRSS MESSAGES 77
a burst alert. If no position is found, then a burst NACK follows. If a position is found, then a
burst position message follows along with a scaled map. In either case (position or no position), a
scaled map is sent down in the ground pass.
The burst alert, position and NACK messages are Null Arrays with all information contained
in the header.
Scaled maps are detector plane images with a single number for each detector representing the
counts in that detector during the time interval used to produce the image of the burst. Other
columns contain diagnostic information about the burst.
A.7.1 BAT GRB Alert
This provides the first evidence that the BAT has detected a potential GRB. The BAT monitors
several quantities that characterize the gamma-ray sky. Each of these quantities defines an index
into a table. The BAT triggers when one or more of these quantities exceeds a pre-defined threshold.
The BAT GRB Alert message information includes:
• the time when the trigger was detected,
• the table index of the trigger with the highest value, and
• the trigger significance.
A description of possible quantities that can initiate a BAT trigger is contained in Fenimore,
et al., ”The Trigger Algorithm for the Burst Alert Telescope on Swift” (astro-ph/0408514;
http://content.aip.org/APCPCS/v662/i1/491 1.html).
A.7.2 GRB Position Message (ACK or NACK)
There are two possible BAT position message types: the “BAT ACK” in which a point source can
be imaged and the “BAT NACK” in which no point source is detected in the BAT image. Both
message types are distributed through the GCN.
After the GRB Alert Message, the FFT-based image reconstruction from the BAT will confirm
(ACK) or not confirm (NACK) that the trigger is from a point source. The ACK message contains
the position of the source, intensity information and the length of the best trigger at the time of
the alert. This can distinguish between fast and slow bursts but doesn’t give the full-burst fluence.
The significance of the detection is included, as well as a flag where each bit gives more detailed
information on the detection. The BAT position message will have an accuracy of 1–4 arcminutes,
depending on the source brightness.
The NACK message is similar in content to the ACK message, but a flag is set to indicate a
point source solution was not found. If the NACK message is received, (a non-confirmed trigger),
no other GCN messages follow, except for the diagnostic scaled map from the BAT.
A.7.3 FoM and S/C Will/Will Not Observe
Following an ACK message for the BAT position, the BAT sends the GRB position to the onboard
FoM. Two messages follow: FoM will/will not observe and S/C will/will not slew. The first reports
whether the new BAT position had sufficient merit to become the next Automated Target (AT)
and to request an autonomous slew. The second gives information about whether the S/C can or