The Neil Gehrels Swift Observatory

Swift TDRSS Messages and the GCN

TDRSS Messages from Swift

In order to quickly inform the GRB and afterglow community of new bursts, for each BAT trigger Swift autonomously sends a series of short informative messages through the Tracking and Data Relay Satellite System (TDRSS) that are received by the Gamma Ray Burst Coordination Network (GCN) and distributed promptly to the community.

This page gives an overview of the Swift TDRSS messages and the GCN data products. These quick products are available to the community within a few seconds to several minutes after the BAT burst trigger. These quantities are not highly processed or extensively calibrated. More robust and complete Swift data arrive later (on a timescale of hours) through different communications systems. These later data are called the "Malindi data", and they are described in detail separately.

Even though the TDRSS data are preliminary, they allow ground-based observers and other interested parties to decide whether or not to follow-up the GRB with a pointed observation, and thus maximize the scientific return of the mission.

TDRSS Messages: Data Flow

When a BAT rate trigger results from a point source it generates a BAT TDRSS alert download, which is sent through TDRSS and received at the GCN. This is then released (after some minimal processing) as a GCN BAT alert.

The Figure below shows the flow of data from the Swift spacecraft, to TDRSS, then via White Sands to the GCN and the world.

TDRSS data flow diagram

The Swift strategy is to slew to each new GRB position as soon as possible and to follow all the GRB afterglows as long as they are visible. To see the earliest phase of the afterglow, new BAT positions trigger an autonomous slew followed by a programmed sequence of observations with the focusing telescopes. The slew time for the Swift baseline is usuall less than approximately 90 s. The XRT and UVOT observations begin while the burst is still in progress for approximately 30% of all bursts.

The following gives a list of some relevant TDRSS messages and a rough timeline for their delivery. It assumes a spacecraft slew time of 75 seconds while the nominal value for slews is about 50 seconds.

Not all messages are generated for every BAT trigger; this depends on what is found by the instruments and by the FoM decisions. Not all possible TDRSS messages are listed here. Spacecraft emergency or instrument emergency messages are sent through TDRSS, but they do not generate GCNs and they are not included here. More information on Swift GCN messages is available on the Swift GCN Message Information page.

BAT TDRSS messages

For BAT, the GRB alert, GRB position, and lightcurve messages are the standard data products being sent via GCN to the community.

BAT GRB Alert Message

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:

  1. the time when the trigger was detected,
  2. the table index of the trigger with the highest value, and
  3. 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"

Example BAT TDRSS Alert Message

Here is an example BAT TDRSS GRB Alert message, translated to FITS. 

TELESCOP= 'SWIFT   '           / Telescope (mission) name
INSTRUME= 'BAT     '           / Instrument name
OBS_ID  = '00020531000'        / Observation ID
TARG_ID = '20531   '           / Target ID
SEG_NUM = '0       '           / Sequence within the target ID
DATE-OBS= '2003-12-24T15:28:47.5' / Date observed
OBJECT  = 'GRB20031224'        / Possible name
DATE-END= '2003-12-24T15:28:47.5' / Date observation ended
TSTART  =         93972527.488 / Observation start time
TSTOP   =         93972527.488 / Observation stop time
TRIGGER =                20531 / Burst Trigger Number
TRIGTIME=         93972527.488 / TRIGger float TIME in seconds
UTCCTIME=                   0. / UTC Correction TIME
TRIGSATF=                   64 / TRIGger SATisFied
SCORE   =                   49 / burst Score
SIGNIF  =                   7. / burst SIGNIFicance
PKTTIME =          93972527.85 / ccsds PacKeT TIME
APID    =                  341 / data derived from ApID
END

The BAT first triggers on a rate increase (rate trigger), then performs an FFT-based image reconstruction of the sky. If it finds a source in this image, it generates an image trigger. This takes 6-12 seconds onboard. The image triggers are more reliable than the rate triggers since they require that the photons come from a point source, so they should eliminate many false triggers based on particle background events, etc.

To avoid confusion, BAT triggers are suppressed for a (commandable) 5 minute interval after a GRB.

GRB Position Message

There are two possible BAT position message types: the "BAT ACK" and the "BAT NACK". Both message types are distributed through the GCN. After the alert, 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 doesn't give the full-burst fluence. The significance of 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.

Example of a BAT TDRSS Position (ACK) Message

Here is an example of a BAT TDRSS Position (ACK) Message for a rate trigger in which a point source has been found by the onboard processing. The message has been translated into FITS. 

TRIGGER =                20531 / Burst Trigger Number
TRIGTIME=         93972527.296 / TRIGger float TIME in seconds
TRIGSATF=                  450 / TRIGger SATisFied
SIGNIF  =     17.2336879396141 / burst SIGNIFicance
IMAGETRG=                    F / Image Trigger occured?
CATSRC  =                    F / Catlog Source?
INTEREST=                    F / Interesting?
GRB     =                    T / Gamma Ray Burst?
PNTSRC  =                    T / Point Source found?
GRB_RA  =     23.1856729830814 / [deg] bat GRB RA location
GRB_DEC =    -41.9685701419874 / [deg] bat GRB DEC location
BATTHETA=     16.1640012557666 / [deg] BAT THETA on detector
BATPHI  =     -45.437922245969 / [deg] BAT PHI on detector
PEAKINT =                  360 / PEAK INTensity
TOTFLUE =                 7156 / burst TOTal FLUEnce
BACKFLUE=                45038 / BACKground FLUEnce
FLUENCE =             1391.136 / burst FLUENCE (background subtracted)
BACKSTRT=             93972512 / [s] BACKground STaRT time
SIGNIF  =     3.16227766016838 / BAT detector SIGNIF
FORETIME=                1.024 / [s] Foreground TIME
TRIGSTOP=          93972528.32 / [s] End of foreground interval
BACKTIME=                   8. / [s] Background TIME

The BAT TDRSS NACK Message

The NACK message is similar in content to the ACK message, but the flag is set for point source solution not found. If the NACK message is receieved,(a non-confirmed trigger), no other GCN messages follow, except for the diagnostic scaled map from the BAT.

FoM and S/C Will/Will Not Observe

Following an ACK message for the BAT position, the BAT sends the GRB position to the FoM. Two messages follow: FoM observe and S/C slew. The first communicates whether or not the new BAT position had sufficient merit to become an Automated Target (AT) and to request a slew. The second gives information about whether the S/C will or will not slew.

BAT Lightcurve

The purpose of the TDRSS lightcurve is to provide a quick indication of what the burst looks like and to allow it to be classified as long or short. BAT TDRSS light curves are not background subtracted. More complete lightcurves of longer bursts will be available from the Malindi data.

A set of BAT lightcurve messages is sent if there is a position for the GRB (the BAT position message is ACK). All lightcurves are produced in four adjustable bandpasses. The current defaults are 15-25 keV, 25-50 keV, 50-100 keV, and 100-350 keV.

To obtain the very important early-time lightcurve information, the BAT sends multiple copies of the four band-limited lightcurves; each copy is sent in a separate TDRSS message. Specifically, there are three lightcurve messages sent via TDRSS (again, each contains four band-limited lightcurves).

First lightcurve message
This is up to 32 seconds long, spanning the BAT rate trigger time T-24 seconds through T+8 seconds. This message is likely to be 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 of that first copy of the first lightcurve packet will contain "UNKNOWN" values).
Updated first lightcurve message
This is a complete 32-second lightcurve 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 lightcurve spanning the burst T-24 seconds through T+8 seconds and sends the message. This is essentially a copy of the first lightcurve message, but it is guaranteed to contain the entire 32 seconds (not cut off or truncated).
Second lightcurve message
This is 128 seconds long, spanning the burst trigger time T-24 seconds through T+104 seconds. This message is sent within a second after the last second in the 128-second lightcurves, and about 200 seconds after the BAT rate trigger.

BAT Scaled Map

To be provided by a BAT team expert. The scaled map is for verification of sources found and/or refined analysis of sources not found.

Burst Detection

The burst trigger algorithm looks for excesses in the detector count rate above expected background and constant sources. It is based on algorithms developed for the HETE-2 GRB observatory, upgraded based on HETE-2 experience. The algorithm continuously applies a large number of criteria that specify the pre-burst background intervals, the order of the extrapolation of the background rate, the duration of the burst emission test interval, the region of the detector plane illuminated, and the energy range. The BAT processor continuously track hundreds of these criteria sets simultaneously. The table of criteria can be adjusted. The burst trigger threshold is commandable, ranging from 4 to 11 sigma above background noise with a typical value of 8 sigma. A key feature of the BAT instrument for burst detection is its imaging capability. Following the burst trigger, the on-board software checks for and requires that the trigger corresponds to a point source, thereby eliminating many sources of background such as magnetospheric particle events and flickering in bright galactic sources. Time-stamping of events within the BAT has a relative accuracy of 100 μs and an absolute accuracy from the spacecraft clock of ∼200 μs. When a burst is detected the sky location and intensity will be immediately sent to the ground and distributed to the community through the Gamma-Ray Burst Coordinates Network (GCN) (Barthelmy et al. 2000).