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Swift GCN and TDRSS Messages

Jump to:

• Overview of TDRSS Messages
• BAT Data in GCN/TDRSS Messages
• UVOT Data in GCN/TDRSS Messages

Overview of TDRSS Messages

The GRB Coordinates Network (GCN) automatically receives Swift TDRSS GRB messages and distributes them to the community. A subset of the Swift TDRSS messages receives value-added processing at the GCN (eg., the UVOT finding chart) before being sent on to users. The table below gives a list of some relevant TDRSS messages, and a rough timeline for their delivery to the user by the GCN. It assumes a spacecraft slew time of 75 seconds.

Not all messages are generated for every BAT trigger; this depends on what is found by the instruments and by the Figure of Merit (FoM) "decisions." Note that while spacecraft emergency or instrument emergency messages are sent through TDRSS, they do not generate GCNs and are not part of a typical timeline, so they are not included.

TDRSS Message Type	Typical Time from BAT rate trigger (s)
BAT GRB Alert	20
BAT Position	20
BAT first lightcurve (4 energy bands)	20
FoM Will/Will Not Observe	70
S/C Will/Will Not Observe	70
BAT updated first light curve (24 s before trigger to 8 s after)	at end of slew
XRT Position	100
XRT Postage Stamp Image	150
BAT second light curve (24 s before trigger to 104 s after)	200
UVOT Finding Chart	270
UVOT Dark Burst Neighborhood Image	TBD
XRT Spectrum	1200
BAT "scaled map" (optional)	after XRT products

BAT Data in GCN/TDRSS Messages

Overview

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

The BAT first triggers on a rate increase (rate trigger), then performs an FFT-based image reconstruction of the sky. If it finds a point 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 receipt of an image trigger should eliminate many false rate triggers due to, for example, particle background events.

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

BAT GRB Alert Message (rate trigger):

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
• 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"

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 point source solution 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.

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 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.

BAT Lightcurve Messages

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 light curve messages, all counts detected by the entire BAT are telemetered. Note that BAT TDRSS light curves are not background subtracted. More complete information to do a faithful background-subtraction on BAT light curves is available from the Malindi data. However, a simple approximation to the BAT background can be made from the BAT TDRSS light curve 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.

The final BAT TDRSS message, the scaled map, is for verification of sources found and/or refined analysis of sources not found.

UVOT Data in GCN/TDRSS Messages

Overview

Once Swift has slewed to point the NFIs at the position of a new burst, two separate UVOT messages are sent through TDRSS. They are called the "source list" and the"image" messages. They are both derived onboard from the same V filter finding chart image. (This finding chart image is currently 100-s, but may change to 200-s or 400-s in the future to allow deeper afterglow detections.)The UVOT TDRSS messages are designed to use the minimum number of bits to communicate the maximum amount of information about the burst and the surrounding field. These "raw" products are the first UVOT data seen by the user. Scripts on the GCN computer operate on these messages, deriving higher-level "processed" products that are sent later via the GCN.

RAW and PROCESSED CONTENTS: Both the source list and the image notices come in 'raw' and 'processed' versions. Raw is the data product as it comes down through TDRSS from the Swift-UVOT instrument. The image is in raw coordinates. It is converted to FITS, GIF, JPEG, Postscript formats. Each 'raw' message contains a single attachment. The processed versions come with four attachments and are most useful for quick analysis. For each message, these include: a UVOT image in sky coordinates; a postscript image of the field, pre-burst, with the UVOT-identified sources circled, and the BAT and XRT error circles overlaid; a list of sources (RA, Dec, magnitudes and errors) found in the UVOT message; and the results of a catalog match performed on the identified source.

Bacodine Message Name		Description	Number of attachments
GCN/SWIFT_UVOT_SRC_LIST	    list of sources	1
GCN/SWIFT_UVOT_SRC_LIST_PROCESSED see above	4
GCN/SWIFT_UVOT_IMAGE	    region around GRB	1
GCN/SWIFT_UVOT_IMAGE_PROCESSED    see abobe	4

We briefly describe the products, and the processing done at the GCN, below.

UVOT Finding Chart Source List

This is really just a list of the positions of the brightest sources in the field. It is used to reconstruct a crude optical finding chart in raw and sky oordinates. The onboard processor first runs a source detection algorithm over the finding chart image. It identifies positions and counts from the brightest sources and sends these through TDRSS as "postage stamps." A postage stamp is a mosaic of 1, 5, or 9 pixels around a source, depending on the source brightness. The number of sources is limited by telemetry.

UVOT Image (sometimes referred to as the GeNIe or "neighborhood" image):

This is a small, 80x80 pixel sub-image from the UVOT detector, centered on the GRB position. and binned 2x2, with a window size of 80x80 arcsec. The advantage of the image over the source list is that it contains all the pixels in a sub-region of the UVOT finding chart. In the majority of cases, this image includes the position of the afterglow, and fairly accurate magnitude (or limiting magnitude) esimates can be made from this image. In the case where the BAT position error is greater than the size of the image, then the full source list products can be examined for new sources in the field.

GCN processing of UVOT TDRSS messages

Software runs on both UVOT TDRSS messages at the GCN and generates a finding chart for ground-based observers. It also locates UVOT sources and attempts to identify them by correlating with the Hubble Guide Star Catalog (GSC) v3.0, and differentiates sources in the field that are likely afterglow candidates. Sources are flagged as afterglow candidates if they have no counterpart in the GSC. These are then ranked by their proximity to the XRT and BAT position. The UVOT GCN script produces the following products:

• uvot_sources_srclist.fits - A list of sources found in the sparse finding chart message. In the interests of getting useful information quickly to ground-based observers, this preliminary list is generated before attempting to correlate sources with those in the catalog.
• uvot_sky_srclist.img.gz - A FITS image from the sparse finding chart message, in sky coordinates.
• uvot_catalog_srclist.fits - An analyzed source list from the sparse finding chart message (positions, magnitudes, GSC counterparts), with afterglow candidates listed first, in order of proximity to GRB position.
• uvot_field_srclist.ps.gz - A postscript finding chart generated from the Digitized Sky Survey, with XRT and BAT error circles included. Sources in the field identified in the sparse finding chart image are circled.
• uvot_sky_image.img.gz - A FITS image from the UVOT sub-image message, in sky coordinates.
• uvot_sources_image.fits - A list of sources found in the UVOT image message. As for the source list message, this preliminary list is generated before attempting to correlate sources with those in the catalog.
• uvot_catalog_image.fits - An analyzed source list from the UVOT image (positions, magnitudes, GSC counterparts), with afterglow candidates listed first, in order of proximity to GRB position. Due to the small size of the image, this list will typically contain a just a few sources.
• uvot_field_image.ps.gz - A postscript file containing the GeNie image with XRT and/or BAT circles overlaid, as well as position and scale information.

Below are examples of the postscript files sent for the source list message and the image message.

Figure 1 (above): Finding Chart postscript file generated from the Digitized Sky Survey, including positional information, BAT and XRT error circles (both green - XRT circle is small dot at center), and XRT position. Known sources close to the BAT position, identified through catalog matching, are circled in blue.

UVOT Finding Chart - Image

Figure 2 (above): Finding Chart postscript file, showing the UVOT GeNie image, XRT error circle (green) and XRT position.

The onboard software that generates the source list can and will get confused by any artifacts in the raw image, such as ghosts and halos. PSF cores that have been affected by coincidence loss will also confuse the onboard algorithm. In both cases, spurious sources may be identified and sent via TDRSS. The UVOT script running at the GCN will list these as unidentified sources, and possible afterglows.