TO: All GCN Notice recipients RE: Swift-BAT Subthreshold Notice type is available DT: 26 Aug 09 INTRODUCTION: The GCN system has been modified to incorporate the distribution of the Swift-BAT Subthreshold Notices. The testing period is over and now the Swift-BAT team is ready to release this new data product to the public. About once per day, the BAT flight software will trigger on a rate increase that produces a peak in the image domain that has a significance below that of normal image-domain threshold (=6.5 sigma), hence the name Subthreshold Notices (image sigma is in the 5.7-6.5 range). The goal is to go after fainter GRBs, some of which might be more distant than the typical normal-trigger GRBs. The trade-off is that the percentage of false positives will greatly increase. For those Subthreshold trigger locations that the spacecraft can slew to (~50% promptly), XRT will be used to confirm GRB-ness if it detects and point source. (These confirmed bursts will be "promoted" to the normal BAT_Position Notices/etc and distributed to the normal set of Swift notice recipients.) ACTION ITEM: If, after reviewing the information below, you want to receive these Subthreshold Notices, please send me a request to have them enabled. Forwarding this email to is useful because then I have your exact "site name". OR Visit the Configuration Change webpage: (You can, of course, take this opportunity to request other changes to your site's configuration.) SUGGESTED USE: Since the vast majority of these Subthreshold triggers will not be real (i.e. just noise in the image domain), it is envisioned that these will only be useful to robotic telescopes (i.e. no immediate consumption of human resources). Of course anyone is free to receive this new notice type. PROCESSING and DISTRIBUTION: The sequence for the on-board software and the ground software for these Subthreshold Notices is as follows: 1) BAT produces a Subthreshold trigger. (Please note that in this process of lowering the trigger threshold, proceedures have been implemented to reduce triggers around the SAA.) 2) The BAT Figure-of-Merit task and the spacecraft (SC) make their usual decisions about slewing to the new location or not. 3) If not, then the sequence terminates after BAT produces the usual set of BAT_Position, FOM_Observe, SC_Slew, and Lightcurve TDRSS messages (and captures some event-by-event data). 4) If the S/C does slew, then the XRT makes its usual set of observations and produces the usual set of real-time Notices. These observations terminate at the point in the orbit when the location becomes unobservable for the first time (1-40 min). (Longer follow-up observations are scheduled manually if a GRB is confirmed.) 5) If you have BAT_Subthreshold enabled, then you will receive these BAT and XRT follow-on Notices if you normally have them enabled. (UVOT does take data for these Subthreshold triggers, but it is not the standard Automated-Target response sequence. This is because of filter wheel rotation lifetime concerns. The UVOT exposures are longer and produce no real-time notices.) If you do not have BAT_Subthreshold enabled, then you will receive none of these Notices that are derived from the original BAT Subthreshold trigger. 6) These notices also go to an automated processing operation (Phil Evans, U.Leicester) which looks for an XRT point source within the BAT position error circle. (The notices used are the XRT_Position, _Image, _Threshpix, and _SPER. The later comes down for a period of time (1-~40 min, see item 4).) 7) If it finds a point source (presumably the afterglow of the GRB), then it sends that result back to GCN. This can take up to 2-45 min as more and more of the SPER data are integrated. 8) GCN then "promotes" all the stored copies of the various notices to "real GRB" status and distributes them to all the sites that have those Notices Types enabled (and meets all their other filter criteria). (If the trigger is not promoted there is no corresponding Nack distributed -- just silence.) TIME DELAY: The time from the trigger to the Subthreshold notice being distributed on the ground is in the 10-20 sec range (the same for normal triggers). POSITION UNCERTAINTY: The location uncertainties are in the 2-4 arcmin range (90% CL); slightly worse than for normal triggers. GRB CONFIDENCE LEVEL: It should be noted that these Subthreshold Notices will contain a very large fraction of false positives. During the testing phase, the rate has been 3 real astrophysical sources (2 GRBs and 1 hard x-ray transient) in about 75 subthreshold triggers (i.e. 96% false positives). MORE INFORMATION: Can be found at: The Notices are archived at: and NOTICE EXAMPLE: Here is an example of a full-format email BAT_Subthreshold Notice. It has all the same fields as a normal (above-threshold) BAT_Position Notice -- only the NOTICE_TYPE field has changed, and there is an extra COMMENT line indicating the low image signifiance nature of these triggers. TITLE: GCN/SWIFT NOTICE NOTICE_DATE: Tue 25 Aug 09 16:59:46 UT NOTICE_TYPE: Swift-BAT Sub-Threshold Position TRIGGER_NUM: 360857, Seg_Num: 0 GRB_RA: 257.802d {+17h 11m 12s} (J2000), 257.897d {+17h 11m 35s} (current), 257.306d {+17h 09m 14s} (1950) GRB_DEC: +28.045d {+28d 02' 41"} (J2000), +28.033d {+28d 02' 01"} (current), +28.105d {+28d 06' 17"} (1950) GRB_ERROR: 4.00 [arcmin radius, statistical only] GRB_INTEN: 7654 [cnts] Image_Peak=193 [image_cnts] TRIGGER_DUR: 2.048 [sec] TRIGGER_INDEX: 264 E_range: 25-100 keV BKG_INTEN: 83797 [cnts] BKG_TIME: 61122.97 SOD {16:58:42.97} UT BKG_DUR: 24 [sec] GRB_DATE: 15068 TJD; 237 DOY; 09/08/25 GRB_TIME: 61155.99 SOD {16:59:15.99} UT GRB_PHI: -156.31 [deg] GRB_THETA: 30.70 [deg] SOLN_STATUS: 0x883 RATE_SIGNIF: 5.38 [sigma] IMAGE_SIGNIF: 6.07 [sigma] MERIT_PARAMS: +1 +0 +0 +1 +2 -4 +0 +0 +10 +1 SUN_POSTN: 154.58d {+10h 18m 20s} +10.54d {+10d 32' 26"} SUN_DIST: 96.54 [deg] Sun_angle= -6.9 [hr] (East of Sun) MOON_POSTN: 217.95d {+14h 31m 49s} -20.31d {-20d 18' 51"} MOON_DIST: 61.87 [deg] MOON_ILLUM: 33 [%] GAL_COORDS: 50.16, 33.17 [deg] galactic lon,lat of the burst (or transient) ECL_COORDS: 252.85, 50.76 [deg] ecliptic lon,lat of the burst (or transient) COMMENTS: SWIFT-BAT Sub-Threshold GRB Coordinates. COMMENTS: This is a rate trigger. COMMENTS: A point_source was found. COMMENTS: This does not match any source in the on-board catalog. COMMENTS: This does not match any source in the ground catalog. COMMENTS: This is a GRB. COMMENTS: COMMENTS: The IMAGE_SIGNIF is VERY LOW, this is very likely not real. COMMENTS: This trigger occurred at longitude,latitude = 74.27,-13.64 [deg]. The socket packets contents can be found at: Sincerely, Scott Scott Barthelmy NASA-GSFC, Code 661, Greenbelt, MD 20771 PHONE: 301-286-3106 (office) CELL: 301-346-3733 FAX: 301-286-1684 (1st choice, -1682 2nd choice) EMAIL: PAGER: WEB: