TITLE: GCN CIRCULAR NUMBER: 21960 SUBJECT: GRB 171003A: Swift detection of a burst or a Galactic Transient DATE: 17/10/03 03:22:38 GMT FROM: Scott Barthelmy at NASA/GSFC S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Swift Team: At 02:37:00 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 171003A (trigger=775946). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 40.910, +61.408, which is RA(J2000) = 02h 43m 38s Dec(J2000) = +61d 24' 28" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, the real-time light curve does not show anything significant. The XRT began observing the field at 02:43:29.0 UT, 388.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 40.9186, 61.4323 which is equivalent to: RA(J2000) = +02h 43m 40.46s Dec(J2000) = +61d 25' 56.3" with an uncertainty of 5.4 arcseconds (radius, 90% containment). This location is 88 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 1.29e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 400 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the large, but uncertain extinction expected. The analysis is complicated by the proximity of a known star with a USNO V magnitude of 12.01 approximately 7.7 arc-seconds from the XRT position. Based on this source's proximity to the Galactic plane (latitude=1.4 degrees) and the duration of the BAT image trigger (>5 minutes), there is a possibility that this is a Galactic transient (i.e. Swift J0243.6+6124) rather than a GRB. The nature of this source will be determined by the full downloaded dataset and further observations. Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)