TITLE: GCN CIRCULAR NUMBER: 13875 SUBJECT: GRB 121017A: Swift detection of a burst DATE: 12/10/17 19:55:10 GMT FROM: Scott Barthelmy at NASA/GSFC D. Grupe (PSU), B. N. Barlow (PSU), S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), E. A. Helder (PSU), S. T. Holland (STScI), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), C. J. Mountford (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and C. A. Swenson (PSU) report on behalf of the Swift Team: At 19:23:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 121017A (trigger=536172). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 288.816, -1.595 which is RA(J2000) = 19h 15m 16s Dec(J2000) = -01d 35' 43" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows at least two peaks with a duration of about 10 sec (possibly longer). The peak count rate was ~2500 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 19:25:03.3 UT, 95.2 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 288.8302, -1.6042 which is equivalent to: RA(J2000) = 19h 15m 19.25s Dec(J2000) = -01d 36' 15.1" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 60 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (2.22 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 3 (+2.89/-2.40) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 1141 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. There is no new source down to a limit of about 19th mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the expected extinction corresponding to E(B-V) of 0.44. XRT and UVOT detections were delayed due to entry into the SAA shortly after the initial trigger. Burst Advocate for this burst is D. Grupe (grupe AT astro.psu.edu). 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.)