TITLE: GCN CIRCULAR NUMBER: 21946 SUBJECT: GRB 171001A: Swift detection of a burst DATE: 17/10/01 18:38:25 GMT FROM: David Palmer at LANL A. Y. Lien (GSFC/UMBC), P. D'Avanzo (INAF-OAB), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Swift Team: At 18:25:40 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 171001A (trigger=775553). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 22.088, -11.893 which is RA(J2000) = 01h 28m 21s Dec(J2000) = -11d 53' 35" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 30 sec. The peak count rate was ~900 counts/sec (15-350 keV), at ~20 sec after the trigger. The XRT began observing the field at 18:28:53.9 UT, 193.7 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 22.0774, -11.8940 which is equivalent to: RA(J2000) = 01h 28m 18.59s Dec(J2000) = -11d 53' 38.5" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 37 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (3.56 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.9 (+3.76/-3.10) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 197 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 expected extinction corresponding to E(B-V) of 0.03. Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)