TITLE: GCN CIRCULAR NUMBER: 21901 SUBJECT: GRB 170921A: Swift detection of a burst DATE: 17/09/21 14:44:31 GMT FROM: Boris Sbarufatti at PSU A. D'Ai (INAF-IASFPA), V. D'Elia (ASDC), P. A. Evans (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), S. J. LaPorte (PSU), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), B. Sbarufatti (PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 14:28:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170921A (trigger=773509). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 26.849, +30.874 which is RA(J2000) = 01h 47m 24s Dec(J2000) = +30d 52' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peaked structure with a duration of about 20 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 14:30:25.8 UT, 97.3 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 26.8484, 30.9069 which is equivalent to: RA(J2000) = 01h 47m 23.63s Dec(J2000) = +30d 54' 24.8" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 118 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 (5.49 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 1.4 (+0.67/-0.55) x 10^22 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 100 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 none of the XRT error circle. 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 0.05. Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.inaf.it). 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.)