TITLE: GCN CIRCULAR NUMBER: 30746 SUBJECT: GRB 210901A: Swift detection of a burst DATE: 21/09/01 15:22:09 GMT FROM: Kim Page at U.of Leicester <klp5@leicester.ac.uk> S. Laha (GSFC/UMBC/CRESST), E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR), P. A. Evans (U Leicester), C. Gronwall (PSU), J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 15:04:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210901A (trigger=1071488). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 295.292, -76.932 which is RA(J2000) = 19h 41m 10s Dec(J2000) = -76d 55' 55" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 60 sec. As this is an image trigger a more complete description of the lightcurve will require the ground pass data. The peak count rate was ~500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 15:07:07.7 UT, 151.4 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 295.2774, -76.9263 which is equivalent to: RA(J2000) = 19h 41m 06.57s Dec(J2000) = -76d 55' 34.8" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 23 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 https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.02 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 1.86e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 157 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 0.200. Burst Advocate for this burst is S. Laha (sib.laha AT gmail.com). 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/)