TITLE: GCN CIRCULAR NUMBER: 30897 SUBJECT: GRB 210930A: Swift detection of a burst DATE: 21/09/30 02:58:45 GMT FROM: Mike Siegel at PSU/Swift MOC <siegel@swift.psu.edu> V. D'Elia (SSDC & INAF-OAR), E. Ambrosi (INAF-IASFPA), S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (U Tampa), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 02:46:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210930A (trigger=1075565). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 197.435, +48.623 which is RA(J2000) = 13h 09m 44s Dec(J2000) = +48d 37' 22" 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 15 sec. The peak count rate was ~800 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 02:49:12.3 UT, 140.1 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 197.40185, 48.62853 which is equivalent to: RA(J2000) = 13h 09m 36.44s Dec(J2000) = +48d 37' 42.7" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 81 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 in excess of the Galactic value (9.55 x 10^19 cm^-2, Willingale et al. 2013), with an excess column of 6.4 (+3.87/-3.20) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 143 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 0.010. Burst Advocate for this burst is V. D'Elia (valerio.delia AT ssdc.asi.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/)