TITLE: GCN CIRCULAR NUMBER: 30846 SUBJECT: GRB 210919A: Swift detection of a short burst DATE: 21/09/19 00:54:18 GMT FROM: Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov> A. Tohuvavohu (U Toronto), S. D. Barthelmy (GSFC), A. Y. Lien (U Tampa) and B. Sbarufatti (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 00:28:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located a short GRB 210919A (trigger=1073893). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 80.230, +1.274 which is RA(J2000) = 05h 20m 55s Dec(J2000) = +01d 16' 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 0.3 sec. The peak count rate was ~3500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 00:30:10.5 UT, 97.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 80.25441, 1.31189 which is equivalent to: RA(J2000) = 05h 21m 01.06s Dec(J2000) = +01d 18' 42.8" with an uncertainty of 6.9 arcseconds (radius, 90% containment). This location is 162 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. 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 consistent with the Galactic value of 1.56 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 99 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 0.142. Burst Advocate for this burst is A. Tohuvavohu (tohuvavohu AT astro.utoronto.ca). 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/)