TITLE: GCN CIRCULAR NUMBER: 32465 SUBJECT: GRB 220813A: Swift detection of a burst with an optical counterpart DATE: 22/08/13 19:35:48 GMT FROM: David Palmer at LANL palmer@lanl.gov

M. G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB), M. Ferro (INAF-OAB), J.D. Gropp (PSU), A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL), T. M. Parsotan (GSFC/UMBC/CRESSTII), T. Sbarrato (INAF-OAB) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 19:23:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220813A (trigger=1120270). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 81.501, -32.985 which is RA(J2000) = 05h 26m 00s Dec(J2000) = -32d 59' 06" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single-peak structure with a duration of about 30 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~2 sec after the trigger.

The XRT began observing the field at 19:24:34.2 UT, 90.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 81.53235, -33.01546 which is equivalent to: RA(J2000) = 05h 26m 07.76s Dec(J2000) = -33d 00' 55.7" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 144 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 (2.32 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.7 (+3.14/-2.65) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 100 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 05:26:07.57 = 81.53155 DEC(J2000) = -33:00:58.9 = -33.01636 with a 90%-confidence error radius of about 0.67 arc sec. This position is 4.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.61 with a 1-sigma error of about 0.17. No correction has been made for the expected extinction corresponding to E(B-V) of 0.022.

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.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/)