TITLE: GCN CIRCULAR NUMBER: 32484 SUBJECT: GRB 220826A: Swift detection of a burst with an optical counterpart DATE: 22/08/26 12:05:48 GMT FROM: David Palmer at LANL palmer@lanl.gov

R. Caputo (GSFC), P. A. Evans (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 11:55:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 220826A (trigger=1121751). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 206.431, -44.044 which is RA(J2000) = 13h 45m 44s Dec(J2000) = -44d 02' 39" 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 20 sec. The peak count rate was ~3500 counts/sec (15-350 keV), at ~7 sec after the trigger.

The XRT began observing the field at 11:56:42.9 UT, 81.2 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 206.41169, -44.03885 which is equivalent to: RA(J2000) = 13h 45m 38.81s Dec(J2000) = -44d 02' 19.9" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 53 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 (9.05 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.8 (+2.60/-2.30) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 90 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 13:45:38.52 = 206.41049 DEC(J2000) = -44:02:19.0 = -44.03862 with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.89 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.110.

Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov). 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/)