TITLE: GCN CIRCULAR NUMBER: 30523 SUBJECT: GRB 210726A: Swift detection of a burst DATE: 21/07/26 19:39:05 GMT FROM: David Palmer at LANL M. G. Bernardini (INAF-OAB), M. A. Baer (PSU), P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII), S. Laha (GSFC/UMBC/CRESST), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), M. J. Moss (GWU), 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 19:19:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210726A (trigger=1061687). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 193.342, +19.185 which is RA(J2000) = 12h 53m 22s Dec(J2000) = +19d 11' 05" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a short spike with a tail with a total duration of about ~3 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. This may be a short GRB. The XRT began observing the field at 19:20:03.6 UT, 60.0 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 193.2909, 19.1898 which is equivalent to: RA(J2000) = 12h 53m 09.81s Dec(J2000) = +19d 11' 23.2" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 174 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 (1.73 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.4 (+2.93/-2.51) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 250 seconds with the U filter starting 276 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.020. 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/)