TITLE: GCN CIRCULAR NUMBER: 30960 SUBJECT: GRB 211023B: Swift detection of a possibly short burst with an optical counterpart DATE: 21/10/23 21:16:59 GMT FROM: David Palmer at LANL <palmer@lanl.gov> S. Dichiara (NASA/GSFC/UMCP), J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 21:05:52 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 211023B (trigger=1080859). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 170.302, +39.137 which is RA(J2000) = 11h 21m 12s Dec(J2000) = +39d 08' 14" 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 2 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 21:07:27.3 UT, 95.3 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 170.31048, 39.13595 which is equivalent to: RA(J2000) = 11h 21m 14.52s Dec(J2000) = +39d 08' 09.4" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 23 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 (2.30 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5 (+2.51/-2.21) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 97 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 11:21:14.31 = 170.30963 DEC(J2000) = +39:08:08.8 = 39.13578 with a 90%-confidence error radius of about 0.79 arc sec. This position is 3.2 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.32 with a 1-sigma error of about 0.16. No correction has been made for the expected extinction corresponding to E(B-V) of 0.024. We note that this is Swift's 1500th GRB. Burst Advocate for this burst is S. Dichiara (dichiara AT umd.edu). 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/)