TITLE: GCN CIRCULAR NUMBER: 35602 SUBJECT: GRB 240123A: Swift detection of a burst with a possible optical counterpart DATE: 24/01/23 11:34:56 GMT FROM: Antonino D'Ai' at IASF-PA <antonino.dai@inaf.it> A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA), V. D'Elia (SSDC & INAF-OAR), J.D. Gropp (PSU), J. A. Kennea (PSU), A. Y. Lien (U Tampa), A. Melandri (INAF-OAR), K. L. Page (U Leicester), T. M. Parsotan (GSFC/UMBC/CRESSTII) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 11:05:46 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 240123A (trigger=1210276). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 199.319, +60.632 which is RA(J2000) = 13h 17m 16s Dec(J2000) = +60d 37' 53" 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 25 sec. The peak count rate was ~500 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 11:08:08.6 UT, 141.8 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source located at RA, Dec 199.34797, 60.61894 which is equivalent to: RA(J2000) = 13h 17m 23.51s Dec(J2000) = +60d 37' 08.2" with an uncertainty of 3.8 arcseconds (radius, 90% containment). This location is 69 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 (1.45 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.7 (+2.69/-2.33) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 7.30e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 151 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 13:17:23.25 = 199.3468 DEC(J2000) = +60:37:04.4 = 60.6179 with a 90%-confidence error radius of about 0.2 arc sec. This position is 4.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.9 with a 1-sigma error of about 0.2. No correction has been made for the expected extinction. Burst Advocate for this burst is A. D'Ai (antonino.dai AT 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/) View this GCN Circular online at https://gcn.nasa.gov/circulars/35602. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...