TITLE: GCN CIRCULAR NUMBER: 35522 SUBJECT: GRB 240112C: Swift detection of a burst with an optical counterpart DATE: 24/01/12 17:51:21 GMT FROM: David Palmer at LANL <palmer@lanl.gov> R. A. J. Eyles-Ferris (U Leicester), J.D. Gropp (PSU), S. Laha (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), M. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 17:37:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 240112C (trigger=1208389). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 270.644, +24.487 which is RA(J2000) = 18h 02m 35s Dec(J2000) = +24d 29' 12" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Due to a telemetry gap, the BAT lightcurve is not immediately available. The XRT began observing the field at 17:39:18.9 UT, 114.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 270.64225, 24.48830 which is equivalent to: RA(J2000) = 18h 02m 34.14s Dec(J2000) = +24d 29' 17.9" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 7.4 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. We cannot determine whether the source is fading at the present time. No spectrum from the promptly downlinked event data is yet available to determine the column density. UVOT took a finding chart exposure of 115 seconds with the White filter starting 120 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 18:02:34.59 = 270.64414 DEC(J2000) = +24:29:21.2 = 24.48923 with a 90%-confidence error radius of about 0.75 arc sec. This position is 7.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 16.73 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.108. Burst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 AT leicester.ac.uk). 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/35522. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...