TITLE: GCN CIRCULAR NUMBER: 43252 SUBJECT: GRB 251230A: Swift detection of a burst with a bright optical counterpart DATE: 25/12/30 01:48:56 GMT FROM: Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu> S. Lanava (PSU), S. Dichiara (PSU) and R. Gupta (NASA GSFC) report on behalf of the Neil Gehrels Swift Observatory Team: At 01:21:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251230A (trigger=1429020). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 179.894, +11.415 which is RA(J2000) = 11h 59m 35s Dec(J2000) = +11d 24' 55" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multiple-peaked structure with a duration of about 80 sec. The peak count rate was ~5000 counts/sec (15-350 keV), at ~30 sec after the trigger. The XRT began observing the field at 01:22:49.3 UT, 68.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 179.89781, 11.43828 which is equivalent to: RA(J2000) = 11h 59m 35.47s Dec(J2000) = +11d 26' 17.8" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 84 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 (1.88 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.4 (+1.47/-1.34) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 4.73e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 76 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 11:59:35.42 = 179.89760 DEC(J2000) = +11:26:19.3 = 11.43870 with a 90%-confidence error radius of about 0.74 arc sec. This position is 4.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 13.74 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.024. Burst Advocate for this burst is S. Lanava (sml7284 AT psu.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/) View this GCN Circular online at https://gcn.nasa.gov/circulars/43252. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...