TITLE: GCN CIRCULAR NUMBER: 39066 SUBJECT: GRB 250129A: Swift detection of a burst with an optical counterpart DATE: 25/01/29 05:12:01 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> A. P. Beardmore (U Leicester), S. Dichiara (PSU) and K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift Observatory Team: At 04:45:09 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250129A (trigger=1285812). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 198.689, +5.039 which is RA(J2000) = 13h 14m 45s Dec(J2000) = +05d 02' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for an image trigger, the BAT light curve shows no significant peaks. The XRT began observing the field at 04:47:47.6 UT, 158.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source with an enhanced position: RA, Dec 198.67654, 5.03019 which is equivalent to: RA(J2000) = 13h 14m 42.37s Dec(J2000) = +05d 01' 48.7" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 54 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. The initial flux in the 0.1 s image was 1.18e-08 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150.000 seconds with the White filter starting 1184 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 13:14:42.42 = 198.67673 DEC(J2000) = +05:01:50.3 = 5.03063 with a 90%-confidence error radius of about 1.10 arc sec. This position is 2.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.88. No correction has been made for the expected extinction corresponding to E(B-V) of 0.037. Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.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/39066. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...