TITLE: GCN CIRCULAR NUMBER: 39058 SUBJECT: GRB 250128B: Swift detection of a short burst DATE: 25/01/28 16:38:33 GMT FROM: P.A. Evans at U. Leicester pae9@leicester.ac.uk

P. A. Evans (U Leicester), R. A. J. Eyles-Ferris (U Leicester), R. Gupta (NASA GSFC), J. A. Kennea (PSU), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 16:22:53 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250128B (trigger=1285677). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 231.420, -0.517 which is RA(J2000) = 15h 25m 41s Dec(J2000) = -00d 31' 01" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peak structure with a duration of about 0.5 sec. The peak count rate was ~5578 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 16:24:12.3 UT, 78.6 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source with an enhanced position: RA, Dec 231.42552, -0.53970 which is equivalent to: RA(J2000) = 15h 25m 42.12s Dec(J2000) = -00d 32' 22.9" 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. We cannot determine whether the source is fading at the present time.

A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 8.00 x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 82 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.121.

Burst Advocate for this burst is P. A. Evans (pae9 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/)

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