TITLE: GCN CIRCULAR NUMBER: 33378 SUBJECT: GRB 230228A: Swift detection of a possibly short burst DATE: 23/02/28 06:03:29 GMT FROM: David Palmer at LANL palmer@lanl.gov

S. Dichiara (PSU), J.D. Gropp (PSU), D. M. Palmer (LANL) and T. M. Parsotan (GSFC/UMBC/CRESSTII) report on behalf of the Neil Gehrels Swift Observatory Team:

At 05:50:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230228A (trigger=1156572). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 18.403, +44.489 which is RA(J2000) = 01h 13m 37s Dec(J2000) = +44d 29' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single symmetrical peak structure with a duration of about 1.5 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 05:52:49.2 UT, 119.0 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 18.37776, 44.48321 which is equivalent to: RA(J2000) = 01h 13m 30.66s Dec(J2000) = +44d 28' 59.6" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 68 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 consistent with the Galactic value of 9.72 x 10^20 cm^-2 (Willingale et al. 2013).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 122 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 0.085.

Burst Advocate for this burst is S. Dichiara (simonedichiara55 AT gmail.com). 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/)