TITLE: GCN CIRCULAR NUMBER: 40292 SUBJECT: GRB 250430A: Swift detection of a burst with optical counterpart DATE: 25/04/30 18:07:07 GMT FROM: Tyler Parsotan at NASA GSFC tyler.parsotan@nasa.gov

T. M. Parsotan (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC), N. J. Klingler (GSFC/UMBC/CRESSTII), M. J. Moss (GSFC), K. L. Page (U Leicester), M. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift Observatory Team:

At 17:31:21 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250430A (trigger=1308754). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 233.428, -18.083 which is RA(J2000) = 15h 33m 43s Dec(J2000) = -18d 04' 59" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi peak structure with a duration of about 20 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 17:33:37.5 UT, 157.3 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 233.38611, -18.11784 which is equivalent to: RA(J2000) = 15h 33m 32.67s Dec(J2000) = -18d 07' 04.2" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 190 arcseconds from the BAT onboard position, outside 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 (9.57 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5 (+2.62/-2.27) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 150 seconds with the White filter starting 161 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 15:33:32.85 = 233.38688 DEC(J2000) = -18:07:06.2 = -18.11840 with a 90%-confidence error radius of about 1.10 arc sec. This position is 3.65 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.44 with a 1-sigma error of about 0.35. No correction has been made for the expected extinction corresponding to E(B-V) of 0.092.

Burst Advocate for this burst is T. M. Parsotan (tyler.parsotan AT nasa.gov). 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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