TITLE: GCN CIRCULAR NUMBER: 34610 SUBJECT: GRB 230903A: Swift detection of a burst DATE: 23/09/03 17:46:24 GMT FROM: K.L. Page at U Leicester klp5@leicester.ac.uk

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA), J.D. Gropp (PSU), J. A. Kennea (PSU), A. Y. Lien (U Tampa), A. Melandri (INAF-OAR), M. J. Moss (GWU), D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift Observatory Team:

At 17:22:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 230903A (trigger=1189514). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 9.926, -40.924 which is RA(J2000) = 00h 39m 42s Dec(J2000) = -40d 55' 25" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 10 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 17:24:47.9 UT, 111.7 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 9.91169, -40.94930 which is equivalent to: RA(J2000) = 00h 39m 38.81s Dec(J2000) = -40d 56' 57.5" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 99 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 (2.68 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 2.6 (+2.17/-1.93) x 10^21 cm^-2 (90% confidence).

UVOT took a finding chart exposure of 250 seconds with the U filter starting 117 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 19.2 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 18.0 mag.

Burst Advocate for this burst is A. D'Ai (antonino.dai AT inaf.it). 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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