TITLE: GCN CIRCULAR NUMBER: 41704 SUBJECT: GRB 250904A: Swift detection of a burst DATE: 25/09/04 17:49:36 GMT FROM: David Palmer at LANL <palmer@lanl.gov> C. Salvaggio (INAF-OAB), R. Brivio (INAF-OAB), S. Dichiara (PSU), R. Gupta (NASA GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 17:37:03 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250904A (trigger=1347179). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 84.358, +3.584 which is RA(J2000) = 05h 37m 26s Dec(J2000) = +03d 35' 04" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single complex peak structure with a duration of about 50 sec. The peak count rate was ~2000 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 17:38:25.4 UT, 82.4 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 84.38001, 3.58520 which is equivalent to: RA(J2000) = 05h 37m 31.20s Dec(J2000) = +03d 35' 06.7" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 79 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 3.34 x 10^21 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 85 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 0.292. Burst Advocate for this burst is C. Salvaggio (chiara.salvaggio 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/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41704. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...