TITLE: GCN CIRCULAR NUMBER: 40407 SUBJECT: GRB 250509A: Swift detection of a burst DATE: 25/05/09 22:49:14 GMT FROM: Mike Siegel at PSU/Swift MOC <siegel@swift.psu.edu> M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 22:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250509A (trigger=1311764). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 46.455, -38.853 which is RA(J2000) = 03h 05m 49s Dec(J2000) = -38d 51' 11" 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 60 sec. The peak count rate was ~2800 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 22:34:41.7 UT, 71.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 46.45032, -38.84173 which is equivalent to: RA(J2000) = 03h 05m 48.08s Dec(J2000) = -38d 50' 30.2" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 42 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 (2.03 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 9.2 (+3.98/-3.41) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 7.21e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150.000 seconds with the White filter starting 80 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. Data from the 2.7'x2.7' sub-image are not available at this time. 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 0.021. Burst Advocate for this burst is M. H. Siegel (siegel AT swift.psu.edu). 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/40407. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...