TITLE: GCN CIRCULAR NUMBER: 18365 SUBJECT: GRB 150925A: Swift detection of a burst DATE: 15/09/25 04:27:53 GMT FROM: David Palmer at LANL J. L. Racusin (NASA/GSFC), A. D'Ai (INAF-IASFPA), A. Maselli (INAF-IASFPA), D. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 04:09:28 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150925A (trigger=656604). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 227.520, -19.628 which is RA(J2000) = 15h 10m 05s Dec(J2000) = -19d 37' 41" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). Only partial BAT lightcurve data is currently available, and so no description of this image trigger can be given. The XRT began observing the field at 04:11:48.9 UT, 140.1 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 227.5340, -19.6325 which is equivalent to: RA(J2000) = 15h 10m 08.16s Dec(J2000) = -19d 37' 57.1" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 50 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 http://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 (1.01 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.7 (+4.53/-2.33) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 3.82e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 148 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 0.10. Burst Advocate for this burst is J. L. Racusin (judith.racusin 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/too.html.)