TITLE: GCN CIRCULAR NUMBER: 11296 SUBJECT: GRB 100925A: Swift detection of a burst DATE: 10/09/25 08:45:18 GMT FROM: Vanessa Mangano at INAF-IASFPA V. Mangano (INAF-IASFPA), E. A. Hoversten (PSU), C. B. Markwardt (NASA/GSFC), B. Sbarufatti (INAF-OAB/IASFPA), R. L. C. Starling (U Leicester) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 08:05:05 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 100925A (trigger=434928). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 254.736, -15.236 which is RA(J2000) = 16h 58m 57s Dec(J2000) = -15d 14' 07" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). This is an image trigger with on-board significance of 8.1 sigma. As is typical for image triggers, there is no obvious structure in the raw light curve. The XRT began observing the field at 08:36:18.0 UT, 1872.2 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 254.7603, -15.2576 which is equivalent to: RA(J2000) = 16h 59m 02.47s Dec(J2000) = -15d 15' 27.3" with an uncertainty of 5.3 arcseconds (radius, 90% containment). This location is 115 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. UVOT took a finding chart exposure of 150 seconds with the White filter starting 1882 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 0.61. Burst Advocate for this burst is V. Mangano (vanessa AT ifc.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/too.html.)