TITLE: GCN CIRCULAR NUMBER: 18264 SUBJECT: GRB 150910A: Swift detection of a burst with an optical afterglow DATE: 15/09/10 09:26:53 GMT FROM: Kim Page at U.of Leicester C. Pagani (U Leicester), A. P. Beardmore (U Leicester), A. A. Breeveld (UCL-MSSL), K. L. Page (U Leicester), P. Romano (INAF-IASFPA) and T. N. Ukwatta (LANL) report on behalf of the Swift Team: At 09:04:48 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150910A (trigger=655097). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 5.675, +33.499 which is RA(J2000) = 00h 22m 42s Dec(J2000) = +33d 29' 55" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers, the real-time light curve does not show anything significant. The XRT began observing the field at 09:07:14.1 UT, 145.3 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 5.6669, 33.4717 which is equivalent to: RA(J2000) = +00h 22m 40.06s Dec(J2000) = +33d 28' 18.1" with an uncertainty of 5.1 arcseconds (radius, 90% containment). This location is 101 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 153 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 00:22:40.14 = 5.66723 DEC(J2000) = +33:28:21.9 = 33.47275 with a 90%-confidence error radius of about 0.71 arc sec. This position is 3.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 20.27 with a 1-sigma error of about 0.18. No correction has been made for the expected extinction corresponding to E(B-V) of 0.05. Burst Advocate for this burst is C. Pagani (cp232 AT star.le.ac.uk). 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.)