TITLE: GCN CIRCULAR NUMBER: 18076 SUBJECT: GRB 150727A: Swift detection of a burst with an optical counterpart DATE: 15/07/27 19:24:14 GMT FROM: Scott Barthelmy at NASA/GSFC S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), J. L. Racusin (NASA/GSFC) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 19:02:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 150727A (trigger=650530). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 203.990, -18.355, which is RA(J2000) = 13h 35m 58s Dec(J2000) = -18d 21' 16" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a large FRED-like peak structure with a duration of about 60 sec. The peak count rate was ~2795 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 19:03:19.5 UT, 77.2 seconds after the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located at RA, Dec 203.9705, -18.3259 which is equivalent to: RA(J2000) = +13h 35m 52.92s Dec(J2000) = -18d 19' 33.2" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 124 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. The initial flux in the 2.5 s image was 8.93e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 86 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 13:35:52.52 = 203.96883 DEC(J2000) = -18:19:31.4 = -18.32539 with a 90%-confidence error radius of about 0.62 arc sec. This position is 6.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.31 with a 1-sigma error of about 0.15. No correction has been made for extinction. Burst Advocate for this burst is S. B. Cenko (brad.cenko 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.)