TITLE: GCN CIRCULAR NUMBER: 9512 SUBJECT: GRB 090618: Swift detection of a bright burst with optical afterglow DATE: 09/06/18 08:48:39 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT P. Schady (MSSL-UCL), W. H. Baumgartner (GSFC/UMBC), A. P. Beardmore (U Leicester), S. Campana (INAF-OAB), P.A. Curran (MSSL-UCL), C. Guidorzi (U Ferrara), J. A. Kennea (PSU), J. Mao (INAF-OAB), R. Margutti (Univ Bicocca&OAB), J. P. Osborne (U Leicester), K. L. Page (U Leicester), P. Romano (INAF-IASFPA), M. H. Siegel (PSU), G. Stratta (ASDC) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 08:28:29 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 090618 (trigger=355083). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 294.021, +78.353 which is RA(J2000) = 19h 36m 05s Dec(J2000) = +78d 21' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed multi peak structure with a duration of about 130 sec. The peak count rate was ~40,000 counts/sec (15-350 keV), at 80 sec after the trigger. The XRT began observing the field at 08:30:30.8 UT, 120.9 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 293.9897, +78.3576 which is equivalent to: RA(J2000) = 19h 35m 57.52s Dec(J2000) = +78d 21' 27.3" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 28 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 128 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 19:35:58.69 = 293.99456 DEC(J2000) = +78:21:24.3 = 78.35676 with a 90%-confidence error radius of about 0.74 arc sec. This position is 4.6 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.36 with a 1-sigma error of about 0.01. No correction has been made for the expected extinction corresponding to E(B-V) of 0.09. Burst Advocate for this burst is P. Schady (ps AT mssl.ucl.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.)