TITLE: GCN CIRCULAR NUMBER: 25945 SUBJECT: GRB 191004A: Swift detection of a burst with an optical detection DATE: 19/10/04 18:30:53 GMT FROM: Scott Barthelmy at NASA/GSFC S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC), J.D. Gropp (PSU), N. J. Klingler (PSU), S Laha (CRESST/GSFC/UMD), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), K. L. Page (U Leicester), B. Sbarufatti (PSU) and A. Tohuvavohu (Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 18:07:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191004A (trigger=927825). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 31.721, -36.932 which is RA(J2000) = 02h 06m 53s Dec(J2000) = -36d 55' 53" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a double-peaked structure with a duration of about 5 sec. The peak count rate was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 18:08:24.2 UT, 81.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 31.66950, -36.93368 which is equivalent to: RA(J2000) = 02h 06m 40.68s Dec(J2000) = -36d 56' 01.2" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 148 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. No spectrum from the promptly downlinked event data is yet available to determine the column density. UVOT took a finding chart exposure of 150 seconds with the White filter starting 84 seconds after the BAT trigger. Results from the list of sources generated on-board shows an optical source consistent with the XRT position. Because there was not automatic aspect solution for the image, the precise position and magnitude will be reported later. The 2.7'x2.7' sub-image covers none of the XRT error circle. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. 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/)