TITLE: GCN CIRCULAR NUMBER: 25947 SUBJECT: GRB 191004B: Swift detection of a burst with an optical counterpart DATE: 19/10/04 21:43:36 GMT FROM: David Palmer at LANL 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), J. A. Kennea (PSU), N. J. Klingler (PSU), N. P. M. Kuin (UCL-MSSL), S Laha (CRESST/GSFC/UMD), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), D. M. Palmer (LANL) and A. Tohuvavohu (Toronto) report on behalf of the Neil Gehrels Swift Observatory Team: At 21:33:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191004B (trigger=927839). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 49.177, -39.613, which is RA(J2000) = 03h 16m 42s Dec(J2000) = -39d 36' 46" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a total duration of about 40 sec. The peak count rate was ~9500 counts/sec (15-350 keV), at ~3 sec after the trigger. The XRT began observing the field at 21:34:39.3 UT, 57.7 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 49.20403, -39.63494 which is equivalent to: RA(J2000) = 03h 16m 48.97s Dec(J2000) = -39d 38' 05.8" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 108 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. The initial flux in the 2.5 s image was 1.81e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 68 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 03:16:49.10 = 49.20458 DEC(J2000) = -39:38:03.3 = -39.63425 with a 90%-confidence error radius of about 0.62 arc sec. This position is 2.9 arc sec. from the center of the XRT error circle. The estimated magnitude is 18.00 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. 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/)