TITLE: GCN CIRCULAR NUMBER: 25988 SUBJECT: GRB 191011A: Swift detection of a burst with an optical counterpart DATE: 19/10/11 04:46:45 GMT FROM: David Palmer at LANL S. L Laha (GSFC/UMBC/CRESST), C. Gronwall (PSU), J.D. Gropp (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester), D. M. Palmer (LANL) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Neil Gehrels Swift Observatory Team: At 04:35:56 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 191011A (trigger=928924). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 44.724, -27.845 which is RA(J2000) = 02h 58m 54s Dec(J2000) = -27d 50' 40" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex peak structure with a duration of about 20 sec. The peak count rate was ~3000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 04:37:11.1 UT, 74.8 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 44.72679, -27.84594 which is equivalent to: RA(J2000) = 02h 58m 54.43s Dec(J2000) = -27d 50' 45.4" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 9.5 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.54 x 10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.8 (+3.35/-2.89) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 150 seconds with the White filter starting 77 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 02:58:54.80 = 44.72832 DEC(J2000) = -27:50:43.5 = -27.84541 with a 90%-confidence error radius of about 0.83 arc sec. This position is 6.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 20.20 with a 1-sigma error of about 0.19. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is S. L Laha (sib.laha AT gmail.com). 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/)