TITLE: GCN CIRCULAR NUMBER: 19887 SUBJECT: GRB 160905A: Swift detection of a burst with an optical counterpart DATE: 16/09/05 11:31:37 GMT FROM: David Palmer at LANL P. D'Avanzo (INAF-OAB), A. P. Beardmore (U Leicester), A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), P.A. Evans (U Leicester), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), P. T. O'Brien (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team: At 11:18:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160905A (trigger=711084). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 162.237, -50.800 which is RA(J2000) = 10h 48m 57s Dec(J2000) = -50d 47' 59" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 50 sec. The peak count rate was ~13000 counts/sec (15-350 keV), at ~18 sec after the trigger. The XRT began observing the field at 11:19:56.0 UT, 57.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 162.2448, -50.8014 which is equivalent to: RA(J2000) = +10h 48m 58.75s Dec(J2000) = -50d 48' 05.0" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 18 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 6.05e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 66 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 10:48:59.18 = 162.24658 DEC(J2000) = -50:48:04.3 = -50.80120 with a 90%-confidence error radius of about 0.61 arc sec. This position is 4.1 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.88 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.46. Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT brera.inaf.it). 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.)