TITLE: GCN CIRCULAR NUMBER: 20064 SUBJECT: GRB 161017A: Swift detection of a burst with a bright optical afterglow DATE: 16/10/17 18:04:12 GMT FROM: David Palmer at LANL E. Troja (NASA/GSFC/UMCP), D. N. Burrows (PSU), P. D'Avanzo (INAF-OAB), N. Gehrels (NASA/GSFC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. N. Ukwatta (LANL) report on behalf of the Swift Team: At 17:51:51 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 161017A (trigger=718023). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 142.765, +43.142 which is RA(J2000) = 09h 31m 04s Dec(J2000) = +43d 08' 30" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a multi-peaked structure with a duration of about 160 sec. The peak count rate was ~2100 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 17:52:48.8 UT, 57.7 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 142.7681, 43.1300 which is equivalent to: RA(J2000) = +09h 31m 4.34s Dec(J2000) = +43d 07' 48.0" with an uncertainty of 4.9 arcseconds (radius, 90% containment). This location is 43 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 3.24e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of nominal 150.000 seconds with the White filter starting 67 seconds after the BAT trigger. There is a candidate afterglow in the list of sources generated on-board at RA(J2000) = 09:31:04.58 = 142.76910 DEC(J2000) = +43:07:36.3 = 43.12675 with a 90%-confidence error radius of about 1.10 arc sec. This position is 12.0 arc sec. from the center of the XRT error circle. The estimated magnitude is 13.86. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. Burst Advocate for this burst is E. Troja (eleonora.troja 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/too.html.)