TITLE: GCN CIRCULAR NUMBER: 19316 SUBJECT: GRB 160417A: Swift detection of a burst with an optical counterpart DATE: 16/04/17 04:34:46 GMT FROM: David Palmer at LANL P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 04:23:41 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 160417A (trigger=683076). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 120.240, +7.671 which is RA(J2000) = 08h 00m 58s Dec(J2000) = +07d 40' 15" 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 30 sec. The peak count rate was ~1100 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 04:25:53.1 UT, 132.2 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source located at RA, Dec 120.25774, 7.66265 which is equivalent to: RA(J2000) = 08h 01m 01.86s Dec(J2000) = +07d 39' 45.5" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 70 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 2.92 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 135 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 08:01:01.64 = 120.25682 DEC(J2000) = +07:39:46.4 = 7.66290 with a 90%-confidence error radius of about 0.64 arc sec. This position is 4.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 19.15 with a 1-sigma error of about 0.15. No correction has been made for the expected extinction corresponding to E(B-V) of 0.02. 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.)