TITLE: GCN CIRCULAR NUMBER: 21214 SUBJECT: GRB 170607A: Swift detection of a burst with a bright optical counterpart DATE: 17/06/07 23:32:47 GMT FROM: David Palmer at LANL A. D'Ai (INAF-IASFPA), D. N. Burrows (PSU), A. Cholden-Brown (PSU), V. D'Elia (ASDC), J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 23:21:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170607A (trigger=756284). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 7.343, +9.238 which is RA(J2000) = 00h 29m 22s Dec(J2000) = +09d 14' 16" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single FRED structure with a duration of about 40 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~1 sec after the trigger. The XRT began observing the field at 23:22:47.1 UT, 73.4 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 7.3655, 9.2425 which is equivalent to: RA(J2000) = +00h 29m 27.72s Dec(J2000) = +09d 14' 33.0" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 81 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 5.32e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 83 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 00:29:27.80 = 7.36582 DEC(J2000) = +09:14:36.2 = 9.24339 with a 90%-confidence error radius of about 0.61 arc sec. This position is 3.4 arc sec. from the center of the XRT error circle. The estimated magnitude is 15.90 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.04. Burst Advocate for this burst is A. D'Ai (antonino.dai AT ifc.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.)