TITLE: GCN CIRCULAR NUMBER: 21191 SUBJECT: GRB 170604A: Swift detection of a burst DATE: 17/06/04 19:29:03 GMT FROM: Kim Page at U.of Leicester K. L. Page (U Leicester), A. Cholden-Brown (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC) and F. E. Marshall (NASA/GSFC) report on behalf of the Swift Team: At 19:08:50 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 170604A (trigger=755867). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 342.645, -15.414 which is RA(J2000) = 22h 50m 35s Dec(J2000) = -15d 24' 50" 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 25 sec. The peak count rate was ~2400 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 19:10:55.1 UT, 124.7 seconds after the BAT trigger. No source was detected in the 2.5-s promptly available image. However, the XRT TDRSS light curve suggests the presence of a bright, fading source; since no Photon Counting mode data were collected, no prompt source localisation can be performed. We are waiting for the full dataset to detect and localise the XRT counterpart. No results from UVOT are available at this time. Burst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk). 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.)