TITLE: GCN CIRCULAR NUMBER: 22807 SUBJECT: GRB 180620B: Swift detection of a burst with an optical counterpart DATE: 18/06/20 16:05:27 GMT FROM: Kim Page at U.of Leicester P. A. Evans (U Leicester), A. A. Breeveld (UCL-MSSL), A. Deich (PSU), J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 15:50:37 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 180620B (trigger=843211). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 357.517, -57.957 which is RA(J2000) = 23h 50m 04s Dec(J2000) = -57d 57' 25" 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 30 sec. The peak count rate was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 15:52:00.3 UT, 83.2 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 357.52114, -57.96306 which is equivalent to: RA(J2000) = 23h 50m 05.07s Dec(J2000) = -57d 57' 47.0" with an uncertainty of 3.7 arcseconds (radius, 90% containment). This location is 23 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 1.39 x 10^20 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 7.04e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 91 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 23:50:05.10 = 357.52126 DEC(J2000) = -57:57:44.1 = -57.96224 with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.7 arc sec. from the center of the XRT error circle. The estimated magnitude is 17.98 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.01. Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.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.)