TITLE: GCN CIRCULAR NUMBER: 30261 SUBJECT: GRB 210619B: Swift detection of a bright burst and optical counterpart DATE: 21/06/20 00:12:00 GMT FROM: David Palmer at LANL P. D'Avanzo (INAF-OAB), M. G. Bernardini (INAF-OAB), A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 23:59:25 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 210619B (trigger=1056757). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 319.713, +33.860 which is RA(J2000) = 21h 18m 51s Dec(J2000) = +33d 51' 35" 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 100 sec. The peak count rate was ~100,000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 00:04:53.4 UT, 328.1 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 319.7161, 33.8495 which is equivalent to: RA(J2000) = 21h 18m 51.86s Dec(J2000) = +33d 50' 58.2" with an uncertainty of 11.1 arcseconds (radius, 90% containment). This location is 38 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 4.27e-09 erg cm^-2 s^-1 (0.2-10 keV). 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) = 21:18:52.39 = 319.71831 DEC(J2000) = +33:51:01.6 = 33.85044 with a 90%-confidence error radius of about 0.61 arc sec. This position is 7.5 arc sec. from the center of the XRT error circle. The estimated magnitude is 14.71 with a 1-sigma error of about 0.14. No correction has been made for the expected extinction corresponding to E(B-V) of 0.173. Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT 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/)