TITLE: GCN CIRCULAR NUMBER: 6560 SUBJECT: GRB 070621: Swift detection of a burst DATE: 07/06/21 23:36:31 GMT FROM: Scott Barthelmy at NASA/GSFC B. Sbarufatti (INAF-IASFPA), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), P. J. Brown (PSU), M. M. Chester (PSU), C. Guidorzi (Univ Bicocca&INAF-OAB), S. T. Holland (CRESST/GSFC/USRA), J. A. Kennea (PSU), C. B. Markwardt (CRESST/GSFC/UMD), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL), P. Romano (Univ. Bicocca & INAF-OAB), M. Stamatikos (NASA/ORAU) and T. N. Ukwatta (GSFC/GWU) report on behalf of the Swift Team: At 23:17:39 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070621 (trigger=282808). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 323.804, -24.806 which is RA(J2000) = 21h 35m 13s Dec(J2000) = -24d 48' 20" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a total duration of at least 40 sec. The peak count rate was ~1500 counts/sec (15-350 keV), at ~8 sec after the trigger. The XRT began observing the field at 23:19:30 UT, 111 seconds after the BAT trigger. XRT found a bright and fading X-ray source located at RA, Dec 323.7909, -24.8181 which is RA(J2000) = 21h 35m 09.8s Dec(J2000) = -24d 49' 05.1" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 61 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 3.7e-09 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 120 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The 3-sigma upper limit is about 19.2 mag. The upper limit has not been corrected for the expected Galactic extinction along the line of sight corresponding to a reddening of E_{B-V} = 0.05 mag.