TITLE: GCN CIRCULAR NUMBER: 6114 SUBJECT: GRB 070220: Swift detection of a burst DATE: 07/02/20 05:05:13 GMT FROM: Scott Barthelmy at NASA/GSFC M. Stamatikos (NASA/ORAU), S. D. Barthelmy (GSFC), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), N. Gehrels (NASA/GSFC), S. Immler (GSFC/USRA), J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL), T. Sakamoto (NASA/ORAU), P. Schady (MSSL-UCL) and D. E. Vanden Berk (PSU) report on behalf of the Swift Team: At 04:44:33 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070220 (trigger=261299). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 34.819, +68.818 which is RA(J2000) = +02h 19m 17s Dec(J2000) = +68d 49' 06" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a duration of about 60 sec. The peak count rate was ~7000 counts/sec (15-350 keV), at ~15 sec after the trigger. The XRT began observing the field at 04:45:51 UT, 79 seconds after the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located at RA, Dec 34.7765, +68.8052 which is RA(J2000) = 02h 19m 06.3s Dec(J2000) = 68d 48' 18.7" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 72 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 4.5e-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 88 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 in a 100s white band exposure is 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.90.