TITLE: GCN CIRCULAR NUMBER: 6425 SUBJECT: GRB 070520B: Swift detection of a burst DATE: 07/05/20 18:14:26 GMT FROM: Scott Barthelmy at NASA/GSFC A. Moretti (INAF-OAB), S. D. Barthelmy (GSFC), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), J. A. Kennea (PSU), V. Mangano (INAF-IASFPA), P. T. O'Brien (U Leicester), D. M. Palmer (LANL), P. Romano (Univ. Bicocca & INAF-OAB), M. Stamatikos (NASA/ORAU), E. Troja (INAF-IASFPA) and S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team: At 17:44:53 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070520B (trigger=279898). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 121.951, +57.617 which is RA(J2000) = 08h 07m 48s Dec(J2000) = +57d 37' 00" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single peak with a duration of about 5 sec. The peak count rate was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. This burst occurred during a Malindi data downlink session which buffered the initial notices for up to 12 minutes. The XRT began observing the field at 17:46:31 UT, 98 seconds after the BAT trigger. Using prompt downlinked data XRT found a bright and flaring uncatalogued X-ray source located at RA, Dec 121.8798, 57.6092 which is RA(J2000) = 08h 07m 31.1's Dec(J2000) = +57d 36' 33.1'' with an uncertainty of 4.3 arcsec (radius, 90% containment). This location is 138 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 1.5e-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 107 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 at the XRT position is 19.5 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 Galactic reddening of E(B-V)=0.04.