TITLE: GCN CIRCULAR NUMBER: 6423 SUBJECT: GRB 070520: Swift detection of a burst DATE: 07/05/20 13:21:43 GMT FROM: Scott Barthelmy at NASA/GSFC P. Romano (Univ. Bicocca & INAF-OAB), S. D. Barthelmy (GSFC), M. M. Chester (PSU), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. Guidorzi (Univ Bicocca&INAF-OAB), V. Mangano (INAF-IASFPA), F. E. Marshall (NASA/GSFC), C. Pagani (PSU), D. M. Palmer (LANL), M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester) and S. D. Vergani (DIAS-DCU) report on behalf of the Swift Team: At 13:05:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070520 (trigger=279817). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 193.316, +74.993 which is RA(J2000) = 12h 53m 16s Dec(J2000) = +74d 59' 36" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). This is an 88-sec image trigger, and as such the TDRSS lightcurve does not show anything significant above background, as is typical for image triggers. The XRT began observing the field at 13:07:48 UT, 159 seconds after the BAT trigger. XRT found a X-ray source located at RA, Dec 193.3636, +74.9909 which is RA(J2000) = 12h 53m 27.2s Dec(J2000) = 74d 59' 27.2" with an uncertainty of 5.4 arcseconds (radius, 90% containment). This location is 45 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 1.1e-09 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 77 seconds with the White (160-650 nm) filter starting 168 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 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 0.02.