TITLE: GCN CIRCULAR NUMBER: 6431 SUBJECT: GRB 070521: Swift detection of a burst DATE: 07/05/21 07:04:35 GMT FROM: Scott Barthelmy at NASA/GSFC C. Guidorzi (Univ Bicocca&INAF-OAB), S. D. Barthelmy (GSFC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), C. Gronwall (PSU), C. B. Markwardt (GSFC/UMD), A. Moretti (INAF-OAB), P. T. O'Brien (U Leicester), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (Univ. Bicocca & INAF-OAB) and G. Tagliaferri (INAF-OAB) report on behalf of the Swift Team: At 06:51:10 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070521 (trigger=279935). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 242.669, +30.241 which is RA(J2000) = 16h 10m 41s Dec(J2000) = +30d 14' 27" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows multiple peaks with a duration of about 40 sec. The peak count rate was ~7000 counts/sec (15-350 keV), at ~20 sec after the trigger. The XRT began taking data at 06:52:27 UT, 77 seconds after the BAT trigger. The XRT on-board centroid algorithm did not find a source in the image and no prompt position is available. However, the downlinked X-ray spectrum and lightcurve show that there is an X-ray object in the field that appears to be fading. Using prompt downlinked data, we find a position RA, Dec 242.6606, 30.2579 which is RA(J2000) = 16h10m 38.5s Dec(J2000) = +30d 15' 28.3'' with an uncertainty of 4.0 arcsec (radius, 90% containment). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 81 seconds after the BAT trigger. No afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 25% of the BAT error circle. The typical 3-sigma upper limit has been about 18.5 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the BAT 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 0.03.