TITLE: GCN CIRCULAR NUMBER: 6137 SUBJECT: GRB 070224: Swift detection of a burst DATE: 07/02/24 20:48:09 GMT FROM: Scott Barthelmy at NASA/GSFC J. L. Racusin (PSU), L. M. Barbier (NASA/GSFC), S. D. Barthelmy (GSFC), M. M. Chester (PSU), G. Cusumano (INAF-IASFPA), D. Grupe (PSU), J. A. Kennea (PSU), D. M. Palmer (LANL), R. L. C. Starling (U Leicester) and L. Vetere (PSU) report on behalf of the Swift Team: At 20:27:58 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 070224 (trigger=261880). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 178.990, -13.340 which is RA(J2000) = 11h 55m 58s Dec(J2000) = -13d 20' 23" 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 25 sec. The peak count rate was ~500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 20:30:21 UT, 143 seconds after the BAT trigger. XRT found a bright, fading, uncatalogued X-ray source located at RA, Dec 179.0257, -13.3294 which is RA(J2000) = 11h 56m 06.2s Dec(J2000) = -13d 19' 46.0" with an uncertainty of 4.2 arcseconds (radius, 90% containment). This location is 131 arcseconds from the BAT on-board position, within the BAT error circle. The initial flux in the 2.5s image was 3.1e-10 erg/cm2/s (0.2-10 keV). UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 153 seconds after the BAT trigger, and a finding chart of nominal 400 seconds with the V filter starting 259 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 White at the XRT position is 19.3 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. No correction has been made for the expected extinction corresponding to E(B-V) of 0.06.