TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5436 SUBJECT: Swift detection of an outburst from the 4U 1246-58 LMXB burster DATE: 06/08/11 03:44:16 GMT FROM: Scott Barthelmy at NASA/GSFC P. Romano (INAF-OAB), S. D. Barthelmy (GSFC), N. Gehrels (NASA/GSFC), C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC), K. M. McLean (LANL/UTD), A. Moretti (INAF-OAB), D. M. Palmer (LANL), G. Sato (GSFC/ISAS), M. Stamatikos (NASA/ORAU) and D. E. Vanden Berk (PSU) report on behalf of the Swift Team: At 02:59:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located a soft burst from near the Galactic plane (trigger=223918). Swift slewed immediately to the burst. The BAT on-board calculated location is RA,Dec 192.420, -59.093 {12h 49m 41s, -59d 05' 32"} (J2000) with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a weak soft peak with a duration of about 20 sec. The peak count rate was ~500 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 03:03:08 UT, 193 seconds after the BAT trigger. XRT found a bright, possibly fading, X-ray source located at RA(J2000) = 12h 49m 39.8s, Dec(J2000) = -59d 05' 16.0", with an estimated uncertainty of 4.7 arcseconds (90% confidence radius). This location is 20 arcseconds from the BAT on-board position, within the BAT error circle. This position is consistent (3.1 arcsec) with the best known X-ray position of the 4U 1246-58 LMXB burster (Bassa et al. 2006, A&A, 446, L17). 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 202 seconds after the BAT trigger. No UV/optical counterpart has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. Because of the density of catalogued stars, no counterpart can be confirmed at this time. There is large, but uncertain extinction in the direction of the source.