TITLE: GCN CIRCULAR NUMBER: 12914 SUBJECT: Swift detection of a burst from LS I +61 303 DATE: 12/02/05 12:25:11 GMT FROM: David Palmer at LANL D. N. Burrows (PSU), M. M. Chester (PSU), V. D'Elia (ASDC), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), C. J. Saxton (UCL-MSSL), E. Sonbas (NASA/GSFC/Adiyaman Univ.), M. Stamatikos (OSU/NASA/GSFC) and G. Stratta (ASDC) report on behalf of the Swift Team: At 11:47:24 UT, the Swift Burst Alert Telescope (BAT) triggered and located a burst consistent with the known High Mass X-Ray binary LS I +61 303 (trigger=513505). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 40.100, +61.220 which is RA(J2000) = 02h 40m 24s Dec(J2000) = +61d 13' 13" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single short spike with a duration of about 64 ms or less. The peak count rate was 1500 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 11:48:42.0 UT, 77.5 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 40.1326, 61.2290 which is equivalent to: RA(J2000) = 02h 40m 31.83s Dec(J2000) = +61d 13' 44.4" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. This position is 1.2 arcseconds from that of a known X-ray source: LS I +61 303, and 65.1 arcseconds from the on-board BAT position. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 7.57 x 10^21 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 80 seconds after the BAT trigger, and a finding chart exposure of 250 seconds with the U filter starting 293 seconds after the BAT trigger. A source consistent with the position of LS I +61 303 is detected, with white = 13.8 and u = 12.5 (1-sigma errors of about 0.1.). The 2.7'x2.7' sub-image covers 100% of the XRT error circle. 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 large, but uncertain extinction expected. The BAT position is within 1.1 arcminutes of the micro-quasar source LS I +61 303, and the XRT position is consistent in both position and observational characteristics to our previous observations of this source. This location has previously been the source of similar magnetar-like bursts (Torres et al. Astrophys. J., 744, 106 (2012)).