TITLE: GCN CIRCULAR NUMBER: 8311 SUBJECT: Trigger 330353: Swift detection of a Galactic short soft burst DATE: 08/10/03 10:18:48 GMT FROM: David Palmer at LANL H. A. Krimm (CRESST/GSFC/USRA), A. P. Beardmore (U Leicester), D. N. Burrows (PSU), M. Capalbi (ASDC), P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC), O. Godet (U Leicester), C. Guidorzi (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), S. D. Hunsberger (PSU), J. A. Kennea (PSU), J. Mao (INAF-OAB), C. B. Markwardt (CRESST/GSFC/UMD), P. T. O'Brien (U Leicester), S. R. Oates (UCL-MSSL), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-IASFPA), R. L. C. Starling (U Leicester), G. Tagliaferri (INAF-OAB) and E. Troja (INAF-IASFPA) report on behalf of the Swift Team: At 09:28:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located a short, soft burst (trigger=330353). Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 237.711, -54.319 which is RA(J2000) = 15h 50m 51s Dec(J2000) = -54d 19' 08" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single peaked structure with a duration of less than 0.1 sec, with most of the flux below 100 keV. The peak count rate was ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 09:29:26.6 UT, 77.9 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 237.72546, -54.30659 which is equivalent to: RA(J2000) = 15h 50m 54.11s Dec(J2000) = -54d 18' 23.7" with an uncertainty of 1.9 arcseconds (radius, 90% containment). This location is 54 arcseconds from the BAT onboard position, within the BAT error circle. 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 1.5 arcsec from (and hence consistent with) a source in the 2XMM catalogue: 2XMM J155054.2-541824. The XRT count rate is significantly higher than that catalogued by XMM, however. A power-law fit to a spectrum formed from promptly downlinked event data does not constrain the column density. UVOT took a finding chart exposure of 100 seconds with the White (160-650 nm) filter starting 82 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 typical 3-sigma upper limit has been about 18.5 mag. No correction has been made for the large, but uncertain extinction expected. The enhanced brightness of the 2XMM source immediately following a BAT detection argues that this is the correct source identification. However, we note that there are additional catalogued X-ray sources in the BAT error circle which are also plausible candidates for the burst source. Given the location of the burst, within 0.14 degrees of the Galactic plane, these are all expected to be Galactic sources. Because this is a short soft burst from a Galactic source, this may be a magnetar, such as an Soft Gamma Repeater (SGR) or an Anomalous X-ray Pulsar (AXP).