TITLE: GCN CIRCULAR NUMBER: 18000 SUBJECT: Swift trigger 648437: A GRB or a burst from 1RXS J125754.7+141850 DATE: 15/07/10 00:56:43 GMT FROM: David Palmer at LANL M. G. Bernardini (INAF-OAB), S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB), A. Melandri (INAF-OAB), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 00:28:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located a short burst consistent with the location of the ROSAT faint source 1RXS J125754.7+14185 (trigger=648437). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 194.497, +14.305 which is RA(J2000) = 12h 57m 59s Dec(J2000) = +14d 18' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single spike structure with a duration of about 0.25 sec. The peak count rate was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 00:28:51.6 UT, 49.2 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source with an enhanced position: RA, Dec 194.4710, 14.3183 which is equivalent to: RA(J2000) = 12h 57m 53.03s Dec(J2000) = +14d 19' 05.8" with an uncertainty of 3.5 arcseconds (radius, 90% containment). This location is 102 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. We cannot determine whether the source is fading at the present time. This position is 28.6 arcseconds from that of a known X-ray source: 1RXS J125754.7+141850 in the ROSAT RASSFSC catalogue. UVOT took a finding chart exposure of 150 seconds with the White filter starting 53 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers none 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 expected extinction corresponding to E(B-V) of 0.03. This source location is consistent with the source 1RXS J125754.7+141850, an object in the ROSAT RASSFSC. This catalog has 106k sources, and so the possibility of a chance coincidence cannot be ruled out. This burst is comparable in hardness and other aspects to a typical short GRB. For this reason, we cannot confirm whether this burst comes from the previously-known source or a new sGRB with the immediately-available data. The complete downlinked dataset and further observations will be required to determine the nature of this source. Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.inaf.it). Please contact the BA by email if you require additional information regarding Swift followup of this burst. In extremely urgent cases, after trying the Burst Advocate, you can contact the Swift PI by phone (see Swift TOO web site for information: http://www.swift.psu.edu/too.html.)