TITLE: GCN CIRCULAR NUMBER: 13831 SUBJECT: Swift Trigger 535026: Galactic Transient or possible GRB DATE: 12/10/01 18:54:05 GMT FROM: Jamie A. Kennea at PSU/Swift-XRT V. D'Elia (ASDC), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), N. Gehrels (NASA/GSFC), S. T. Holland (STScI), J. A. Kennea (PSU), C. B. Markwardt (NASA/GSFC), F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB/PSU), M. H. Siegel (PSU), E. Sonbas (NASA/GSFC/Adiyaman Univ.) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift Team: At 18:23:02 UT, the Swift Burst Alert Telescope (BAT) triggered and located trigger 535026. Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 276.033, -5.667 which is RA(J2000) = 18h 24m 08s Dec(J2000) = -05d 40' 00" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a weak multi-peaked structure with a duration of about 25 sec. The peak count rate was ~1200 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 18:25:03.7 UT, 121.5 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 276.0325, -5.6654 which is equivalent to: RA(J2000) = 18h 24m 07.80s Dec(J2000) = -05d 39' 55.5" with an uncertainty of 2.7 arcseconds (radius, 90% containment). This location is 6.0 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (4.53 x 10^21 cm^-2, Kalberla et al. 2005), with an excess column of 5.5 (+4.26/-3.49) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 5.49e-10 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 287 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. 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 18.0 mag. No correction has been made for the large, but uncertain extinction expected. The classification of this trigger is uncertain, but it's quite likely that it is a galactic transient. The duration (~20 sec) and low energy emission (<25 keV) are consistent with it being a thermonuclear X-ray burst from a neutron star. The location is 3.5 degrees from the galactic plane. Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.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.)