TITLE: GCN CIRCULAR NUMBER: 42556 SUBJECT: GRB 251103B: Swift detection of a burst DATE: 25/11/03 17:03:31 GMT FROM: K.L. Page at U Leicester S. Lanava (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC), M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 16:46:36 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 251103B (trigger=1409824). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 105.864, -24.843 which is RA(J2000) = 07h 03m 27s Dec(J2000) = -24d 50' 35" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a complex structure with a duration of about 200 sec. The peak count rate was ~4000 counts/sec (15-350 keV), at ~2 sec after the trigger. The XRT began observing the field at 16:47:49.2 UT, 72.9 seconds after the BAT trigger. Using promptly downlinked data we find a bright, uncatalogued X-ray source located at RA, Dec 105.87352, -24.87083 which is equivalent to: RA(J2000) = 07h 03m 29.64s Dec(J2000) = -24d 52' 15.0" with an uncertainty of 3.6 arcseconds (radius, 90% containment). This location is 104 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 https://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 (3.33 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.5 (+2.76/-2.40) x 10^21 cm^-2 (90% confidence). The initial flux in the 2.5 s image was 2.78e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 250 seconds with the U filter starting 139 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 expected extinction corresponding to E(B-V) of 0.255. Burst Advocate for this burst is S. Lanava (sml7284 AT psu.edu). 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/)