TITLE: GCN CIRCULAR NUMBER: 16788 SUBJECT: GRB 140907A: Swift detection of a burst with an optical counterpart DATE: 14/09/07 16:34:45 GMT FROM: David Palmer at LANL H. A. Krimm (CRESST/GSFC/USRA), V. D'Elia (ASDC), C. Gronwall (PSU), L. Izzo (URoma/ICRA), K. L. Page (U Leicester), D. M. Palmer (LANL) and T. N. Ukwatta (LANL) report on behalf of the Swift Team: At 16:07:08 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 140907A (trigger=611933). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 48.147, +46.595 which is RA(J2000) = 03h 12m 35s Dec(J2000) = +46d 35' 42" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed a single spiky peak structure with a duration of about 40 sec. The peak count rate was ~3100 counts/sec (15-350 keV), at ~15 sec after the trigger. The XRT began observing the field at 16:08:32.3 UT, 83.6 seconds after the BAT trigger. Using promptly downlinked data we find a bright, fading, uncatalogued X-ray source with an enhanced position: RA, Dec 48.1461, 46.6051 which is equivalent to: RA(J2000) = 03h 12m 35.06s Dec(J2000) = +46d 36' 18.2" with an uncertainty of 2.2 arcseconds (radius, 90% containment). This location is 36 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 consistent with the Galactic value of 3.13 x 10^21 cm^-2 (Willingale et al. 2013). The initial flux in the 2.5 s image was 5.46e-10 erg cm^-2 s^-1 (0.2-10 keV). Automated analysis of the UVOT data products is not available, however there appears to be a new bright optical source at the XRT position in the prompt finding chart exposure. More information will be available when the full dataset is received. Burst Advocate for this burst is H. A. Krimm (hans.krimm AT nasa.gov). 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.)