GCN Circular 18251
J.P. Osborne (U. Leicester), D.N. Burrows (PSU), J.A. Kennea (PSU), G. Cusumano (INAF-IASF PA), M. Perri (ASDC) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: The XRT began observing the field of GRB 150907B at 00:16:37.8 UT, 3010.0 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 255.30809, -63.78591 which is equivalent to: RA(J2000) = 17h 01m 13.94s Dec(J2000) = -63d 47' 09.3" with an uncertainty of 4.0 arcseconds (radius, 90% containment). This location is 185 arcseconds from the BAT onboard position, outside 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.52 x 10^21 cm^-2 (Willingale et al. 2013).