GCN Circular 18217
P.A. Evans (U. Leicester), J.P. Osborne (U. Leicester), D.N. Burrows (PSU), J.A. Kennea (PSU), S. Campana (INAF-OAB) and G. Cusumano (INAF-IASF PA) report on behalf of the Swift-XRT team: The XRT began observing the field of GRB 150831B at 23:10:52.8 UT, 3085.5 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 271.03936, -27.25933 which is equivalent to: RA(J2000) = 18h 04m 09.45s Dec(J2000) = -27d 15' 33.6" with an uncertainty of 3.9 arcseconds (radius, 90% containment). This location is 64 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. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 5.13 x 10^21 cm^-2 (Willingale et al. 2013).