TITLE: GCN CIRCULAR NUMBER: 8789 SUBJECT: GRB 090107B: Swift-XRT detection of possible afterglow DATE: 09/01/07 21:09:39 GMT FROM: Matteo Perri at ISAC/ASDC M. Perri (ASDC) and P. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift began observing the field of the INTEGRAL GRB 090107B (Gotz et al., GCN Circ. 8786) 4.2 ks after the trigger. A single uncatalogued source was identified within the IBIS/ISGRI error circle. Using 1314 s of XRT Photon Counting mode data and 2 UVOT images, we find an astrometrically corrected X-ray position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 284.81750, 59.59510 which is equivalent to: RA (J2000): 18 59 16.21 Dec (J2000): +59 35 42.3 with an uncertainty of 1.8 arcsec (radius, 90% confidence). The source lies 21 arcsec from the IBIS/ISGRI centroid and has a count rate of ~0.25 counts/s during this observation. Currently we cannot determine if this source is fading, and therefore cannot confirm if this source is associated with GRB 090107B. Observations of this source are on-going. This circular is an official product of the Swift-XRT team.