TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3956 SUBJECT: GRB 050906: Possible X-ray Counterpart DATE: 05/09/09 18:16:53 GMT FROM: Derek Fox at PSU D. B. Fox (PSU), C. Pagani (PSU/INAF-OAB), L. Angelini (GSFC), D. N. Burrows (PSU), J. P. Osborne (U. Leicester), and V. La Parola (INAF-IASF) report: "We have analyzed the Swift/XRT observations of the BAT localization region for the short-hard burst GRB 050906 (Parsons et al., GCN 3935). During the first orbit only, there is evidence for an excess of counts near the position: RA 03:31:15.6, Dec -14:36:37 (J2000) where we have corrected the XRT coordinates by roughly 6-arcsec from the native astrometry using a bright star / X-ray source in the field. Within a 20-pixel (47-arcsec) radius of this position, 6 photons are detected where 0.9 photons are expected from the background, representing a detection at 99.7%-confidence when considered over the full XRT field of view. If due to an astrophysical source, the source's estimated count rate is 7.5 +/- 5.2 cts/ksec (90%-confidence) over the 798 seconds of observation, and the source positional uncertainty is 18 arcsec (90%-confidence radius). Subsequent XRT observations reveal no further excess count detections within this region. The source location is not coincident with either the VLA source (Cameron & Frail, GCN 3933) or the bright galaxy (Levan & Tanvir, GCN 3927; Parsons et al., GCN 3935) that have been identified in observations of this burst. We note that: (1) The location of the excess is 13 arcsec outside the 2.6-arcmin radius (90%-confidence) BAT localization region (Parsons et al., GCN 3935); (2) The temporal distribution of the counts during the first orbit is consistent with the temporal distribution of the detector background counts (88%-confidence via two-sided K-S test); (3) The spatial distribution of the counts appears flat rather than PSF-like (unquantified); and (4) The source region lies within a region of increased local background, towards the end of the first orbit, due to emission from the bright Earth limb (two counts arrive during the last 100 seconds; however, the event grades are distinct from those associated with the bright Earth limb). Given these caveats, we are not confident that the counts excess represents the detection of an astrophysical source. However, if it is due to a real source, it is the most likely X-ray counterpart to GRB 050906."