TITLE: GCN CIRCULAR NUMBER: 7159 SUBJECT: Bright X-ray Transient in NGC 2770 - A low-luminosity XRF? DATE: 08/01/10 16:12:03 GMT FROM: Edo Berger at Princton U E. Berger and A.M. Soderberg (Princeton University) report: "Follow-up observations of SN2007uy with the Swift/XRT reveal a new transient source about 95 arcsec away from the SN position at RA=09:09:30.7, Dec=+33:08:19 (J2000). This position coincides with the outskirts of the host galaxy of SN2007uy (NGC 2770). The object was not detected in the previous observation of SN2007uy (ATEL #1350) and is detected only in the first 1 ksec of the new observation. During that time the light curve is FRED-like with a rise time of about 50 sec and a decay to the background level by about 600 sec. The source is not detected in the subsequent 4 ksec of data that intermittently cover 5-24 ksec after the flare. The average count rate is about 1 cps, which at the distance of NGC2770 (d=27 Mpc) corresponds to a luminosity of about 4x10^42 erg/s. This is in excess of 10^4 times the Eddington luminosity of a solar mass object. No coincident object is detected in the simultaneous Swift/UVOT data or in archival DSS and 2MASS images. Alternatively, this may be a foreground bright stellar flare (with L_X/L_bol=10^-3), but this would require a solar-type star at a distance of only 25 pc or an M dwarf at 2 pc, both of which would be detectable in optical and near-IR images. An extreme stellar flare with L_X=L_bol would require distances of about 500 and 50 pc, respectively. Spectral fitting indicates a best-fit absorbed power law spectrum with NH=4x10^21 cm^-2 (in excess of the Galactic value of 2x10^20 cm^-2) and a photon index of 2.1. The unabsorbed luminosity is 5x10^42 erg/s. The high column density likely rules out a Galactic origin. Given the high luminosity at the distance of NGC 2770, the light curve shape, and the integrated energy of about 10^45 erg, we hypothesize that this object may be a weak X-ray Flash, perhaps analogous to GRB980425/SN1998bw. Deep optical and near-IR observations to search for a counterpart are encouraged. Radio observations are in progress."