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Extremely Bright X-ray Transient Likely in NGC 2770

ATel #1353; Edo Berger and Alicia Soderberg (Princeton University)
on 10 Jan 2008; 13:52 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Alicia M. Soderberg (ams@astro.caltech.edu)

Subjects: X-ray, Transient

Referred to by ATel #: 1356, 1368

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. 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.