[vsnet-grb-info 5493] Bright X-ray Transient in NGC 2770 - A low-luminosity XRF?

[GCN Circulars]

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  <eberger at astro.princeton.edu>

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."