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A Giant X-ray Flare in NGC 2770

ATel #1355; A. K.H. Kong (National Tsing Hua Univ., Taiwan) and T. J. Maccarone (Southampton)
on 10 Jan 2008; 15:30 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Albert Kong (akong@cfa.harvard.edu)

Subjects: X-ray, Soft Gamma-ray Repeater, Transient

Referred to by ATel #: 1356, 1368

We report a giant X-ray flare event detected in a Swift observation of SN2007uy. On 2008 Jan 9 UT, Swift performed a 5 ks follow-up observation of SN2007uy (ATel #1350). While SN2007uy was not detected with XRT, a bright X-ray point source was found ~90 arcsec from SN2007uy. The derived position of this new X-ray source is R.A.=9h09m30.6s, Decl.=+33:08:18 (J2000) with a 3.6 arcsec error circle (90% confidence) and the source is located near the edge of the edge-on galaxy NGC 2770. The source has over 600 counts in the 0.3-10 keV band and the light curve indicates that all X-ray events are from the first ~600 sec. The X-ray light curve shows a fast-rise-exponential-decay profile with a rise time of ~50 sec and an e-folding time of ~160 sec. The X-ray spectrum can be fit with a power-law model with N_H=6e21 cm^-2 and a photon index of 2.1. We note that the Galactic N_H to the direction of NGC 2770 is 2e20 cm^-2. If the source is associated with NGC 2770 (d=29 Mpc), the unabsorbed 0.3-10 keV luminosity is 8.3e42 erg/s.

We can rule out a foreground object due to the relatively hard X-ray spectrum and the high absorption. We can also rule out an AGN because a ~10-20 second doubling timescale for the luminosity early in the flare requires the black hole mass be less than 10^6 solar masses by light crossing timescale arguments. Finally, we propose that the event may be a giant flare from a soft gamma-ray repeater similar to SGR 1806-20 (Hurley et al. 2005).