  NGC 247 is a nearby late-type bulgeless spiral galaxy that contains an
inactive nucleus. We report a serendipitous discovery of an X-ray flare from
the galaxy center with a luminosity up to 2*10^39 erg/s in the 0.3-10 keV band
with XMM-Newton. A Chandra observation confirms that the new X-ray source is
spatially coincident with the galaxy nucleus. The XMM-Newton data revealed a
hard power-law spectrum with a spectral break near 3-4 keV, no pulsations on
timescales longer than 150 ms, and a flat power spectrum consistent with
Poisson noise from 1 mHz to nearly 10 Hz. Follow-up observations with Swift
detected a second flux peak followed by a luminosity drop by factor of almost
20. The spectral and temporal behaviors of the nuclear source are well
consistent with the scenario that the flare was due to an outburst of a
low-mass X-ray binary that contains a stellar-mass black hole emitting near its
Eddington limit at the peak. However, it cannot be ruled out that the sudden
brightening in the nucleus was due to accretion onto a possible low-mass
nuclear black hole, fed by a tidally disrupted star or a gas cloud; the MAXI
observations limit the peak luminosity of the flare to less than ~10^43 erg/s,
suggesting that it is either a low mass black hole or an inefficient tidal
disruption event (TDE).
