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NGC 247 is a nearby late-type bulgeless spiral galaxy that contains aninactive nucleus. We report a serendipitous discovery of an X-ray flare fromthe galaxy center with a luminosity up to 2*10^39 erg/s in the 0.3-10 keV bandwith XMM-Newton. A Chandra observation confirms that the new X-ray source isspatially coincident with the galaxy nucleus. The XMM-Newton data revealed ahard power-law spectrum with a spectral break near 3-4 keV, no pulsations ontimescales longer than 150 ms, and a flat power spectrum consistent withPoisson noise from 1 mHz to nearly 10 Hz. Follow-up observations with Swiftdetected a second flux peak followed by a luminosity drop by factor of almost20. The spectral and temporal behaviors of the nuclear source are wellconsistent with the scenario that the flare was due to an outburst of alow-mass X-ray binary that contains a stellar-mass black hole emitting near itsEddington limit at the peak. However, it cannot be ruled out that the suddenbrightening in the nucleus was due to accretion onto a possible low-massnuclear black hole, fed by a tidally disrupted star or a gas cloud; the MAXIobservations 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 tidaldisruption event (TDE).

A Luminous X-ray Flare From The Nucleus of The Dormant Bulgeless Spiral Galaxy NGC 247