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

TL;DR

A luminous X-ray flare was detected from the nucleus of the dormant, bulgeless galaxy NGC 247, likely caused by a low-mass X-ray binary outburst or a low-mass black hole accretion event, with detailed spectral and temporal analysis supporting these scenarios.

Contribution

This study reports the first detection of an X-ray flare from the nucleus of NGC 247, providing insights into possible low-mass black hole activity or stellar disruption events in bulgeless galaxies.

Findings

X-ray flare luminosity up to 2×10^39 erg/s observed

Spectral analysis shows a hard power-law with a break near 3-4 keV

Temporal data suggest the flare is consistent with a low-mass X-ray binary or a TDE

Abstract

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).