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

Jun 2, 2015
7 pages
Published in:
  • Astrophys.J. 807 (2015) 2, 185
  • Published: Jul 9, 2015
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Abstract: (IOP)
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 of up to 2×10392\times {10}^{39} erg s(−)(1) 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 a factor of almost 20. The spectral and temporal behaviors of the nuclear source are 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 Monitor of All-sky X-ray Image observations limit the peak luminosity of the flare to less than 1043\sim {10}^{43} erg s(−)(1), suggesting that it is either a low-mass black hole or an inefficient tidal disruption event.
Note:
  • accepted for publication in ApJ
  • galaxies: nuclei
  • X-rays: galaxies
  • black hole physics