A LUMINOUS X-RAY FLARE from the NUCLEUS of the DORMANT BULGELESS SPIRAL GALAXY NGC 247

Hua Feng, Luis C. Ho, Philip Kaaret, Lian Tao, Kazutaka Yamaoka, Shuo Zhang, Fabien Grisé

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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 of up to 2 × 1039 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 34 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 erg s-1, suggesting that it is either a low-mass black hole or an inefficient tidal disruption event.

Original languageEnglish (US)
Article number185
JournalAstrophysical Journal
Volume807
Issue number2
DOIs
StatePublished - Jul 10 2015

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

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