X-ray study of HLX1: intermediate-mass black hole or foreground neutron star?

Abstract

We re-assess the XMM-Newton and Swift observations of HLX1, to examine the evidence for its identification as an intermediate-mass black hole. We show that the X-ray spectral and timing properties are equally consistent with an intermediate-mass black hole in a high state, or with a foreground neutron star with a luminosity of about a few times 10^{32} erg/s ~ 10^{-6} L_{Edd}, located at a distance of about 1.5 to 3 kpc. Contrary to previously published results, we find that the X-ray spectral change between the two XMM-Newton observations of 2004 and 2008 (going from power-law dominated to thermal dominated) is not associated with a change in the X-ray luminosity. The thermal component becomes more dominant (and hotter) during the 2009 outburst seen by Swift, but in a way that is consistent with either scenario.

10 pages, accepted by MNRAS. Corrected LZ's affiliation and updated 1 reference

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• A Redshift for the Intermediate Mass Black Hole Candidate HLX-1: Confirmation of its Association with the Galaxy ESO 243-49
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• The X-ray spectral properties of very-faint persistent neutron star X-ray binaries
• Daily, multiwavelength Swift monitoring of the neutron star low-mass X-ray binary Cen X-4: evidence for accretion and reprocessing during quiescence
• A direct measurement of the heat release in the outer crust of the transiently accreting neutron star XTE J1709-267
• Continued Neutron Star Crust Cooling of the 11 Hz X-Ray Pulsar in Terzan 5: A Challenge to Heating and Cooling Models?
• On the Constancy of the Photon Index of X-ray spectra of 4U~1728-34 through all spectral states
• XMM-Newton and Swift spectroscopy of the newly discovered very-faint X-ray transient IGR J17494-3030
• A low-level accretion flare during the quiescent state of the neutron-star X-ray transient SAX J1750.8-2900
• Outbursts of the intermediate-mass black hole HLX-1: a wind instability scenario
• Models of Ultra-Luminous X-ray transient sources
• Spectral variability in Swift and Chandra observations of the Ultraluminous source NGC 55 ULX1
• Eccentricity of HLX-1
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• Identification of Intermediate-mass Black Hole Candidates Among a Sample of Sd Galaxies
• Accretion disk parameters in HLX-1