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The missing link : a low mass X-ray binary in M31 seen as an ultraluminous X-ray source.

Middleton, M.J. and Sutton, A.D. and Roberts, T.P. and Jackson, F.E. and Done, C. (2012) 'The missing link : a low mass X-ray binary in M31 seen as an ultraluminous X-ray source.', Monthly notices of the Royal Astronomical Society., 420 (4). pp. 2969-2977.


A new, transient ultraluminous X-ray source (ULX) was recently discovered by Chandra in M31 with a luminosity at ∼5 × 1039 erg s−1. Here we analyse a series of five subsequent XMM–Newton observations. These show a steady decline in X-ray luminosity over 1.5 months, from 1.8 × 1039 to 0.6 × 1039 erg s−1, giving an observed e-fold time-scale of ∼40 d. This is similar to the decay time-scales seen in multiple soft X-ray transients in our own Galaxy, supporting the interpretation of this ULX as a stellar mass black hole in a low-mass X-ray binary (LMXB), accreting at super-Eddington rates. This is further supported by the lack of detection of an O/B star in quiescence and the spectral behaviour of the XMM–Newton data being dominated by a disc-like component rather than the power law expected from a sub-Eddington intermediate-mass black hole. These data give the best sequence of high Eddington fraction spectra ever assembled due to the combination of low absorption column to M31 and well-calibrated bandpass down to 0.3 keV of XMM–Newton in full frame mode. The spectra can be roughly described by our best current disc model, BHSPEC, assuming a 10 M⊙ black hole with best-fitting spin ∼0.4, declining from L/LEdd= 0.75 to 0.25. However, the data are better described by a two-component model, where the disc emission is significantly affected by advection, and with an additional low-temperature Comptonization component at high energies which becomes more important at high luminosities. This could simply indicate the limitations of our current disc models, though changes in the energy-dependent variability also weakly supports a two-component interpretation of the data. Irrespective of the detailed interpretation of the spectral properties, these data support the presence of accretion on to a stellar mass black hole in a LMXB accreting in the Eddington regime. This allows an unambiguous connection of this object, and, by extension, similar low-luminosity ULXs, to ‘standard’ X-ray binaries.

Item Type:Article
Keywords:Accretion Discs, Black hole physics, X-rays, Binaries.
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS Published by Oxford University Press on behalf of Royal Astronomical Society. All rights reserved.
Date accepted:No date available
Date deposited:28 January 2014
Date of first online publication:March 2012
Date first made open access:No date available

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