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

Abstract

A new, transient ultraluminous X-ray source (ULX) was recently discovered by Chandra in M31 with a luminosity at ∼5 × 10 39 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 × 10 39 to 0.6 × 10 39 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/L Edd = 0.75 to 0.25. However, the data are better described by a twocomponent 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 twocomponent 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.

Revealing the Nature of the Ulx and X-Ray Population of the Spiral Galaxy NGC 4088

“…A hint of a spectral cut-off at ∼5 keV seems to be observed in the residuals of the power-law model. This feature is similar to that observed in other ULXs (e.g., Dewangan et al 2006Dewangan et al , 2010Stobbart et al 2006;Kajava et al 2012;Sutton et al 2013), suggesting that N4088-X1 could be in the ultraluminous state (e.g., Roberts 2007;Kajava & Poutanen 2009;Sutton et al 2013) with a thermal spectrum described by Compton scattering of soft photons (e.g., Middleton et al 2012;Soria et al 2012;Straub et al 2013). The good fit provided by the thermal Comptonization model together with the statistically significant cut-off indicated by the F-test support this possibility, although the Swift data are not of high enough S/N to confirm it.…”

Spectral state transitions of the Ultraluminous X-ray Source IC 342 X-1

“…# denotes parameter held fixed during fitting tonization in addition to a slim disc component rather than the slim disc alone (e.g. Middleton et al 2011Middleton et al , 2012. We find that the Chandra spectrum is well fit by the Kawaguchi (2003) model number 7 which includes a Comptonized local spectrum and relativistic effects.…”

Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

“…Possibly, these faintest ULXs represent a transition phase in the evolution of ULXs at around the Eddington accretion rate, where an optically thick corona obscures the central disc (e.g. Middleton, Sutton & Roberts 2011b;Middleton et al 2012). However, Dotan & Shaviv (2011) suggest that characteristic super-Eddington-like properties would emerge as the Eddington rate is approached, in which case even the faintest ULXs may have emerging 'classic' ultraluminous spectra, with the soft X-ray emission originating in the photosphere of a wind.…”