Proceedings of the International Astronomical Union

Populations of High Energy Sources in Galaxies
Contributed Papers

XMM-Newton reveals $\sim$100 new LMXBs in M31 from variability studies

R. Barnard a1, L. Shaw Greening a1, C. Tonkin a1, U. Kolb a1 and J.P. Osborne a2
a1 The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK email:,,
a2 University of Leicester, University Road, Leicester LE1 7RH, UK email:

Article author query
barnard r   [Google Scholar] 
greening ls   [Google Scholar] 
tonkin c   [Google Scholar] 
kolb u   [Google Scholar] 
osborne jp   [Google Scholar] 
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We have conducted a survey of X-ray sources in XMM-Newton observations of M31, examining their power density spectra (PDS) and spectral energy distributions (SEDs). Our automated source detection yielded 535 good X-ray sources; to date, we have studied 225 of them. In particular, we examined the PDS because low mass X-ray binaries (LMXBs) exhibit two distinctive types of PDS. At low accretion rates, the PDS is characterised by a broken power law, with the spectral index changing from $\sim$0 to $\sim$1 at some frequency in the range $\sim$0.01–1 Hz; we refer to such PDS as Type A. At higher accretion rates, the PDS is described by a simple power law; we call these PDS Type B. Of the 225 sources studied to date, 75 exhibit Type A variability, and are almost certainly LMXBs, while 6 show Type B but not Type A, and are likely LMXBs. Of these 81 candidate LMXBs, 71 are newly identified in this survey; furthermore, they are mostly found near the centre of M31. Furthermore, most of the X-ray population in the disc are associated with the spiral arms, making them likely high mass X-ray binaries (HMXBs). In general these HMXBs do not exhibit Type A variability, while many central X-ray sources (LMXBs) in the same luminosity range do. Hence the PDS may distinguish between LMXBs and HMXBs in this luminosity range.

(Published Online May 12 2006)

Key Words: X-rays: binaries; galaxies: individual (M31); accretion discs; black hole physics.