|Populations of High Energy Sources in Galaxies|
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: R.Barnard@open.ac.uk, L.Shaw-Greening@open.ac.uk., U.C.Kolb@open.a.c.uk
a2 University of Leicester, University Road, Leicester LE1 7RH, UK email: email@example.com
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.