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 .