XMM-Newton reveals ∼100 new LMXBs in M31 from variability studies

Barnard, R.; Shaw Greening, L.; Tonkin, C.; Kolb, U. and Osborne, J. P. (2006). XMM-Newton reveals ∼100 new LMXBs in M31 from variability studies. In: Populations of high energy sources in galaxies: proceedings of the 230th symposium of the International Astronomical Union, 15-19 Aug 2005, Dublin, Ireland.

DOI: https://doi.org/10.1017/S174392130600809X


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 ∼0 to ∼1 at some frequency in the range ∼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.

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