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

Published online by Cambridge University Press:  12 May 2006

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

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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.

Type
Contributed Papers
Copyright
2006 International Astronomical Union