Spectral properties of X-ray binaries in Centaurus A

Burke, M.J., Raychaudhury, S., Kraft, R.P., Forman, W.R., Jones, C., Murray, S.S., Birkinshaw, M., Evans, D.A., Jordán, A., Worrall, D.M., Maccarone, T. J., Croston, J.H., Brassington, Nicola, Hardcastle, M.J., Goodger, J.L., Kainulainen, J., Woodley, K.A., Sivakoff, G.R., Gilfanov, M., Zhang, Z., Sarazin, C.L. and Voss, R. (2013) Spectral properties of X-ray binaries in Centaurus A. The Astrophysical Journal, 766 (2): 88. ISSN 0004-637X

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We present a spectral investigation of X-ray binaries (XBs) in NGC 5128 (Cen A), using six 100 ks Chandra observations taken over two months in 2007. We divide our sample into thermally and non-thermally dominated states based on the behavior of the fitted absorption column N , and present the spectral parameters of sources with L ≳ 2 × 10 erg s. The majority of sources are consistent with being neutron star low-mass X-ray binaries (NS LMXBs) and we identify three transient black hole (BH) LMXB candidates coincident with the dust lane, which is the remnant of a small late-type galaxy. Our results also provide tentative support for the apparent "gap" in the mass distribution of compact objects between ∼2-5 M . We propose that BH LMXBs are preferentially found in the dust lane, and suggest this is because of the younger stellar population. The majority (∼70%-80%) of potential Roche lobe filling donors in the Cen A halo are ≳ 12 Gyr old, while BH LMXBs require donors ≳ 1 M to produce the observed peak luminosities. This requirement for more massive donors may also explain recent results that claim a steepening of the X-ray luminosity function with age at L ≥ 5 × 10 erg s for the XB population of early-type galaxies; for older stellar populations, there are fewer stars ≳ 1 M , which are required to form the more luminous sources.