Spectral properties of X-ray binaries in Centaurus A

M.J. Burke, S. Raychaudhury, R.P. Kraft, W.R. Forman, C. Jones, S.S. Murray, M. Birkinshaw, D.A. Evans, A. Jordán, D.M. Worrall, T. J. Maccarone, J.H. Croston, Nicola Brassington, M.J. Hardcastle, J.L. Goodger, J. Kainulainen, K.A. Woodley, G.R. Sivakoff, M. Gilfanov, Z. ZhangC.L. Sarazin, R. Voss

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Abstract

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.
Original languageEnglish
Article number88
JournalThe Astrophysical Journal
Volume766
Issue number2
DOIs
Publication statusPublished - 1 Apr 2013

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