Spectral Properties of X-ray Binaries in Centaurus A

Mark J. Burke, Somak Raychaudhury, Ralph P. Kraft, Thomas J. Maccarone, Nicola J. Brassington, Martin J. Hardcastle, Jouni Kainulainen, Kristin A. Woodley, Joanna L. Goodger, Gregory R. Sivakoff, William R. Forman, Christine Jones, Stephen S. Murray, Mark Birkinshaw, Judith H. Croston, Daniel A. Evans, Marat Gilfanov, Andres Jordan, Craig L. Sarazin, Rasmus Voss, Diana M. Worrall, Zhongli Zhang

Published 2013-02-04Version 1

We present a spectral investigation of X-ray binaries 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, and present the spectral parameters of sources with L >2x10^37 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 Msol. 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 Msol 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 Lx >= 5x10^38 erg/s for the XB population of early-type galaxies; for older stellar populations, there are fewer stars >1 Msol, which are required to form the more luminous sources.