SPECTRAL PROPERTIES OF X-RAY BINARIES IN CENTAURUS A

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-H, and present the spectral parameters of sources with L-x greater than or similar to 2 x 10(37) erg s(-1). 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 similar to 2-5M(circle dot). We propose that BH LMXBs are preferentially found in the dust lane, and suggest this is because of the younger stellar population. The majority (similar to 70%-80%) of potential Roche lobe filling donors in the Cen A halo are greater than or similar to 12 Gyr old, while BH LMXBs require donors greater than or similar to 1 M-circle dot 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-x >= 5 x 10(38) erg s(-1) for the XB population of early-type galaxies; for older stellar populations, there are fewer stars greater than or similar to 1 M-circle dot, which are required to form the more luminous sources.