Populations of low mass X-ray binaries in the galaxies Centaurus A and Andromeda

We have studied the X-ray point source population of Centaurus A (NGC 5128) using data from four archival CHANDRA observations. We detected 272 point-like X-ray sources within a radius of 10 arcmin from the centre. Approximately half of these are CXB sources, with the remaining half being LMXBs. The spatial distribution of the LMXBs, both azimuthally averaged and 2-D, is consistent with the distribution of the K -band light observed in the 2MASS survey. After correction for the incompleteness effect we constrain the LMXB luminosity function down to ∼ 2× 1036 erg s−1, much lower than previous studies of LMXBs in elliptical galaxies. The obtained XLF flattens significantly below LX ∼ 5 × 1037 erg s−1 and follows the dN/dL ∝ L−1 law in agreement with the behaviour found earlier for LMXBs in the Milky Way and in the bulge of M31.