X-ray Luminosity Function and Total Luminosity of LMXB in Early Type Galaxies

Abstract

We have derived bias-corrected X-ray luminosity functions (XLFs) of sources detected in a uniformly selected sample of 14 E and S0 galaxies observed with Chandra ACIS-S3. The entire sample yields 985 point-like X-ray sources, with typical detection of 30-140 sources per galaxy. After correcting for incompleteness, the individual XLFs are statistically consistent with a single power-law of a (differential) XLF slope b = 1.8 - 2.2 (with a typical error of 0.2-0.3). A break at or near LX,Eddington, as reported in the literature for some of these galaxies, is not required in any case. Although the combined XLF is marginally consistent with a single power-law (with b = 2.1 +- 0.1), a broken power-law gives an improved fit. The best-fit slope is b = 1.8 +- 0.2 in the low-luminosity range LX = a few x 1037 - 5 x 1038 ergs s-1. At higher luminosities, the slope is steeper, b = 2.8 +- 0.6. The break luminosity is 5 +- 1.6 x 1038 ergs s-1 (with an error at 90%), which may be consistent with the Eddington luminosity of neutron stars with the largest possible mass (3 Mo), of He-enriched neutron star binaries, or low-mass stellar mass black holes. If the change in XLF slope at high luminosities is real, our result would imply a different population of high luminosity sources, instead of a beaming effect. We confirm that the total X-ray luminosity of LMXBs is correlated with the optical and more tightly with the near-IR luminosities, but in both cases the scatter exceeds that expected from measurement errors. We find that the scatter in LX(LMXB)/LK is marginally correlated with the specific frequency of globular clusters.