Based on the recently completed Chandra /ACIS survey of X-ray point sources in nearby galaxies , we study the X-ray luminosity functions ( XLFs ) for X-ray point sources in different types of galaxies and the statistical properties of ultraluminous X-ray sources ( ULXs ) .
Uniform procedures are developed to compute the detection threshold , to estimate the foreground/background contamination , and to calculate the XLFs for individual galaxies and groups of galaxies , resulting in an XLF library for 343 galaxies of different types .
With the large number of surveyed galaxies , we have studied the XLFs and ULX properties across different host galaxy types , and confirm with good statistics that the XLF slope flattens from lenticular ( \alpha \sim 1.50 \pm 0.07 ) to elliptical ( \sim 1.21 \pm 0.02 ) , to spirals ( \sim 0.80 \pm 0.02 ) , to peculiars ( \sim 0.55 \pm 0.30 ) , and to irregulars ( \sim 0.26 \pm 0.10 ) .
The XLF break dividing the neutron star and black hole binaries is also confirmed , albeit at quite different break luminosities for different types of galaxies .
A radial dependency is found for ellipticals , with a flatter XLF slope for sources located between D _ { 25 } and 2 D _ { 25 } , suggesting the XLF slopes in the outer region of early-type galaxies are dominated by low-mass X-ray binaries in globular clusters .
This study shows that the ULX rate in early-type galaxies is 0.24 \pm 0.05 ULXs per surveyed galaxy , on a 5 \sigma confidence level .
The XLF for ULXs in late-type galaxies extends smoothly until it drops abruptly around 4 \times 10 ^ { 40 } erg s ^ { -1 } , and this break may suggest a mild boundary between the stellar black hole population possibly including 30 M _ { \odot } black holes with super-Eddington radiation and intermediate mass black holes .