ChandraObservations of “The Antennae” Galaxies (NGC 4038/4039). IV. The X‐Ray Source Luminosity Function and the Nature of Ultraluminous X‐Ray Sources

We derive the X-ray luminosity function (XLF) of the X-ray source populationdetected in the Chandra observation of NGC4038/39 (the Antennae). We explicitly include photon counting and spectral parameter uncertainties inour calculations. The cumulative XLF is well represented by a flat power law($\alpha=-0.47$), similar to those describing the XLFs of other star-formingsystems (e.g. M82, the disk of M81), but different from those of early typegalaxies. This result associates the X-ray source population in the Antennaewith young High Mass X-ray Binaries. In comparison with less activelystar-forming galaxies, the XLF of the Antennae has a highly significant excessof sources with luminosities above 10^{39} erg\s (Ultra Luminous Sources;ULXs). We discuss the nature of these sources, based on the XLF and on theirgeneral spectral properties, as well as their optical counterparts discussed inPaper III. We conclude that the majority of the ULXs cannot be intermediatemass black-holes (M > 10-1000 \msun) binaries, unless they are linked to theremnants of massive Population III stars (the Madau & Rees model). Instead,their spatial and multiwavelength properties can be well explained by beamedemission as a consequence of supercritical accretion. Binaries with a neutron star or moderate mass black-hole (up to 20\msun), andB2 to A type star companions would be consistent with our data. In the beamingscenario, the XLF should exibit caracteristic breaks that will be visible infuture deeper observations of the Antennae.Comment: 15 pages, submitted to Ap