  [abridged] We investigate the nature of the luminous X-ray source population
detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/39, the Antennae
galaxies. We derive the average X-ray spectral properties of sources in
different luminosity ranges, and we correlate the X-ray positions with radio,
IR, and optical HST data. The X-ray sources are predominantly associated with
young stellar clusters, indicating that they belong to the young stellar
population. Based on both their co-added X-ray spectrum, and on the lack of
associated radio emission, we conclude that the Ultra Luminous X-ray sources
(ULXs), with Lx>10^{39} erg\s, are not young compact Supernova Remnants (SNR),
but accretion binaries. While their spectrum is consistent with those of ULXs
studied in nearby galaxies, and interpreted as the counterparts of intermediate
mass black-holes (M >10-1000 \msun), comparison with the position of
star-clusters suggests that some of the ULXs may be runaway binaries, thus
suggesting lower-mass binary systems. The co-added spectrum of the sources in
the 3x10^{38}-10^{39} erg\s luminosity range is consistent with those of
Galactic black-hole candidates. These sources are also on average displaced
from neighboring star clusters. The softer spectrum of the less luminous
sources suggests the presence of SNRs or of hot interstellar medium (ISM) in
the Chandra source extraction area. Comparison with HI and CO observations
shows that most sources are detected in the outskirts of large concentrations
of gas. The absorbing columns inferred from these observations would indeed
absorb X-rays up to 5 keV, so there may be several hidden X-ray sources.
Associated with these obscured regions we find 6 sources with heavily absorbed
X-ray spectra and absorption-corrected luminosities in the ULX range.

Astrophysics