2002ApJ...577..710Z

We investigate the nature of the luminous X-ray source population detected in a (72 ks) Chandra ACIS-S observation of NGC 4038/4039, 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 (Hubble Space Telescope) 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 the lack of associated radio emission, we conclude that the ultraluminous X-ray sources (ULXs), with L_X≥10³⁹ ergs.s^–1, are not young compact supernova remnants (SNRs) 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 M_☉), 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 3x10³⁸ to 10³⁹ ergs.s^–1 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 in the Chandra source extraction area. Comparison with H I 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 six sources with heavily absorbed X-ray spectra and absorption-corrected luminosities in the ULX range. We detect the nuclei of both galaxies, with luminosities in the 10³⁹ ergs.s^–1 range and soft, possibly thermal X-ray spectra.