ChandraMonitoring Observations of the Antennae Galaxies. II. X‐Ray Luminosity Functions

Abstract

We present the X-ray luminosity functions (XLFs) of the X-ray source population detected in the Chandra monitoring observations of NGC 4038/4039 (the Antennae). The seven individual XLFs are well described by a flat power law with a cumulative slope α ~ 0.5-0.8. A similar slope (α = 0.48) is measured for the sources detected in the co-added observation, which reaches a limiting luminosity of ~1037 erg s-1. In our analysis we account for observational biases by deriving incompleteness functions and including them in the fitting process. We do not detect significant variations between the shape of the XLF of the seven observations. The two shorter exposures appear to have steeper XLFs, but these are still consistent with the other observations. These results indicate that the XLFs of star-forming galaxies are indeed flatter than those of more evolved stellar populations, even down to the typical luminosities of X-ray binaries. Based on this, as well as the X-ray variability and spectral properties of the X-ray sources, we suggest that the observed population down to our detection limit consists predominantly of X-ray binaries accreting close to their Eddington limit, similar to the high or very high states of Galactic X-ray binaries. In the case of ultraluminous X-ray sources (LX > 1039 erg s-1), we cannot rule out the contribution of a beamed component (because of either mechanical focusing or Doppler boosting) in their observed emission. However, even without beaming, we estimate that the maximum observed luminosity (LX ~ 1040 erg s-1) could be produced by a ~ 80 M☉ black hole accreting at its Eddington limit; such black holes can be the result of regular stellar evolution of double stellar systems.