XMM-Newton Spectroscopy of Bright ULX Sources in the Antennae Galaxies (NGC 4038/4039)

Abstract

We report the results of spectral fits to four bright ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039) observed for 41 ksec with XMM-Newton. Although emission regions are not resolved as well as in prior Chandra observations, at least four ULXs (X-11, X-16, X-37, and X-44 in the Zezas and Fabbiano scheme) are sufficiently bright and well-separated with XMM-Newton that reliable extractions and spectral analyses are possible. We find that the single component multi-color disk blackbody models cannot describe any of the spectra. Sources X-11 and X-16 are acceptably fit with simple power-law models. A thermal bremsstrahlung model provides a better fit to the spectrum of X-44. Including a disk blackbody component to the spectrum of X-37 improves the fit and reveals an apparently cool disk (kT = 0.13 +/- 0.02 keV). This would suggest a parallel to cool disks recently found in other very luminous ULXs, which may contain intermediate mass black holes; however, the complex diffuse emission of the Antennae demands this finding be regarded cautiously.