Bright X - ray source populations in the starburst galaxies NGC 4038 / 4039

Assuming a naive star formation history, we construct the synthetic X-ray source populations for comparison with the X-ray luminosity function (XLF) of the interacting galaxies NGC 4038/4039 using a population synthesis code. We have considered highand intermediate-mass X-ray binaries, young rotation-powered pulsars and fallback disc-fed black holes in modelling the bright X-ray sources detected. To examine the effects of the input parameters on the calculated results we produce a series of hybrid models. We find that for typical binary evolution parameters, it is difficult to match the observed XLF shape, and the predicted XLFs seem to be steeper than those of starburst and late type spiral galaxies. We note that the shape of the XLFs depends critically on the existence of XLF break for young populations, which seems to be a popular feature in theoretical population synthesis works. We discuss possible reasons and implications on the discrepancy of the calculated and observational XLFs.