High-mass X-ray binaries in the Small Magellanic Cloud: the luminosity function

We study a population of compact X-ray sources in the Small Magellanic Cloud (SMC) using archival data from the XMM-Newton observatory. The total area of the survey is ~1.5 deg² with the limiting sensitivity of ~10^-14 erg s^-1 cm^-2, corresponding to a luminosity of ~4.3 × 10³³ erg s^-1 at the SMC distance. Out of ~150 point sources detected in the 2-8 keV energy band, ~3/4 are cosmic X-ray background sources, observed through the SMC. Based on the properties of the optical and near-infrared counterparts of the detected sources, we identified likely high-mass X-ray binary (HMXB) candidates and sources whose nature is uncertain, thus providing lower and upper limits on the luminosity distribution of HMXBs in the observed part of the SMC.

The observed number of HMXBs is consistent with the prediction based on star formation rate (SFR) estimates derived from the supernovae frequency and an analysis of colour-magnitude diagrams for the stellar population. If, on the contrary, the true value of the SFR is better represented by far-infrared-, Hα- and ultraviolet-based estimators, then the abundance of HMXBs in the SMC may significantly (by a factor of as much as ~10) exceed the value derived for the Milky Way and other nearby galaxies. The shape of the observed distribution at the bright end is consistent with the universal HMXB X-ray luminosity function. At the faint end, L_X<~ 2 × 10³⁵ erg s^-1, the upper limit on the luminosity function is consistent with the L^-0.6 power law, while the lower limit is significantly flatter than this.