Paper detail

High mass X-ray binaries in the SMC: the luminosity function

We study population of compact X-ray sources in the Small Magellanic Cloud using the archival data of XMM-Newton observatory. The total area of the survey is ~1.5 square degrees with the limiting sensitivity of ~10^{-14} erg/s/cm^2, corresponding to the luminosity of ~4.3\cdot 10^{33} erg/s at the SMC distance. Out of ~150 point sources detected in the 2--8 keV energy band, ~3/4 are background CXB sources, observed through the SMC. Based on the properties of the optical and near-infrared counterparts of the detected sources we identified likely HMXB candidates, and sources, whose nature is uncertain, thus, providing a 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 SFR estimates derived from the supernovae frequency and analysis of color-magnitude diagrams of the stellar population. If, on the contrary, the true value of the SFR is better represented by FIR, H_alpha and UV 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 XLF. At the faint end, L_X>2* 10^{34} erg/sec, the upper limit on the luminosity function is consistent with while the lower limit is significantly flatter than the L^{-0.6} power law.