High mass X-ray binaries in the LMC: dependence on the stellar population age and the "propeller" effect

Abstract

We study population of compact X-ray sources in the Large Magellanic Cloud using the archival data of XMM-Newton observatory. The total area of the survey is approximately 3.8 square degrees with the limiting sensitivity of about 10^{-14} erg/s/cm^2, corresponding to the luminosity of 3*10^{33} erg/s at the LMC distance. Out of 460 point sources detected in the 2--8 keV energy band,the vast majority are background CXB sources, observed through LMC. Based on the properties of the optical and near-infrared counterparts of the detected sources we identified 9 likely HMXB candidates, and 19 sources, which nature is uncertain, thus, providing a lower and upper limits on the luminosity distribution of HMXBs in the observed part of LMC. When considered globally, the bright end of this distribution is consistent within statistical and systematic uncertainties with extrapolation of the universal luminosity function of HMXBs. However, there seems to be fewer low luminosity sources, log(Lx)<35.5, than predicted. We consider the impact of the ``propeller effect'' on the HMXB luminosity distribution and show, that it can qualitatively explain the observed deficit of low luminosity sources. We found significant field-to-field variations in the number of HMXBs across the LMC, which appear to be uncorrelated with the star formation rates inferred by the FIR and Halpha emission. We suggest that these variations are caused by the dependence of the HMXB number on the age of the underlying stellar population. Using the existence of large coeval stellar aggregates in the LMC, we constrain the number of HMXBs as a function of time elapsed since the star formation event in the range of from 1-2 Myr to 10-12 Myr.