High mass X-ray binaries in the LMC: Dependence on the stellar population age and the “propeller” effect

¹ Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia e-mail: pav_sht@hea.iki.rssi.ru
² Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany

Received: 12 April 2004
Accepted: 18 October 2004

Abstract

We study the population of compact X-ray sources in the Large Magellanic Cloud using the archival data of the XMM-Newton observatory. The total area of the survey is 3.8 square degrees with a limiting sensitivity of ≈10^-14 erg/s/cm², corresponding to a luminosity of ≈ 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 the 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, whose nature is uncertain, thus providing 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, , 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 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 τ elapsed since the star formation event in the range of τ from ~ Myr to ~ Myr.