The Hard X‐Ray Luminosity of OB Star Populations: Implications for The Contribution of Star Formation to the Cosmic X‐Ray Background

We present an empirical analysis of the integrated X-ray luminosity arisingfrom populations of OB stars. In particular, we utilize results from theAll-Sky Monitor on RXTE, along with archival data from previous missions, toassess the mean integrated output of X-rays in the 2-10 keV band from accretingearly-type binaries within 3 kpc of the Sun. Using a recent OB star census ofthe Solar neighborhood, we then calculate the specific X-ray luminosity per Ostar from accretion-powered systems. We also assess the contribution to thetotal X-ray luminosity of an OB population from associated T Tauri stars,stellar winds, and supernovae. We repeat this exercise for the major LocalGroup galaxies, concluding that the total X-ray luminosity per O star spans abroad range from 2 to 20e34 erg/s. Contrary to previous results, we do not finda consistent trend with metallicity; in fact, the specific luminosities for M31and the SMC are equal, despite having metallicities which differ by an order ofmagnitude. In light of these results, we assess the fraction of the observed2-10 keV emission from starburst galaxies that arises directly from their OBstar populations, concluding that, while binaries can explain most of the hardX-ray emission in many local starbursts, a significant additional component orcomponents must be present in some systems. A discussion of the nature of thisadditional emission, along with its implications for the contribution ofstarbursts to the cosmic X-ray background, concludes our report.Comment: aastex, 30 pages including 2 tables and 1 figure. To appear in Ap