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 arising from populations of OB stars. In particular, we utilize results from the A ll-Sky Monitor on RXTE, along with archival data from previous missions, to assess the mean integrated output of X-rays in the 2-10 keV band from accr eting early-type binaries within 3 kpc of the Sun. Using a recent OB star c ensus of the solar neighborhood, we then calculate the specific X-ray lumin osity per O star from accretion-powered systems. We also assess the contrib ution to the total X-ray luminosity of an OB population from associated T T auri stars, stellar winds, and supernovae. We repeat this exercise for the major Local Group galaxies, concluding that the total X-ray luminosity per O star spans a broad range from 2 to 20 x 10(34) ergs s(-1). Contrary to pr evious results, we do not find a consistent trend with metallicity; in fact , the specific luminosities for M31 and the SMC are equal, despite having m etallicities that differ by an order of magnitude. In light of these result s, we assess the fraction of the observed 2-10 keV emission from starburst galaxies that arises directly from their OB star populations, concluding th at, while binaries can explain most of the hard X-ray emission in many loca l starbursts, a significant additional component or components must be pres ent in some systems. A discussion of the nature of this additional emission , along with its implications for the contribution of starbursts to the cos mic X-ray background, concludes our report.