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D.J. Helfand (Columbia University and University of Cambridge), E.C. Moran (University of California, Berkeley)
Early-type stars are themselves modest X-ray emitters, with a mean output in the 0.1-10 keV band roughly 10-7 of their optical/UV flux. Thus, even a population of a million OB stars, typical of that found in a galaxy undergoing a vigorous starburst, will only produce an X-ray luminosity of ~1038 ergs s-1. A single young neutron star similar to that in the Crab Nebula or a single O-star binary with a neutron star or black hole companion will thus outshine the entire population of main sequence stars. In order to determine the expected contribution of a young stellar population to the X-ray luminosity of a galaxy, then, it is necssary to estimate accurately the specific X-ray luminosity per O star, most of which comes from the deceased segment of the population. We have calculated the mean X-ray luminosity of over fifty High Mass X-ray Binaries (HMXBs) in the solar neighborhood using a variety of archival data as well as the All-Sky Monitor data from RXTE; similar censuses have been compiled for other members of the Local Group. We also calculate the expected contribution from thermal and nonthermal supernova remnants produced by a steady-state population of OB stars. We compare these results with the 2-10 keV X-ray luminosities for several starburst galaxies observed by ASCA, and conclude that a population of HMXBs is insufficient to account for the observed luminosities of most systems. We go on to explore the consequences of this result for the contribution star formation can make to the cosmic X-ray background in the 5-50 keV band.
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