Stellar contributors to the hard X-ray background?

We use simple energetic arguments to estimate the contribution of massive X -ray binaries and supernova remnants to the cosmic X-ray background (XRB) a t energies in excess of 2 keV. Recent surveys have shown that active galact ic nuclei (AGN) probably account for most of the hard XRB (E >2 keV), but t here have been many suggestions that star-forming galaxies could emerge at fainter fluxes and perhaps account for a significant fraction of the soft a nd hard X-ray energy density. Assuming that the formation rate of massive X -ray binaries (MXRBs) traces the global star-formation rate, we find that t heir integrated contribution to the hard XRB can be estimated and is shown to be small (at less than the 1 per cent level). Similarly, the integrated flux of supernovae (SN) is also shown to be insignificant, or at most compa rable to MXRBs. AGN therefore remain the most viable candidates for produci ng the hard XRB, unless additional processes can be shown to dominate the g lobal hard X-ray emission in distant starburst galaxies.