X-ray emission from dark clusters of MACHOs

MACHOs (Massive Astrophysical Compact Halo Objects) - as discovered by microlensing experiments towards the LMC - provide a natural explanation for the galactic halo dark matter. A realistic possibility is that MACHOs are brown dwarfs of mass $\sim 0.1M_{\odot }$. Various arguments suggest that brown dwarfs should have a coronal X-ray emission of $\sim {10}^{27}$ erg s${}^{-1}$. As MACHOs are presumably clumped into dark clusters (DCs), each DC is expected to have a total X-ray luminosity of $\sim {10}^{29}-{10}^{32}$ erg s${}^{-1}$. We discuss the possibility that dark clusters contribute to the diffuse X-ray background (XRB) or show up as discrete sources in very deep field X-ray satellite observations. Moreover, from the observed diffuse XRB we infer that the amount of virialized diffuse gas present in the galactic halo can at most make up 5% of the halo dark matter.