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 0.1M. Various arguments suggest that brown dwarfs should have a coronal X-ray emission of 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 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.
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