The faintest accretors

Recent X-ray observations have detected a class of very faint X-ray transients in the Galaxy which probably reveal a previously unrecognized type of accretion on to neutron stars or black holes. We show that these systems cannot have descended from binaries with stellar-mass components of normal composition. Accretion of hydrogen-depleted matter on to stellar-mass black holes can account for individual systems, but requires that these transients should be observed to repeat within a few years, and does not explain why the class is distinctly faint. Two other explanations appear to be quite natural. One invokes accretion by neutron stars or stellar-mass black holes from companions that were already brown dwarfs or planets when the systems formed, i.e. which did not descend from low-mass stars. The other possibility is that these systems are the endpoints of primordial (zero-metallicity) binaries in which the primary was extremely massive, and collapsed to a black hole of mass ≳1000 M⊙. The (primordial) companion must by now have reached an extremely low mass (≲0.01 M⊙) and be transferring mass at a very low rate to the black hole. This picture avoids the main difficulty encountered by models invoking intermediate-mass black hole formation at non-primordial metallicities, and is a natural consequence of some current ideas about Population III star formation.