NL 82 Web.indb

Accretion onto compact stellar objects (white dwarfs, neutron stars, and black holes) from companions in close binaries is the primary beacon for study of the astrophysics of the extreme: from endpoints of stellar and binary evolution, to the physical processes in the extremes of gravity, radiation, or magnetic fi elds. Accretion from the interstellar medium on the presumed still larger population of isolated compact objects should, at least for relatively more massive and low-velocity stellar black holes, also be observable in certain conditions, yet never has been. Given the fundamental role that accretion plays in so many key problems of interest in current astrophysics, it is surprising that the space density and luminosity function(s) of the most common accretion-powered compact objects—white dwarfs accreting from low-mass stellar companions, or cataclysmic variables (CVs)—has not been measured to better than a factor of ~10 in the solar neighborhood, and they have even less well-known spatial distributions in the Galaxy.