SOLITARY, WANDERING BLACK HOLES, unheralded by any bright accretion disk or rapidly orbiting stars or gas, have been detected through the process of gravitational microlensing. The Massive Compact Halo Object (MACHO) collaboration regularly views millions of stars in the direction of the dense bulge of our galaxy hoping to observe, every now and then, stars brightening courtesy of the lensing caused by the passage of some nonluminous object (hovering in the galaxy's halo) between us and the star. The brightening can last as short as two days or as long as 1000. Longer durations suggest either large or very slow lensing objects. David Bennett of Notre Dame reported at the AAS meeting on two such long-duration events in which the mass of the lens was calculated to be roughly 6 solar masses, too heavy to be a neutron star and more likely to be a black hole. Bennett speculates that the lone-wolf black holes form from supernova collapse and might be as common as neutron stars in the galaxy.
