Emission from isolated black holes and MACHOs accreting from the interstellar medium

Adopting the advection-dominated accretion flow (ADAF) model, we have reexamined the radiation emitted by isolated stellar-mass black holes accreting from the interstellar medium of our Galaxy. Two distinct types of black holes are given consideration: (1) remnants of conventional massive star evolution (remnant black holes [RBHs], mass similar to 10 M.) and (2) low-mass objects that possibly constitute the dark halo and cause the observed microlensing events (MACHOBHs, mass similar to 0.5 M.). In contrast to previous studies incorporating spherical accretion models, the characteristic hard X-ray emission from ADAFs serves as an important observational signature. Standard estimates for the present density of RBHs suggest that a fair number of objects, either in nearby molecular clouds (such as Orion) or in the general Galactic disk, may be detected by near-future X-ray instruments. Their observation may also be feasible at lower frequencies, particularly in the infrared and optical bands. On the other hand, the smaller mass and larger velocity on average of MACHOBHs compared with RBHs make them unlikely to be observable in the foreseeable future.