Black Hole Candidates

Abstract

The state of the searches for isolated black holes, non-accreting black holes in binary systems and, finally, the accreting black holes in the X-ray binaries is presented. The third category is, by far, the most important source of information about the Galactic black holes and it is the main topic of this review. Different proposed signatures of an accreting black hole are reviewed (including the growing evidence for the presence of the event horizon in some systems). The list of 51 black hole candidates compiled with the help of these signatures is presented. To obtain a conclusive evidence that the candidate is really a black hole, one needs a dynamical mass estimate indicating that the mass of the compact object is greater than about 3 solar masses. The observational methods of obtaining such estimates are briefly discussed. The application of these methods leads to the list of 18 objects that could be considered confirmed black holes. Their estimated masses are found to fall in the range 3.5 to 16 solar masses. Finally, some general remarks concerning relation of black hole candidates to different classes of X-ray binaries such as X-ray Novae, microquasars and high/low mass X-ray binaries are presented. The implications of the properties of black hole binaries for the theory of stellar evolution are also discussed.