Recent Progress in Observational Identifications of Stellar Mass Black Holes

After exhausting their thermal nuclear energy,stars with different masses may eventually collapse to form different types of compact objects,such as white dwarfs,neutron stars,or black holes.Since the 1930s,observations and identifications of black holes have remained active research frontiers in astrophysics.In this article we first briefly overview the research history of the formation of black holes and their candidates.We then outline how to identify stellar mass black holes observationally with five criteria;these criteria satisfy the highest standards for recognizing new discoveries in physics experiments and astronomical observations.This is followed by detailed discussions on how to measure the mass and spin parameters of stellar mass black holes.We suggest that the inclination angle of the accretion disk,which may not be exactly co-aligned with the orbital plane of a black hole binary system,may be determined by the polarization measurement of the disk emission.We propose a method of observing the orbital motion of the black hole by detecting the orbital Doppler shift of the absorption lines of the accretion disk wind.Combined with the orbital Doppler shift of the emission lines of the companion star and its mass,the black hole mass and orbital inclination angle can be determined reliably.The improved measurements of the black hole mass and accretion disk inclination can in turn be used to improve the black hole spin measurement with the X-ray continuum fitting method we proposed previously.We also discuss some methods of determining the distances of X-ray binaries and some other methods of black hole spin measurements.Finally we summarize the recent progress in observations and identifications of stellar mass black holes,including the evolution of the inner disk radius when the disk luminosity changes over several orders of magnitudes.We show that outflow dominates over inflow at accretion rates exceeding about a fraction of Eddington limit,by comparing the observations of neutron star and black hole X-ray binaries.Finally we conclude that there is now sufficient evidence to claim positive identifications of stellar mass black holes in many accreting X-ray binaries.