X-ray line diagnostics of hot accretion flows around black holes

We compute X-ray emission Lines from thermal plasma in hot accretion flows. We show that line profiles are strong probes of the gas dynamics, and we p resent line-ratio diagnostics which are sensitive to the distribution of ma ss with temperature in the flow. We show how these can be used to constrain the run of density with radius and the size of the hot region. We also pre sent diagnostics which are primarily sensitive to the importance of recombi nation versus collisional ionization and which could help discriminate adve ction-dominated accretion flows from photoionization-dominated accretion di sk coronae. We apply our results to the Galactic center source Sagittarius A* and to the nucleus of M87. We find that the brightest predicted lines ar e within the detection capability of current X-ray instruments.