Abstract
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 present line-ratio diagnostics which are sensitive to the distribution of mass 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 present diagnostics which are primarily sensitive to the importance of recombination versus collisional ionization and which could help discriminate advection-dominated accretion flows from photoionization-dominated accretion disk 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 are within the detection capability of current X-ray instruments.
Highlights
Many X-ray sources in the sky, such as stellar mass black holes in X-ray binaries, and supermassive black holes in the nucleus of our Galaxy and in other galaxies, are believed to be powered by gas falling into the black hole
We have shown that high-resolution observations of line profiles have the potential to probe the dynamics of thermal gas in accretion flows around black holes
In the standard advection-dominated accretion flows (ADAFs) model, velocities are independent of the mass of the central object and, line profiles are independent of it
Summary
Many X-ray sources in the sky, such as stellar mass black holes in X-ray binaries, and supermassive black holes in the nucleus of our Galaxy and in other galaxies, are believed to be powered by gas falling into the black hole. Some line ratios are very sensitive to these aspects of the flow, and they can be used to set constraints on them This is useful for the case of SgrA∗, where both models with and without a wind are consistent with the data (but require different assumptions for the microscopic parameters of the flow; see, e.g., Quataert & Narayan 1999), and there is no evidence of an outer disk. In this case, we show that relative line intensities can be a useful, direct probe of the amount of mass lost to a wind during the process of accretion. We show that line profiles can be used to constrain the amount of rotation of the flow and, (see §2.1) learn about some of its underlying microphysics
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