Spectral Properties of Three‐dimensional Magnetohydrodynamic Accretion Flows

Abstract

In spite of a large number of global three-dimensional MHD simulations of accretion flows and jets being made recently, their astrophysical relevance for realistic situations is not well known. In order to examine to what extent the simulated MHD flows can account for the observed SED of Sgr A*, for the first time we calculate the emergent spectra from three-dimensional MHD flows in a wide range of wavelengths (from radio to X-ray) by solving the three-dimensional radiative transfer equations. We use the simulation data by Kato and coworkers and perform Monte Carlo radiative transfer simulations, in which synchrotron emission/absorption, free-free emission/absorption, and Compton/inverse Compton scattering are taken into account. We assume two-temperature plasmas and calculate electron temperatures by solving the electron energy equation. Only thermal electrons are considered. It is found that the three-dimensional MHD flow generally overproduces X-rays by means of bremsstrahlung radiation from the regions at large radii. A flatter density profile, ρ ∝ r-a with a < 1, than that of the ADAF, ρ ∝ r-3/2, is the main reason for this. If we restrict the size of the emission region to be as small as ~10rS, where rS is the Schwarzschild radius, the MHD model can reproduce the basic features of the observed SED of Sgr A* during its flaring state. Yet, the spectrum in the quiescent state remains to be understood. We discuss how to resolve this issue in the context of MHD flow models. Possibilities include modifications of the MHD flow structure by the inclusion of radiative cooling and/or significant contributions by nonthermal electrons. It is also possible that the present spectral results may be influenced by particular initial conditions. We also calculate the time-dependent spectral changes, finding that the fluxes fluctuate in a wide range of the frequency and the flux at each wavelength does not always vary coherently.