Spectral properties of Three-dimensional Magneto-hydrodynamical Accretion Flows

In spite of a large number of global three-dimensional (3D) magneto-hydrodynamical (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 spectral energy distribution (SED) of Sagittarius A* (Sgr A*), for the first time we calculate the emergent spectra from 3D MHD flows in a wide range of wavelengths (from radio to X-ray) by solving the 3D radiative transfer equations. We use the simulation data by Kato, Mineshige, and Shibata (2004) 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 3D MHD flow generally over-produces X-rays by means of bremsstrahlung radiation from the regions at large radii. A flatter density profile, r^{-a} with a<1, than that of the advection-dominated accretion flow (ADAF), r^{-3/2}, is the main reason for this. If we restrict the size of the emission region to be as small as 10r_s, where r_s 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 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.