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By solving the radiative transfer equations, we examine the three-dimensional radiative properties of a magnetohydrodynamic accretion flow model conforming to the observed spectrum of Sgr A* in the vicinity of the supermassive black hole at the Galactic Centre. As a result, we find that the core of the radio emission is larger than the size of the event horizon shadow and its peak location is shifted from the gravitational centre. We also find that the self-absorbed synchrotron emission arising from the superposition of thermal electrons within a few tens of the Schwartzschild radius can account for low-frequency spectra below the critical frequency . Above the critical frequency, the synchrotron self-Compton emission from thermal electrons can account for the variable emission found in recent near-infrared observations. In contrast to a previous study by Ohsuga, Kato & Mineshige, we found that the X-ray spectra arising from bremsstrahlung emission of thermal electrons for the different mass accretion rates can be consistent with both the flaring state and the quiescent state of Sgr A* observed by Chandra. [ABSTRACT FROM AUTHOR]

Spectral properties of super-Eddington accretion flows are investigated by means of a parallel line-of-sight calculation. The subjacent model, taken from the two-dimensional radiation hydrodynamic simulations by Ohsuga et al. (2005) , consists of a disc accretion region and an extended atmosphere with high-velocity outflows. The non-grey radiative transfer equation is solved, including relativistic effects, by applying the flux-limited diffusion approximation. The calculated spectrum is composed of a thermal, blackbody-like emission from the disc which depends sensitively on the inclination angle, and of high-energy X-ray and gamma-ray emission from the atmosphere. We find mild beaming effects in the thermal radiation for small inclination angles. If we compare the face-on case with the edge-on case, the average photon energy is larger by a factor of ∼1.7 due mainly to Doppler boosting, while the photon number density is larger by a factor of ∼3.7 due mainly to anisotropic matter distribution around the central black hole. This gives an explanation for the observed X-ray temperatures of ULXs which are too high to be explained in the framework of intermediate-mass black holes. While the main features of the thermal spectral component are consistent with more detailed calculations of slim accretion discs, the atmosphere induces major changes in the high-energy part, which cannot be reproduced by existing models. We also conclude that, in order to interpret the observational data properly, simple approaches like the Eddington–Barbier approximation cannot be applied. [ABSTRACT FROM AUTHOR]