Spectral and imaging properties of Sgr A^∗ from high-resolution 3D GRMHD simulations with radiative cooling

The candidate supermassive black hole in the Galactic Centre, Sagittarius A∗ (Sgr A^∗), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A^∗. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general-relativistic magnetohydrodynamical simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-millimetre images generated from the accretion flow around Sgr A^∗. These simulations solve the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. We find that radiative cooling plays an increasingly important role in the dynamics of the accretion flow as the accretion rate increases: the mid-plane density grows and the infalling gas is less turbulent as cooling becomes stronger. The changes in the dynamical evolution become important when the accretion rate is larger than 10⁻⁸ M☉ yr⁻¹ (≳ 10⁻⁷M. _Edd, where M. _Edd is the Eddington accretion rate). The resulting spectra in the cooled models also differ from those in the non-cooled models: the overall flux, including the peak values at the sub-mm and the far-UV, is slightly lower as a consequence of a decrease in the electron temperature. Our results suggest that radiative cooling should be carefully taken into account in modelling Sgr A^∗ and other low-luminosity active galactic nuclei that have a mass accretion rate of M. > 10⁻⁷ M.