Coupled jet-disk model for Sagittarius A☆: explaining the flat-spectrum radio core with GRMHD simulations of jets

Context. The supermassive black hole in the center of the Milky Way, Sgr A*, displays a nearly flat radio spectrum that is typical for jets in active galactic nuclei. Indeed, time-dependent magnetized models of radiatively inefficient accretion flows (RIAFs), which are commonly used to explain the millimeter, near-infrared, and X-ray emission of Sgr A*, often also produce jet-like outflows. However, the emission from these models has so far failed to reproduce the flat radio spectrum. Aims. We investigate whether current accretion simulations can produce the compact flat spectrum emission by simply using a different prescription for the heating of the radiating particles in the jet. Methods. We studied the radiative properties of accretion flows onto a black hole produced in time-dependent general-relativistic magnetohydrodynamic (GRMHD) simulations. A crucial free parameter in these models has always been the electron temperature, and here we allowed for variations in the proton-to-electron temperature ratios in the jet and disk. Results. We found that the flat spectrum is readily reproduced by a standard GRMHD model if one has an almost isothermal jet coupled to a two-temperature accretion flow. The low-frequency radio emission comes from the outflowing sheath of matter surrounding the strongly magnetized nearly empty jet. The model is consistent with the radio sizes and spectrum of Sgr A*. Conclusions. Hence, GRMHD models of accreting black holes can in principle naturally reproduce jets that match observed characteristics. For Sgr A* the model fit to the spectrum predicts higher mass-accretion rates when a jet is included than without a jet. Hence, the impact of the recently discovered G2 cloud that is expected to be accreted onto Sgr A* might be less severe than currently thought.