The role of electron heating physics in images and variability of the Galactic Centre black hole Sagittarius A*

Abstract

The accretion flow around the Galactic Center black hole Sagittarius A* (Sgr\nA*) is expected to have an electron temperature that is distinct from the ion\ntemperature, due to weak Coulomb coupling in the low-density plasma. We present\nfour two-temperature general relativistic radiative magnetohydrodynamic\n(GRRMHD) simulations of Sgr A* performed with the code KORAL. These simulations\nuse different electron heating prescriptions, motivated by different models of\nthe underlying plasma microphysics. We compare the Landau-damped turbulent\ncascade model used in previous work with a new prescription we introduce based\non the results of particle-in-cell simulations of magnetic reconnection. With\nthe turbulent heating model, electrons are preferentially heated in the polar\noutflow, whereas with the reconnection model electrons are heated by nearly the\nsame fraction everywhere in the accretion flow. The spectra of the two models\nare similar around the submillimetre synchrotron peak, but the models heated by\nmagnetic reconnection produce variability more consistent with the level\nobserved from Sgr A*. All models produce 230~GHz images with distinct black\nhole shadows which are consistent with the image size measured by the Event\nHorizon Telescope, but only the turbulent heating produces an anisotropic\n`disc-jet' structure where the image is dominated by a polar outflow or jet at\nfrequencies below the synchrotron peak. None of our models can reproduce the\nobserved radio spectral slope, the large near-infrared and X-ray flares, or the\nnear-infrared spectral index, all of which suggest non-thermal electrons are\nneeded to fully explain the emission from Sgr A*.\n