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

A parameter survey of Sgr A* radiative models from GRMHD simulations with self-consistent electron heating

“…Other combinations underproduce or overproduce hot electrons for the black hole spin and inclination angles tried. The SANE/turbulence models have been proposed previously and result in a "diskjet" morphology where jet emission becomes prominent at longer 3mm and 7mm wavelengths (Mościbrodzka et al 2014;Ressler et al 2017;Chael et al 2018).…”

“…Ressler et al (2015) introduced the method used here for evolving multiple electron temperatures with sub-grid heating prescriptions. This method has been used in recent work on Sgr A* (Ressler et al 2017;Chael et al 2018) and radiation GRMHD models of M87 (Ryan et al 2018;Chael et al 2019). Compared to those studies, we have considered a larger parameter space with multiple electron heating models, varying black hole spin, and considering both MAD and SANE solutions.…”

Multimessenger tests of cosmic-ray acceleration in radiatively inefficient accretion flows

“…We assume pure proton composition for simplicity. The magnetic field strength in the hot accretion flows depends on the configuration of the magnetic field: β ∼ 10 − 100 for standard and normal evolution (SANE) flows, whereas β ∼ 1 − 10 for magnetically arrested disks (e.g., [95,97,100,101]). We use β ∼ 3.2 as a reference value because lower β plasma are suitable for producing non-thermal particles [102].…”

On the comparison of AGN with GRMHD simulations: I. Sgr A*

“…This may be part of the reason why two-temperature GRMHD simulations of Sgr A* have thus far under-produced the low frequency radio emission of Sgr A*(Ressler et al 2017;Chael et al 2018), as that emission is often assumed to be powered by the magnetized outflow. Alternatively, a small population of non-thermal particles can also explain the ∼ flat low frequency radio spectrum(Özel et al 2000;Yuan et al 2002).…”