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

Abstract

The Galactic center black hole candidate Sgr A* is the best target for studies of lowluminosity accretion physics, including with near-infrared and submillimeter wavelength long baseline interferometry experiments. Here we compare images and spectra generated from a parameter survey of general relativistic MHD simulations to a set of radio to near-infrared observations of Sgr A*. Our models span the limits of weak and strong magnetization and use a range of sub-grid prescriptions for electron heating. We find two classes of scenarios can explain the broad shape of the submillimeter spectral peak and the highly variable near-infrared flaring emission. Weakly magnetized "disk-jet" models where most of the emission is produced near the jet wall, consistent with past work, as well as strongly magnetized (magnetically arrested disk) models where hot electrons are present everywhere. Disk-jet models are strongly depolarized at submillimeter wavelengths as a result of strong Faraday rotation, inconsistent with observations of Sgr A*. We instead favor the strongly magnetized models, which provide a good description of the median and highly variable linear polarization signal. The same models can also explain the observed mean Faraday rotation measure and potentially the polarization signals seen recently in Sgr A* near-infrared flares.

The Jet–disk Boundary Layer in Black Hole Accretion

“…Each simulation was run for at least 20,000 GM c −3 and has an initial transient phase during which the initial torus relaxes, and magnetic winding and the Rayleigh-Taylor instability and the KHI operate. The transient phase is followed at each radius by a turbulent quasi-equilibrium, with equilibrium radius, defined as the largest radius where   dM dr 0, increasing as r eq ∼ t 2/3 (see, e.g., Penna et al 2010;Dexter et al 2020 for a discussion). Beyond r eq , the flow is strongly dependent on initial conditions, so we consider information only from r < r eq .…”

Constraining particle acceleration in Sgr A⋆with simultaneous GRAVITY,Spitzer,NuSTAR, andChandraobservations

“…Typically discussed candidate mechanisms are either electron acceleration through magnetic reconnection, turbulent heating in shocks induced by a misalignment of BH spin and accretion flow or in shocks along an outflow/jet (Dodds-Eden et al 2009;Dexter & Fragile 2012). Large-scale simulations of the accretion flow do not have the resolution to trace individual reconnection events, but several strategies have been developed to try to account for this (Dexter et al 2020;Chatterjee et al 2021). Particles in cell simulations of plasmas show that turbulence heating and magnetic reconnection can create significantly nonthermal, power-law electron distributions (Sironi & Beloborodov 2020;Wong et al 2020;Werner & Uzdensky 2021).…”

GRMHD Simulations and Modeling for Jet Formation and Acceleration Region in AGNs

“…GRMHD simulations in the MAD regime exhibit violent episodes of flux escape from the black hole magnetosphere (e.g., [116,117]). These magnetic flux eruptions could explain the flare events observed in Sgr A* because they are associated with magnetic reconnection, which provides particle heating and acceleration during the flare events and contains enough energy to power flares.…”

“…A more self-consistent approach for obtaining electron temperature from GRMHD simulations involves coupling with electron thermodynamics, where one evolves an electron entropy equation which takes into account local sub-grid electron heating [116,147,149,[192][193][194][195]. In this approach, the back reaction of electron pressure on the dynamics of the accretion flow is neglected (see [192]).…”