Modelling the black hole silhouette in Sgr A* with ion tori

TL;DR

This paper presents an analytic, relativistic model of the accretion structure around Sgr A* as an ion torus, successfully reproducing key observational features of the black hole's silhouette and spectrum.

Contribution

It introduces a simple, fully analytic ion torus model with eight parameters that captures the main features of Sgr A*'s accretion environment.

Findings

The model reproduces the observed black hole silhouette.

It accounts for the main spectral features of Sgr A*.

The model's simplicity allows for efficient parameter exploration.

Abstract

We calculate the "observed at infinity" image and spectrum of the accretion structure in Sgr A*, by modelling it as an optically thin, constant angular momentum ion torus in hydrodynamic equilibrium. The physics we consider includes a two-temperature plasma, a toroidal magnetic field, as well as radiative cooling by bremsstrahlung, synchrotron and inverse Compton processes. Our relativistic model has the virtue of being fully analytic and very simple, depending only on eight tunable parameters: the black hole spin and the inclination of the spin axis to our line of sight, the torus angular momentum, the polytropic index, the magnetic to total pressure ratio, the central values of density and electron temperature and the ratio of electron to ion temperatures. The observed image and spectrum are calculated numerically using the ray-tracing code GYOTO. Our results demonstrate that the ion torus model is able to account for the main features of the accretion structure surrounding Sgr A*.