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

O. Straub¹, F. H. Vincent^2,3, M. A. Abramowicz^1,4,5, E. Gourgoulhon³ and T. Paumard²

¹ Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
e-mail: odele@camk.edu.pl
² LESIA, CNRS UMR 8109, Observatoire de Paris, Université Pierre et Marie Curie, Université Paris Diderot, 92190 Meudon, France
³ LUTH, CNRS UMR 8102, Observatoire de Paris, Université Paris Diderot, 92190 Meudon, France
⁴ Department of Physics, Göteborg University, 412-96 Göteborg, Sweden
⁵ Institute of Physics, Silesian University, 746-01 Opava, Czech Republic

Received: 12 March 2012
Accepted: 16 May 2012

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*.