A magnetized torus for modeling Sgr A* millimeter images and spectra
  Context. The supermassive black hole, Sagittarius (Sgr) A*, in the centre of
our Galaxy has the largest angular size in the sky among all astrophysical
black holes. Its shadow, assuming no rotation, spans ~ 50 microarcsec.
Resolving such dimensions has long been out of reach for astronomical
instruments until a new generation of interferometers being operational during
this decade. Of particular interest is the Event Horizon Telescope (EHT) with
resolution ~ 20 microarcsec in the millimeter-wavelength range 0.87 mm - 1.3
mm. Aims. We investigate the ability of the fully general relativistic
Komissarov (2006) analytical magnetized torus model to account for observable
constraints at Sgr A* in the centimeter and millimeter domains. The impact of
the magnetic field geometry on the observables is also studied. Methods. We
calculate ray-traced centimeter- and millimeter-wavelength synchrotron spectra
and images of a magnetized accretion torus surrounding the central black hole
in Sgr A*. We assume stationarity, axial symmetry, constant specific angular
momentum and polytropic equation of state. A hybrid population of thermal and
non-thermal electrons is considered. Results. We show that the torus model is
capable of reproducing spectral constraints in the millimeter domain, and in
particular in the observable domain of the EHT. However, the torus model is not
yet able to fit the centimeter spectrum. 1.3 mm images at high inclinations are
in agreement with observable constraints. Conclusions. The ability of the torus
model to account for observations of Sgr A* in the millimeter domain is
interesting in the perspective of the future EHT. Such an analytical model
allows very fast computations. It will thus be a suitable test bed for
investigating large domains of physical parameters, as well as non-black-hole
compact object candidates and alternative theories of gravity.
