A magnetized torus for modeling Sagittarius A^∗millimeter images and spectra

Abstract

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 µas. 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 µas in the millimeter-wavelength range 0.87 mm-1.3 mm. Aims. We investigate the ability of the fully general relativistic Komissarov (2006, MNRAS, 368, 993) 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.

Astrophysical imaging of Kerr black holes with scalar hair

“…The observer is located at some fixed inclination θ ¼ 85°, where θ is the angle between the axis of rotation and the observer. This particular value of inclination is inherited from a previous study [29] where it was shown to be able to reproduce well the observed features of Sgr A Ã modeled as a Kerr BH. The observer is modeled by a screen, with every pixel corresponding to some direction of photon incidence.…”

“…This model is the same as presented in [29] and recently applied to boson stars [30]. It is based on the magnetized torus model of [41].…”

“…It is based on the magnetized torus model of [41]. We will only present this accretion model very briefly and refer to [29] for details. The accretion torus model that we consider is made of a non-self-gravitating perfect polytropic fluid circularly orbiting with constant specific angular momentum l ¼ −u φ =u t , where u is the fluid 4-velocity, which is completely fixed by imposing the constancy of l and the circular motion.…”

Testing SgrA*with the spectrum of its accretion structure

“…Following Refs. [49,50], we employ the Polish doughnut model (reviewed in the previous section) to describe the macroscopic structure of the accretion disk and a gas pressure dominated optically thin 2-temperature plasma model for the microscopic physics. In this section, we just report the results, while more details can be found in Refs.…”

“…As proposed in Refs. [49,50], such an accretion structure may be described by an optically thin and constant angular momentum ion torus within the Polish doughnut model [51,52] and a 2-temperature plasma in which the emission mechanisms are bremsstrahlung, synchrotron radiation, and inverse Compton scattering. Since the structure of the torus is determined by the spacetime geometry, the comparison between theoretical predictions with observations can potentially constrain the metric around SgrA * .…”