General relativistic magnetohydrodynamic simulations of accretion on to Sgr A*: how important are radiative losses?

Abstract

We present general relativistic magnetohydrodynamic numerical simulations of the accretion flow around the supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*). The simulations include for the first time radiative cooling processes (synchrotron, bremsstrahlung and inverse Compton) self‐consistently in the dynamics, allowing us to test the common simplification of ignoring all cooling losses in the modelling of Sgr A*. We confirm that for Sgr A*, neglecting the cooling losses is a reasonable approximation if the Galactic Centre is accreting below ∼10−8 M⊙ yr−1, i.e. Ṁ<10−7trueṀ Edd . However, above this limit, we show that radiative losses should be taken into account as significant differences appear in the dynamics and the resulting spectra when comparing simulations with and without cooling. This limit implies that most nearby low‐luminosity active galactic nuclei are in the regime where cooling should be taken into account. We further make a parameter study of axisymmetric gas accretion around the supermassive black hole at the Galactic Centre. This approach allows us to investigate the physics of gas accretion in general, while confronting our results with the well‐studied and observed source, Sgr A*, as a test case. We confirm that the nature of the accretion flow and outflow is strongly dependent on the initial geometry of the magnetic field. For example, we find it difficult, even with very high spins, to generate powerful outflows from discs threaded with multiple, separate poloidal field loops.

General Overview of Black Hole Accretion Theory

“…Simulating photon bubbles in the presence of MRI turbulence has proved computationally challenging in the radiation pressure dominated regime due partly to the small length scales (of order the gas pressure scale height) that must be resolved, and partly by the fact that Parker instabilities in the magnetically dominated surface layers also produce significant inhomogeneity. Models of slim disks with porous outer layers and winds have recently been developed by Dotan & Shaviv (2011). It has also been suggested that accretion flows in the radiation dominated regime might be highly inhomogeneous structures that are not well-described by any of the standard accretion flow models discussed in section 2.1 (Dexter & Agol 2011).…”

Kinetic and radiative power from optically thin accretion flows

“…Numerical simulations of the transitional regime were performed, although with much simpler treatment of radiation, also by Dibi et al (2012) and Wu et al (2016).…”

Using infrared/X-ray flare statistics to probe the emission regions near the event horizon of Sgr A*

“…Genzel et al 2003;Schödel et al 2011;Shahzamanian et al 2015). Even though Sgr A * is the least luminous black hole ever observed (in Eddington-scaled units), radiative losses can still play a role in the dynamics of the system above an accretion rate of 10 −8 M yr −1 (Dibi et al 2012). The extremely low luminosity of Sgr A * is also an opportunity to observe intense and intriguing variability that is not observed in such a way in any other black hole system on comparable timescales.…”