GRMHD simulations of accretion onto Sgr A*: How important are radiative losses?

TL;DR

This study uses GRMHD simulations including radiative cooling to assess its impact on accretion flows around Sgr A*, revealing that cooling effects are significant above certain accretion rates and depend on magnetic field geometry.

Contribution

First self-consistent GRMHD simulations incorporating radiative cooling processes for Sgr A*'s accretion flow, highlighting the importance of cooling at higher accretion rates and the influence of magnetic field geometry.

Findings

Cooling losses are negligible below ~10^{-8} Msun/yr.

Radiative cooling significantly affects dynamics and spectra above this accretion rate.

Magnetic field geometry critically influences outflow strength and structure.

Abstract

We present general relativistic magnetohydrodynamic (GRMHD) 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 modeling 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} Msun/yr i.e. Mdot < 10^{-7} Mdot_Edd. But 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.