Doosoo Yoon

The supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A∗), the only black hole for which we can directly resolve both the event horizon as well as its outer feeding conditions, serves as a unique laboratory for numerical models of accretion and outflows. It is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rates. RIAF accretion occurs when the accretion rate is Mdot << 10^-5 Mdot_Edd, where Mdot_Edd is the Eddington accretion rate, which is small enough that the cooling time of electrons is much longer than the time required for them to fall into the black hole. Consequently, radiative cooling losses have, in general, been overlooked in the numerical study of Sgr A∗. However, the radiative properties and the non-linear nature of the plasma in RIAFs are poorly understood. For instance, how radiative cooling losses affect the turbulence features of the disk, and thus the angular momentum transport of the accreting plasma? How does radiative cooling together with the effects of general relativity result in the observed spectra from Sgr A*?

In this work, using full 3D general-relativistic magneto-hydrodynamical (GRMHD) simulations, we study the impact of radiative cooling on the dynamical evolution of the accreting plasma, presenting spectral energy distributions and synthetic sub-millimeter images generated from the accretion flow around Sgr A∗. We set the direction of the jet to be aligned with the angular momentum of the accreting gas (Fig. 1). These simulations calculate the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. Fig. 2 describes the computed cooling rates for each process. Given the typical electron temperature of 10^11 K in the accretion flow, bremsstrahlung is relatively weak, while synchrotron and inverse Compton scattering are dominant up to 10 Rg, where Rg is the gravitational radius.

We find that radiative cooling plays an increasingly important role in the dynamics of the accretion flow as the accretion rate increases: the mid-plane density grows and the infalling gas is less turbulent as cooling becomes stronger (Fig. 3). The changes in the dynamical evolution become important when the accretion rate is larger than 10^-8 Msun yr^-1 (i.e., > 10^-7 Mdot_Edd). Fig. 4 shows the disk thickness, H/R, as a function of radius. It shows that as cooling becomes stronger, the disk becomes thinner. Fig. 5 shows the density-weighted radius as a function of time, which indicates how the disk spreads out through the angular momentum transport. It shows that as cooling becomes stronger, angular momentum transport is more suppressed. This is because magneto-rotational instability is reduced as a consequence of the relatively ordered magnetic field (Fig. 3).

The resulting image at 230 GHz and spectra in the cooled model also differ from that in the non-cooled model: the intensity at the brightest region in the cooled model is slightly dimmer than in the non-cooled model (Fig. 6). The overall flux including the peak values at the sub-mm and the far-UV is also slightly lower as a consequence of a decrease in the electron temperature (Fig. 7). Although the difference is not significant, our results suggest that radiative cooling needs to be taken into account in modeling Sgr A*.

Theoretical understanding of black hole Astrophysics is very timely since the upcoming interferometric observation of Sgr A* with the Event Horizon Telescope (EHT) will enable us to stand on another breakthrough, imaging the black hole shadow around Sgr A*. Our study will contribute to predicting the images and understanding the physics of the accreting flows.