THE POWER OF IMAGING: CONSTRAINING THE PLASMA PROPERTIES OF GRMHD SIMULATIONS USING EHT OBSERVATIONS OF Sgr A*

Abstract

Recent advances in general relativistic magnetohydrodynamic simulations have expanded and improved our understanding of the dynamics of black-hole accretion disks. However, current simulations do not capture the thermodynamics of electrons in the low density accreting plasma. This poses a significant challenge in predicting accretion flow images and spectra from first principles. Because of this, simplified emission models have often been used, with widely different configurations (e.g., disk-versus jet-dominated emission), and were able to account for the observed spectral properties of accreting black holes. Exploring the large parameter space introduced by such models, however, requires significant computational power that exceeds conventional computational facilities. In this paper, we use GRay, a fast graphics processing unit (GPU) based ray-tracing algorithm, on the GPU cluster El Gato, to compute images and spectra for a set of six general relativistic magnetohydrodynamic simulations with different magnetic field configurations and black-hole spins. We also employ two different parametric models for the plasma thermodynamics in each of the simulations. We show that, if only the spectral properties of Sgr A * are used, all 12 models tested here can fit the spectra equally well. However, when combined with the measurement of the image size of the emission using the Event Horizon Telescope, current observations rule out all models with strong funnel emission, because the funnels are typically very extended. Our study shows that images of accretion flows with horizon-scale resolution offer a powerful tool in understanding accretion flows around black holes and their thermodynamic properties.

Jet Signatures in the Spectra of Accreting Black Holes

“…The values of these ratios depend on poorly understood electron thermodynamics. However, assuming that (i) the dissipation of turbulence mainly heats the protons, (ii) the cooling time for the electrons is shorter than that of the protons, and (iii) the electron cooling time is shorter than the timescale for significant energy exchange between the electrons and protons, we expect these temperature ratios to be greater than unity (Yuan & Narayan 2014;Chan et al 2015a). Furthermore, because of the similarities between AGN and the low/hard state in XRBs, we assume that the physics of electron heating and cooling is the same across these systems.…”

“…Furthermore, because of the similarities between AGN and the low/hard state in XRBs, we assume that the physics of electron heating and cooling is the same across these systems. We therefore choose a range of values of d  and j  motivated by fitting to Sgr A * and M87, since these are the only sources whose spectra have been fitted to constrain these parameters Mościbrodzka et al 2014;Mościbrodzka & Falcke 2013;Chan et al 2015a;Moscibrodzka et al 2015).…”

“…The disadvantage of this approach is that it is only applicable in regimes in which the radiation is dynamically unimportant. These codes have been used by many authors to calculate the observational signatures of low luminosity systems in which the radiation pressure can be neglected (e.g., Chan et al 2009Chan et al , 2015aChan et al , 2015bMościbrodzka et al 2009;Mościbrodzka et al 2014;Shcherbakov et al 2012;Mościbrodzka & Falcke 2013;Shcherbakov & McKinney 2013). These works mainly focussed on reproducing the spectra and variability properties of Sgr A * , and place important constraints on quantities such as the black hole spin, proton-to-electron temperature ratio, and inclination angle.…”

Variations in emission from episodic plasmoid ejecta around black holes

“…Tracking and resolving magnetic reconnection events and their subsequent flaring within GRMHD simulations (e.g. De Villiers et al 2003;Mościbrodzka et al 2014;Chan et al 2015) will naturally provide a more realistic dynamical model for the plasmoid motion and ejection. Nevertheless, self-consistently coupling such simulations with radiative transfer calculations is beyond the scope of this work and it is more appropriate to leave it for future studies.…”

Modeling Polarized Emission from Black Hole Jets: Application to M87 Core Jet

“…Until recently, the least understood part of these models was a proper treatment of radiating electrons. Finally, however, this issue has been addressed [13][14][15][16][17][18][19]. With new and more detailed observations, we are beginning to directly compare the numerical simulations of accretion flows to real astronomical systems such as Galactic center Sgr A* and the core of M87 galaxy [20].…”