Spectral and Imaging properties of Sgr A* from High-Resolution 3D GRMHD Simulations with Radiative Cooling (2009.14227v1)

Abstract: The candidate supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*), is known to be fed by a radiatively inefficient accretion flow (RIAF), inferred by its low accretion rate. Consequently, radiative cooling has in general been overlooked in the study of Sgr A*. However, the radiative properties of the plasma in RIAFs are poorly understood. In this work, using full 3D general-relativistic magneto-hydrodynamical 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*. These simulations solve the approximated equations for radiative cooling processes self-consistently, including synchrotron, bremsstrahlung, and inverse Compton processes. 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. The changes in the dynamical evolution become important when the accretion rate is larger than $10^{-8}\,M_{\odot}~{\rm yr}^{-1}$ ($\gtrsim 10^{-7} \dot{M}{\rm Edd}$, where $\dot{M}{\rm Edd}$ is the Eddington accretion rate). The resulting spectra in the cooled models also differ from those in the non-cooled models: the overall flux, including the peak values at the sub-mm and the far-UV, is slightly lower as a consequence of a decrease in the electron temperature. Our results suggest that radiative cooling should be carefully taken into account in modelling Sgr A* and other low-luminosity active galactic nuclei that have a mass accretion rate of $\dot{M} > 10^{-7}\,\dot{M}_{\rm Edd}$.

• The paper demonstrates that radiative cooling significantly alters accretion dynamics in Sgr A*, reducing electron temperatures and stabilizing magnetic fields.
• The paper employs GPU-accelerated 3D GRMHD simulations with the H-AMR code to generate synthetic spectra and sub-mm images, enabling direct comparison with observations.
• The paper indicates that incorporating radiative cooling refines models of low-luminosity AGNs and guides future research on SMBH accretion physics.

Spectral and Imaging Properties of Sagittarius A* from High-Resolution 3D GRMHD Simulations with Radiative Cooling

The paper "Spectral and Imaging properties of Sagittarius A* from High-Resolution 3D GRMHD Simulations with Radiative Cooling" by Yoon et al. provides a comprehensive analysis of the accretion flows around the supermassive black hole (SMBH) at the Galactic Center, commonly referred to as Sagittarius A* (Sgr A*). Utilizing high-resolution 3D general-relativistic magneto-hydrodynamics (GRMHD) simulations with incorporated radiative cooling, this research offers insights into the influence that radiative processes exert on the dynamics, spectra, and imaging of Sgr A*.

Core Objectives and Methodology

The principal aim of the research is to evaluate the role of radiative cooling in the dynamics of accretion flows and the accompanying emission spectra. Prior studies largely overlooked radiative cooling, assuming it to be negligible for radiatively inefficient accretion flows (RIAFs) which characterize the low-luminosity environments of Sgr A*. The authors challenge this assumption by conducting GRMHD simulations that include detailed radiative cooling processes, such as synchrotron radiation, bremsstrahlung, and inverse Compton scattering.

To achieve this, they leverage the H-AMR code, optimized with GPU acceleration and sophisticated numerical techniques, enabling high-fidelity simulations of the highly magnetized and relativistic flows surrounding Sgr A*. The spectral energy distribution (SED) and synthetic sub-millimeter images are generated to facilitate a direct comparison with observational data, notably from the Event Horizon Telescope.

Key Findings

Several significant findings emerge from the simulations:

• Impact of Radiative Cooling: The paper reveals that radiative cooling significantly alters the dynamics of accretion flows at higher accretion rates, particularly above $10^{-8} M_{\odot} \text{yr}^{-1}$. This includes increased mid-plane density and reduced turbulence due to cooling, which stabilizes magnetic fields and hinders angular momentum transport via Magneto-Rotational Instability (MRI).
• Spectral Differences: Simulations incorporating radiative cooling display distinct spectral variations compared to those that do not. The cooled models exhibit reduced overall flux and a decrease in electron temperature, affecting both the sub-mm and far-UV spectral peaks.
• Effects on Imaging: The observational appearance of Sgr A* at sub-mm wavelengths, critical for imaging by facilities like the Event Horizon Telescope, is affected by cooling. The paper's GRRT-generated images highlight the variation in brightness and structure, offering predictions for observable signatures of cooling effects.

Implications and Future Directions

These findings underscore the necessity of incorporating radiative cooling in models of Sgr A* and similar low-luminosity active galactic nuclei (AGNs) to accurately predict observable properties and improve our understanding of accretion physics in such environments. The implications extend further to other SMBHs where higher accretion rates may make such effects even more pronounced.

The paper opens up several avenues for future research:

• Broader Parameter Space: Future simulations should explore a wider range of parameters, including different black hole spins, magnetic field configurations, and electron temperature prescriptions.
• Advanced Electron Models: Incorporating non-thermal electron populations may provide better alignment with observed spectral features, particularly in explaining flaring phenomena that are frequently observed in Sgr A*.
• Turbulence and Variability: Detailed exploration of the interplay between turbulence, magnetic fields, and radiative cooling could yield deeper insights into the variability patterns observed across different wavelengths.
• Comparison with Observational Data: Continued comparisons with increasingly detailed observational data will refine models and help bridge theoretical predictions with empirical findings.

Conclusion

The research presents a nuanced view of the accretion processes and radiative phenomena in Sgr A*, demonstrating the critical role of radiative cooling in shaping both spectral and imaging properties. Such insights are pivotal as burgeoning observational techniques hone in on resolving the intricate environments surrounding SMBHs, providing clearer windows into their complex and dynamic behaviors.