Monte Carlo simulations of global Compton cooling in inner regions of hot accretion flows

TL;DR

This study uses advanced general relativistic Monte Carlo simulations to analyze global Compton cooling in hot accretion flows, revealing significant impacts on electron temperature, radiation spectrum, and the importance of relativistic effects for accurate modeling.

Contribution

It extends previous models by providing self-consistent global solutions of Compton cooling in hot accretion flows using relativistic Monte Carlo methods.

Findings

Cooling rate and electron temperature are significantly reduced at radii >=10 Rg.

The radiation spectrum shape remains similar, but the high-energy cut-off is lower and luminosity decreases.

Global Compton scattering effects are crucial for matching observed spectra of black-hole binaries.

Abstract

Hot accretion flows such as advection-dominated accretion flows are generally optically thin in the radial direction. Thus photons generated at some radii can cool or heat electrons at other radii via Compton scattering. Such global Compton scattering has previously been shown to be important for the dynamics of accretion flows. Here, we extend previous treatments of this problem by using accurate global general relativistic Monte Carlo simulations. We focus on an inner region of the accretion flow (R < 600R_g), for which we obtain a global self-consistent solution. As compared to the initial, not self-consistent solution, the final solution has both the cooling rate and the electron temperature significantly reduced at radii >=10 gravitational radii. On the other hand, the radiation spectrum of the self-consistent solution has the shape similar to that of the initial iteration, except for the high-energy cut-off being at an energy lower by a factor of ~2 and the bolometric luminosity decreased by a factor of ~2. We also compare the global Compton scattering model with local models in spherical and slab geometry. We find that the slab model approximates the global model significantly better than the spherical one. Still, neither local model gives a good approximation to the radial profile of the cooling rate, and the differences can be up to two orders of magnitude. The local slab model underestimates the cooling rate at outer regions whereas it overestimates that rate at inner regions. We compare our modelling results to observed hard-state spectra of black-hole binaries and find an overall good agreement provided any disc outflow is weak. We find that general-relativistic effects in flows which dynamics is modified by global Comptonization is crucial in approaching this agreement.