Angle-dependent Compton reflection of X-rays and gamma-rays

TLDR

In Compton reflection, X‑rays and γ‑rays emitted by a source strike a slab (e.g., an accretion disc) and re‑emerge with a spectrum altered by Compton scattering and bound‑free absorption. The study derives approximations to Green’s functions for Compton reflection of X‑rays and γ‑rays by cold electrons using a Monte Carlo method. Monte Carlo simulations were employed to compute Green’s functions for Compton reflection of X‑rays and γ‑rays by cold electrons. The resulting Green’s functions vary with viewing angle, hardening in hard X‑rays and soft γ‑rays, an effect that is crucial for modelling γ‑ray spectra of active galactic nuclei and Galactic black hole candidates.

Abstract

Using a Monte Carlo method, we derive approximations to Green's functions for the Compton reflection of X-rays and γ-rays by cold electrons. In Compton reflection, X-rays and γ-rays emitted by a source impinge upon a slab (e.g., an accretion disc) and re-emerge with a spectrum altered by Compton scattering and bound-free absorption. The obtained Green's functions are dependent on the viewing angle of the reflecting slab, which extends the previous treatments of the problem, in which the reflected spectra were integrated over all viewing angles. The dependence on the viewing angle is especially important in hard X-rays and soft γ-rays, in which regime the reflected spectrum strongly hardens with increasing viewing angle. This is an important effect for modelling γ-ray spectra of active galactic nuclei and Galactic black hole candidates, where the presence of Compton reflection has been established before from X-ray data.