Monte Carlo simulation of the Compton scattering and disk reflection of a cylinder with hot electrons moving away from a black hole

TL;DR

This work addresses the coexistence of weak disk reflection with strong hard X-ray emission in accreting black holes by modeling a disk-seed jet-like cylindrical corona (the disk-seed jet-scattering scenario). Seed photons are assumed to originate from a multicolor blackbody accretion disk and are Compton up-scattered in a cylindrical corona using a Geant4-based Monte Carlo code, while disk reflection is computed with the xilconv XSPEC model. The results show that increasing jet bulk velocity suppresses disk illumination and hence the reflection, while the Compton component of disk photons remains prominent at $E \gtrsim 100$ keV, yielding a robust double-hump spectrum. These findings suggest that a weak reflection does not rule out external disk seed photons and provide a natural explanation for observations like Swift J1727-1613; they also offer a path to distinguish disk-seed jet-scattering from SSC via spectral timing correlations.

Abstract

For accreting black holes (BHs), the lamp-post scenario is a simple and popular model: a hot and point-like corona is located above the black hole, irradiating the accretion disk with hard X-ray radiation, which is believed to be generated by inverse Compton scattering in the corona. Although the lamp-post model successfully explains the disk reflection component, it fails to address the origin of seed photons and the geometry of the hot corona, because this model simplistically treats the corona as a point-like source generating a cutoff powerlaw spectrum. In this paper, we make simulations on a possible physical realization of the lamp-post scenario: the shape of the scattering zone is set to be a cylinder, corresponding to the jet base near a BH. The source of seed photons in this system is assumed to be the multicolor blackbody radiation of the accretion disk. In our simulations, the Compton scattering process is simulated with a custom Monte Carlo program based on the Geant4 package and the disk reflection process is simulated with the xilconv model in XSPEC. Our simulation results have confirmed that the relativistic motion of the jet can weaken or even completely suppress the reflection of the accretion disk, and simultaneously, the Comptonization of disk photons in the jet can still make a major contribution to the observed X-ray spectrum in high energy range. We discuss the implications of our simulation results, in light of the recent observations of a very weak reflection component in the presence of a strong hard X-ray radiation from the outburst of Swift J1727.8-1613.

Figures (27)

• Figure 1: A flow chart of the Comptonization MC program.
• Figure 1: Comparisons of the energy spectra of disk and Compton scattering between disk outer radii of $200R_{\rm{in}}$ and $500R_{\rm{in}}$, in energy range $0.1-200$ keV. The left panel shows the energy spectra of direct disk radiation without Compton scattering with different outer disk radii. The right panel shows the energy spectra of pure Compton radiations with different outer disk radii.
• Figure 1: The simulated spectra at inclination angle 25°. The red lines are the thermal component emitted by the accretion disk; the yellow lines are the Compton component; the green lines are the reflection component; the blue lines are the sum of all components, i.e., the observed spectra. Panels (a) to (f) correspond to jet velocity $0 c$, $0.1 c$, $0.2 c$, $0.4 c$, $0.7 c$, $0.99 c$, respectively.
• Figure 1: The left panel shows comparisons between a cutoffpl (red) and our input spectrum (blue). The right panel shows the reflection spectra obtained by convolving both the cutoff power law spectrum and the return-to-disk photons spectrum with the xilconv model.
• Figure 1: The simulated spectra at inclination angle 35°. The red lines are the thermal component emitted by the accretion disk; the yellow lines are the Compton component; the green lines are the reflection component; the blue lines are the sum of all components, i.e., the observed spectra. Panels (a) to (f) correspond to jet velocity $0 c$, $0.1 c$, $0.2 c$, $0.4 c$, $0.7 c$, $0.99 c$, respectively.