Monte Carlo Simulation of the Compton Scattering and Disk Reflection of a Cylinder with Hot Electrons Moving Away from a Black Hole

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 BH, 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 power law spectrum. In this paper, we make simulations for 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 the 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.