Pair-Rich Corona of an Accreting Kerr Black Hole

TL;DR

This paper presents a self-consistent model of a warm scattering corona around an accreting Kerr black hole, incorporating relativistic effects, pair creation, and polarization, matching observed X-ray spectra.

Contribution

It introduces an iterative Monte Carlo method that models Compton scattering, pair creation, and relativistic effects in the corona of a Kerr black hole.

Findings

The model's spectra fit binary black hole data in the hardest spectral state.

Polarization degree reaches 4-10% in the 2-8 keV band.

Pair creation significantly increases charge density near the black hole.

Abstract

We build a self-consistent model of a warm scattering corona near an accreting black hole in Kerr geometry, in the regime of slow ($\sim 0.01$ Eddington) mass accretion. An iterative Monte Carlo procedure is developed that incorporates self-consistently the effects of Compton scattering and electron-positron pair creation, as well as general relativistic lensing and frame dragging effects. Soft thermal photons are seeded in the inner disk and the velocity dispersion of the electrons and positrons adjusted to yield a fixed seed luminosity amplification through Compton scattering. A simple kinematic prescription is also added for bulk outflow. Pair creation by photon collisions raises significantly the density of scattering charges in and around the innermost ion disk, which is assumed to be geometrically thick and rarefied compared with the disk outside 10 gravitational radii. The self-consistent pair cloud is concentrated closer to the BH. The spectrum and polarization of the escaping X-rays are recorded as a function of the observer's orientation. The temperature and Compton parameter measured from the output spectra using the compPS package are consistent with fits to binary BH data in the hardest spectral state; the polarization degree rises to $4-10\%$ through the 2-8 keV band with allowance for $e^\pm$ upflow from the BH equator.