X-Ray spectra from accretion disks illuminated by protons

TL;DR

This paper models the X-ray spectra from accretion disks heated by protons, revealing a heated surface layer with specific temperature profiles that resemble but differ from observed spectra, and introduces the concept of a 'warm disk' near the inner edge.

Contribution

It provides a detailed calculation of proton-heated accretion disk spectra, including the formation of a hot surface layer and the 'warm disk' region, advancing understanding of X-ray emission mechanisms.

Findings

Heated surface layer temperatures between 60-90 keV.

Spectra resemble hard state but are slightly too steep.

Inner disk regions may form a 'warm disk' with temperatures of several hundred keV.

Abstract

The X-ray spectrum from a cool accretion disk heated by virialized protons is computed. The cool disk is either embedded in a magnetically heated accretion disk corona or partly extends into an ion supported torus (or ADAF). We calculate the stationary equilibrium between proton heating, electron thermal conduction and the radiative losses by bremsstrahlung and Compton scattering. A heated surface layer on top of the accretion disk is produced with temperatures between 60--90 keV above a cool layer with temperatures of 0.01 keV (AGN) and 1keV (galactic black hole candidates). The spectra produced by the surface layer are reminiscent of hard state spectra, but a bit too steep, especially for AGN's. Near the inner edge of the disk, where the optical depth of the disk $\tau lesssim 1$, we find that the cool component of the disk disappears. Instead, the hot protons from the corona/ADAF heat the disk, on a dynamical time-scale, to temperatures of several 100 keV, limited by pair production. This region, here called a `warm disk', could contribute significantly to the hard X-ray spectra and could be important for feeding material into an ADAF.