X-ray spectra from accretion disks illuminated by protons

Abstract

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 1 keV (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 τ . 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.

Excitation of low-frequency QPOs in black hole accretion

“…In addition to this purely radiative interaction, the hot flow also heats the cool disk by "proton illumination" (Deufel et al 2001;Deufel et al 2002;. In this process, part of the ISAF condenses onto the disk in the overlap region.…”

The transition from a cool disk to an ion supported flow

“…A fully relativistic treatment of the Coulomb interactions in a plasma has been given by Stepney & Guilbert (1983). We have compared the classical treatment according to Spitzer's theory with this relativistic result in Deufel et al (2001Deufel et al ( , 2002, and found it to be accurate to better than 5% for proton temperatures <100 MeV. The classical approximation in Spitzer's analysis therefore does not introduce a significant error for the problem considered here.…”

“…Radiative coupling was investigated by Haardt & Maraschi (1991, 1993 in the context of their "twophase model" for the hard X-ray spectra of accreting black holes. Explanation of these spectra as resulting from the coupling between an ISAF and a cool disk via hot the protons was proposed by Spruit (1997) and Spruit & Haardt (2000), and studied in greater detail by Deufel & Spruit (2000) (henceforth Paper I) and Deufel et al (2002) (henceforth Paper II). The physics of this process has been studied before in the context of accretion onto a neutron star surface by Zel'dovich & Shakura (1969), Alme & Wilson (1973), Deufel et al (2001).…”

“…An alternative interpretation of the spectra is given by the "magnetic flare" or disk corona model (Haardt & Maraschi 1991;Maraschi & Haardt 1997 and references therein;di Matteo et al 1999;Merloni et al 2000 and references therein), in which Energy channels in a cool accretion disk embedded in a hot region. The energy dissipation in the cool disk is assumed small compared the atmosphere (a corona or an ion supported ADAF, here called ISAF).…”

Soft X-ray components in the hard state of accreting black holes

“…The energy from the protons is sufficient to evaporate the upper layers of the disk into a hot corona (called here the "hot layer"), whose temperature is set by a balance between heating from the ions and cooling, predominantly through inverse Compton scattering of disk photons. The work in (13) found that the optical depth of the hot layer is around unity, varying only weakly with irradiating flux and distance from the hole.…”