The Structure and X‐Ray Recombination Emission of a Centrally Illuminated Accretion Disk Atmosphere and Corona

Abstract

We model an accretion disk atmosphere and corona photoionized by a central X-ray continuum source. We calculate the opacity and one-dimensional radiation transfer for an array of disk radii to obtain the twodimensional structure of the disk and its X-ray recombination emission. The atmospheric structure is extremely insensitive to the viscosity � . We find a feedback mechanism between the disk structure and the central illumination, which expands the disk and increases the solid angle subtended by the atmosphere. We apply the model to the disk of a neutron star X-ray binary. The model is in agreement with the � 12 � disk half-angle measured from optical light curves. We map the temperature, density, and ionization structure of the disk, and we simulate high-resolution spectra expected from the Chandra and XMM-Newton grating spectrometers. X-ray emission lines from the disk atmosphere are detectable, especially for high-inclination binary systems. The grating observations of two classes of X-ray binary systems already reveal important spectral similarities with our models. The model spectrum is dominated by double-peaked lines of H-like and He-like ions plus weak Fe L. The line flux is proportional to the luminosity and is dominated by the outer radii. Species with a broad range of ionization levels coexist at each radius, from Fe xxvi in the hot corona to C vi at the base of the atmosphere. The line spectrum is very sensitive to the temperature, ionization, and emission measure of each atmospheric layer, and it probes the heating mechanisms in the disk. We assume a hydrostatic disk dominated by gas pressure, in thermal balance, and in ionization equilibrium. As boundary conditions, we take a Compton temperature corona and an underlying Shakura-Sunyaev disk. The choice of thermally stable solutions strongly affects the spectrum since a thermal instability is present in the regime where X-ray recombination emission is most intense. Subject headings: accretion, accretion disks — atomic processes — instabilities — line: formation — X-rays: binaries On-line material: color figures