Iron lines in model disk spectra of galactic black hole binaries

TL;DR

This paper models the spectra of accretion disks around stellar-mass black holes, including detailed iron line features and the effects of external irradiation, to better understand X-ray binary observations.

Contribution

It provides comprehensive angle-dependent spectra of slim accretion disks with detailed radiative transfer, including iron line complexes and irradiation effects, for different black hole spins.

Findings

Irradiation causes iron resonance lines to appear in emission.

Disks exhibit limb-brightening in X-ray reflected continua.

Strong absorption iron lines are produced in non-irradiated disks.

Abstract

We present angle-dependent, broad-band intensity spectra from accretion disks around black holes of 10 M$_\odot$. In our computations disks are assumed to be slim, which means that the radial advection is taken into account while computing effective temperature of the disk. We attempt to reconstruct continuum and line spectra of X-ray binaries in soft state, i.e. dominated by the disk component of multitemperature shape. We follow how the iron line complex depends on the external irradiation, an accretion rate and a black hole spin. Full radiative transfer is solved including effects of Compton scattering, free-free and all important bound-free transitions of 10 main elements. Moreover, we include here the fundamental series of iron lines from helium-like and hydrogen-like ions, and fluorescent K$_{\alpha}$ and K$_{\beta}$ lines from low ionized iron. We consider two cases: non-rotating black hole, and black hole rotating with almost maximum spin a=0.98, and obtain spectra for five accretion disks from hard X-rays to the infrared. In non irradiated disks, resonance lines from He-like and H-like iron appear mostly in absorption. Such disk spectra exhibit limb-darkening in the whole energy range. External irradiation causes that iron resonance lines appear in emission. Furthermore, depending on disk effective temperature, fluorescent iron K$_{\alpha}$ and K$_{\beta}$ lines are present in disk emitting spectra.All models with irradiation exhibit limb-brightening in their X-ray reflected continua. We show, that the disk around stellar black hole itself is hot enought to produce strong absorption resonance lines of iron. Emission lines can be observed only if heating by external X-rays dominates over thermal processess in hot disk atmosphere. Irradiated disks are usually brighter in X-ray continuum when seen edge on, and fainter when seen face on.