Iron lines in model disk spectra of Galactic black hole binaries

¹ N. Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
e-mail: agata@camk.edu.pl
² Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
e-mail: jm@astrouw.edu.pl
³ Toruń Centre for Astronomy, N. Copernicus University, Gagarina 11, 87-100 Torun, Poland

Received: 22 August 2010
Accepted: 1 November 2010

Abstract

Context. We present angle-dependent, broad-band intensity spectra from accretion disks around black holes of 10 M_⊙. In our computations disks are assumed to be slim, which means that the radial advection is taken into account while computing the effective temperature of the disk.

Aims. 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.

Methods. Full radiative transfer is solved including effects of Compton scattering, free-free and all important bound-free transitions of 10 main elements. We assume the LTE equation of state. Moreover, we include here the fundamental series of iron lines from helium-like and hydrogen-like ions, and fluorescent K_α and K_β lines from low ionized iron. We consider two cases: nonrotating 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.

Results. In nonirradiated 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_α and K_β lines are present in disk emitting spectra. All models with irradiation exhibit limb brightening in their X-ray reflected continua.

Conclusions. We show that the disk around stellar black hole itself is hot enough to produce strong-absorption resonance lines of iron. Emission lines can only be observed if heating by external X-rays dominates thermal processess in a hot disk atmosphere. Irradiated disks are usually brighter in X-ray continuum when seen edge on, and fainter when seen face on.