Iron lines from transient and persisting hot spots on AGN accretion disks

Abstract

[abridged] We model the X-ray reprocessing from a strong co-rotating flare above an accretion disk in active galactic nuclei. We explore the horizontal structure and evolution of the underlying hot spot. To obtain the spectral evolution seen by a distant observer, we apply a general relativity ray-tracing technique. We concentrate on the energy band around the iron K-line, where the relativistic effects are most pronounced. Persistent flares lasting for a significant fraction of the orbital time scale and short, transient flares are considered. In our time-resolved analysis, the spectra recorded by a distant observer depend on the position of the flare/spot with respect to the central black hole. If the flare duration significantly exceeds the light travel time across the spot, then the spot horizontal stratification is unimportant. On the other hand, if the flare duration is comparable to the light travel time across the spot radius, the lightcurves exhibit a typical asymmetry in their time profiles. The sequence of dynamical spectra proceeds from more strongly to less strongly ionized re-emission. At all locations within the spot the spectral intensity increases towards edge-on emission angles, revealing the limb brightening effect. Future X-ray observatories with significantly larger effective collecting areas will enable to spectroscopically map out the azimuthal irradiation structure of the accretion disk and to localize persistent flares. If the hot spot is not located too close to the marginally stable orbit of the black hole, it will be possible to probe the reflecting medium via the sub-structure of the iron K-line. Indications for transient flares will only be obtained from analyzing the observed lightcurves on the gravitational time scale of the accreting supermassive black hole.

15 pages, 8 figures, accepted by Astronomy & Astrophysics

References in corpus (9)

• Surface Structure in an Accretion Disk Annulus with Comparable Radiation and Gas Pressure
• Statistics of relativistically broadened Fe K-alpha lines in AGN
• Variable iron-line emission near the black hole of Markarian 766
• The structure and X-ray radiation spectra of illuminated accretion disks in AGN. III. Modeling fractional variability
• Correlated modulation between the redshifted Fe K alpha line and the continuum emission in NGC 3783
• Exploring the X-ray spectral variability of MCG-6-30-15 with XMM-Newton
• Anisotropic X-ray emission in AGN accretion discs
• Modeling time delays in the X-ray spectrum of the Seyfert galaxy MCG-6-30-15
• Magnetic flares in Active Galactic Nuclei: Modeling the iron Kalpha-line

Cited by in corpus (4)

• X-ray Absorption and Reflection in Active Galactic Nuclei
• A deep X-ray view of the bare AGN Ark 120. II. Evidence for Fe K emission transients
• Variation in the primary and reprocessed radiation from an orbiting spot around a black hole
• The flare model for X-ray variability of NGC 4258