Iron Kα Fluorescent Line Profiles from Spiral Accretion Flows in Active Galactic Nuclei

Abstract

We present 6.4 keV iron K-alpha fluorescent line profiles predicted for a relativistic black hole accretion disk in the presence of a spiral motion in Kerr geometry, the work extended from an earlier literature motivated by recent magnetohydrodynamic (MHD) simulations. The velocity field of the spiral motion, superposed on the background Keplerian flow, results in a complicated redshift distribution in the accretion disk. An X-ray source attributed to a localized flaring region on the black hole symmetry axis illuminates the iron in the disk. The emissivity form becomes very steep because of the light bending effect from the primary X-ray source to the disk. The predicted line profile is calculated for various spiral waves, and we found, regardless of the source height, that: (i) a multiple-peak along with a classical double-peak structure generally appears, (ii) such a multiple-peak can be categorized into two types, sharp sub-peaks and periodic spiky peaks, (iii) a tightly-packed spiral wave tends to produce more spiky multiple peaks, whereas (iv) a spiral wave with a larger amplitude seems to generate more sharp sub-peaks, (v) the effect seems to be less significant when the spiral wave is centrally concentrated, (vi) the line shape may show a drastic change (forming a double-peak, triple-peak or multiple-peak feature) as the spiral wave rotates with the disk. Our results emphasize that around a rapidly-rotating black hole an extremely redshifted iron line profile with a noticeable spike-like feature can be realized in the presence of the spiral wave. Future X-ray observations, from {\it Astro-E2} for example, will have sufficient spectral resolution for testing our spiral wave model which exhibits unique spike-like features.