The influence of microlensing on the shape of the AGN Fe K_α line

¹ Astronomical Observatory, Volgina 7, 11160 Belgrade 74, Serbia
² Isaac Newton Institute of Chile, Yugoslavia Branch
³ Instituto de Astrofísica de Canarias, 382005 La Laguna, Tenerife, Spain

Corresponding author: L. Č. Popović, lpopovic@aob.bg.ac.yu

Received: 12 July 2002
Accepted: 14 November 2002

Abstract

We study the influence of gravitational microlensing on the AGN Fe K_α line confirming that unexpected enhancements recently detected in the iron line of some AGNs can be produced by this effect. We use a ray tracing method to study the influence of microlensing in the emission coming from a compact accretion disc considering both geometries, Schwarzschild and Kerr.
Thanks to the small dimensions of the region producing the AGN Fe K_α line, the Einstein Ring Radii associated to even very small compact objects have size comparable to the accretion disc hence producing noticeable changes in the line profiles. Asymmetrical enhancements contributing differently to the peaks or to the core of the line are produced by a microlens, off-centered with respect to the accretion disc.
In the standard configuration of microlensing by a compact object in an intervening galaxy, we found that the effects on the iron line are two orders of magnitude larger than those expected in the optical or UV emission lines. In particular, microlensing can satisfactorily explain the excess in the iron line emission found very recently in two gravitational lens systems, H 1413+117 and MG J0414+0534. 
Exploring other physical scenarios for microlensing, we found that compact objects (of the order of one Solar mass) which belong to the bulge or the halo of the host galaxy can also produce significant changes in the Fe K_α line profile of an AGN. However, the optical depth estimated for this type of microlensing is very small, , even in a favorable case.