The influence of microlensing on the shape of the AGN Fe K_αline

Abstract

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, offcentered 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, τ ∼ 0.001, even in a favorable case.

On the contribution of microlensing to X-ray variability of high-redshifted QSOs

“…So, using the lower limit for the AGN bulge radius and the total mass estimation M tot , we obtain an upper limit of the optical depth for microlensing by bulge stellar mass objects of τ bulge ∼ 3.5 × 10 −5 . This upper limit is about the value evaluated earlier (Popović et al 2003) and the contribution to the total optical depth for microlensing is small. Microlensing would thus be detectable only in a small fraction of quasars.…”

“…As was mentioned earlier by Popović et al (2003) the probability of microlensing by stars or other compact objects in halos and bulges of quasars is very low (about 10 −4 −10 −3 ). However, for cosmologically distributed microlenses it could reach 10 −2 −0.1 at z ∼ 2.…”

“…However, it would be appropriate to present some more accurate estimates for optical depths for microlensing by bulge/halo objects assuming reasonable values for density distribution of QSO bulges/halos. This is because, as we mentioned above, the X-ray emission regions are much smaller than UV/optical ones, and even small mass deflectors from a QSO bulge/halo can produce significant magnification in the X-ray radiation (Popović et al 2003), while it will not happen in the UV/optical band.…”

“…Popović et al (2003) showed that objects in a foreground galaxy with very small masses can cause strong changes in the X-ray line profile. This fact may indicate that the observational probability of X-ray variation due to microlensing events is higher than in the UV and optical radiation of AGNs.…”

“…For qualitative discussions of the optical depth range we make the assumption of constant mass density (see also Popović et al 2003). Evaluating this integral, we obtain…”