Perturbations in accretion disk emissivity

Standard model of accretion disk cannot explain the observed variability in emission of some AGN. For example, in addition to the stable 6.4 keV core of the Fe Kα line, a variable "red" feature of the line at 6.1 keV is also detected in X-ray spectrum of Seyfert galaxy NGC 3516 (see below figure), observed by XMM-Newton satellite. Such systematical and periodical variability supports the hypothesis of an orbiting spot in the accretion disk.

NGC 3516
Light curves of Seyfert galaxy NGC 3516 for the 0.3 - 10 keV band, the Fe Kα line red feature and the line core (picture is taken from this paper).

Several physical mechanisms could be responsible for for bright spot formations (i.e. perturbations in accretion disk emissivity), such as disk self-gravity, baroclinic vorticity, disk-star collisions, tidal disruptions of stars by central black hole and fragmented spiral arms of the disk (see e.g. this paper and references therein).

We developed a model of the disk perturbing region in the form of a single bright spot (or flare) by a modification of the power law disk emissivity (as shown in below figure), and used this model to simulate the line profiles.

Bright spot in accretion disk
Shape of the perturbed power law emissivity of an accretion disk in Schwarzschild metric.

Two examples of a bright spot moving along the recedeing and approaching side of an accretion disk and the corresponding perturbed and unperturbed Fe Kα line profiles are presented in below figure. As it can be seen from this figure, when the bright spot moves along the receding side of the disk (positive direction of x-axis) it affects only "red" wing of the line, while the "blue" wing and the line core stay nearly constant. In the case when the bright spot moves along the approaching side of the disk (negative direction of x-axis), it affects only "blue" wing of the line.

Bright spot on the receding side of the disk
Bright spot on the approaching side of the disk
Variations of the perturbed Fe Kα line profiles for different positions of bright spot on the receding (top) and approaching side of the disk (bottom). In both cases, the positions of bright spot are varied from the inner radius of the disk towards its outer radius.

This model has been used to fit the observed Hβ line profiles of quasar 3C 390.3, observed from 1995 to 1999. The observed variations of the Hβ line, including two large amplitude outbursts observed during the analyzed period, are successfully reproduced by this model, indicating that they are probably caused by perturbations in the disk emissivity (see this paper for more information).

Last updated on October 15th, 2010.