Main research of Dr. Predrag Jovanović


My main investigations are in the following fields:

  1. Relativistic accretion disks around supermassive black holes
  2. Gravitational microlensing and AGN variability
  3. Perturbations in accretion disk emissivity

Relativistic accretion disks around supermassive black holes

It is now widely accepted that Active Galactic Nuclei (AGN) derive their extraordinary luminosities from energy release by matter accreting towards, and falling into, a central supermassive black hole through an accretion disk. Such a relativistic accretion disk represents an efficient mechanism for extracting gravitational potential energy and converting it into radiation.

Since 2001, I study emission from the relativistic accretion disks around supermassive black holes using numerical simulations based on ray-tracing in Kerr metric. In this method only photon trajectories reaching the observer's sky plane are taken into account. One divides the image of the disk on the observer's sky into a number of small elements (pixels) and for each pixel the photon trajectory is traced backward from the observer by following the geodesics in a Kerr space-time, until it crosses the plane of the disk (see below figure). Then, the flux density of the radiation emitted by the disk at that point, as well as the redshift factor of the photon are calculated. In that way, one can obtain the color images of the accretion disk which a distant observer would see by a high resolution telescope. The simulated line profiles can be calculated taking into account the intensities and received photon energies of all pixels of the corresponding disk image.

Ray-tracing in Kerr metric
Schematic illustration of the ray-tracing method in the Kerr metric, showing a light ray emitted from some radius of accretion disk in reference frame defined by a rotating black hole with spin a, and observed at a pixel with coordinates (impact parameters) α, β on the disk image in the observer's reference frame. For more information about ray-tracing in Kerr metric see this paper and references therein.

The following illustrations of the accretion disk and the line shape are obtained using numerical simulations based on ray-tracing in Kerr metric.

Schwarzschild metric
Kerr metric
Illustrations of accretion disk (left) and the corresponding Fe Kα line profiles (right) in the case of Schwarzschild (top) and Kerr metric with angular momentum parameter a = 0.998 (bottom). The disk inclination is i = 35o and its inner and outer radii are Rin = Rms (Rms - radius of marginally stable orbit) and Rout = 20 Rg (Rg - gravitational radius), respectively. For more information see this article, it's preprint or it's review.


Schwarzschild metric
Kerr metric
Numerical simulations of an accretion disk for different inclination angles i (left) and the corresponding profiles of the Fe Kα line (right) in the case of Schwarzschild (top) and Kerr metric with angular momentum parameter a = 0.998 (bottom). For more information see this article, it's preprint or it's review.


From Schwarzschild to Kerr metric
Numerical simulations of a highly inclined accretion disk (i = 75o) for different values of angular momentum parameter a (left) and the corresponding profiles of the Fe Kα line (right). For more information see this article, it's preprint or it's review.


Narrow ring (closer to BH)
Narrow ring (farther away from BH)
Left: illustrations of the Fe Kα line emitting region in form of narrow ring which is 1 Rg wide and located at the following distances from black hole: 10 Rg (top) and 50 Rg (bottom). Right: the corresponding Fe Kα line profiles. For more information see this article, it's preprint or it's review.

Comparisons between the simulated line profiles (such as those above) with the high resolution observations could be used as a tool for investigating the plasma conditions, space-time geometry (metric) and effects of strong gravity in vicinity of the supermassive black holes. For more information see e.g. this article, it's preprint or it's review.


Last updated on October 15th, 2010.