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The template for preparation of abstracts

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\rightline{\it Invited lecture}

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\centerline{\bf CENTRAL ENGINES OF ACTIVE GALAXIES AS THE MOST}

\centerline{\bf POWERFUL SOURCES OF X-RAY RADIATION IN UNIVERSE}

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\centerline{\bf P. Jovanovi\' c and L. \v C. Popovi{\'c}}

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\centerline{\it Astronomical Observatory, Volgina 7, 11060
Belgrade 38, Serbia}

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\centerline{\it E-mail\/: pjovanovic@aob.bg.ac.yu,
lpopovic@aob.bg.ac.yu}

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Active galaxies are galaxies hosting a small, highly variable and
very bright core (Active Galactic Nucleus), embedded in an
otherwise typical galaxy. Active Galactic Nuclei (AGN) are
powerful sources of radiation in a wide spectral range: from
$\gamma$ rays to radio waves. They derive their extraordinary
luminosities (sometimes more than $10^4$ times higher than
luminosities of "ordinary" galaxies) from energy release by matter
accreting towards, and falling into, a central supermassive black
hole (BH). According to the unification model of AGN, their
central engine consists of a supermassive BH ($10^7 - 10^9
M_\odot$) which is surrounded by an accretion disk that radiates
in the X-ray band. Accretion disks could have different forms,
dimensions, and emission, depending on the type of central BH,
whether it is rotating (Kerr metric) or nonrotating (Schwarzschild
metric). They represent an efficient mechanism for extracting
gravitational potential energy and converting it into radiation,
giving us the most probable explanation for the main
characteristics of AGN (high luminosity, compactness, jet
formation, rapid time variations in radiation and the profile of
the Fe K$\alpha$ spectral line). A broad emission line Fe
K$\alpha$ at 6.4 keV with asymmetric profile (narrow bright blue
peak and a wide faint red wing) has been observed in a number of
type 1 AGN. In some cases the line width corresponds to one third
of speed of light, indicating that its emitters rotate with
relativistic velocities. Therefore, the line is probably produced
in a very compact region near the central BH of AGN and can
provide us some essential information about the plasma conditions
and the space-time geometry in vicinity of the BH. The X-ray
radiation of AGN, in the line, as well as in the continuum, has
rapid and irregular variability. This variability could be due to
disk instability, resulting in perturbations of disk emissivity,
or it could be caused by some external effects, such as
gravitational microlensing (especially in the case of
gravitationally lensed quasars) and absorption. Here we present a
short overview and main results of our recent investigations of
the Fe K$\alpha$ spectral line and X-ray continuum variability.

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