| Abstract |
Active Galactic Nuclei (AGN) are luminous and exotic celestial objects. It is generally
accepted that AGN are powered by the accretion of mass onto a super-massive black hole,
which is located at their center. As matter accretes in the form of a disk, energy is liberated
and radiated as black body emission in the optical and ultraviolet wavebands. The observed
X-rays of AGN are considered to be the result of thermal Comptonization of soft photons in
a hot plasma region, usually called the X-ray "corona".
Despite being extensively studied for more than 40 years, many of the physical processes
in AGN are still unclear. A reason for that is the really small (angular) size of the AGN, which
does not allow a direct observation of their core region with the currently used instruments.
As a reasult, the only way to get information about those objects is the simultaneous multiwavelength
observations.
The results from such studies so far have been suggestive, and not conclusive. In general,
the ultraviolet/optical emission is well correlated, with delays which increase with the wavelength.
However, the correlation between X-ray and ultraviolet variations is rather low. The
corresponding time lag suggests that X-ray variations lead the ultraviolet/optical ones, although
the results are also consistent, within the errors, with the ultraviolet emission leading
the X-rays.
A possible explanation of the small correlation between the X-rays and the ultraviolet flux
is that the observed X-rays (in 0.3–10 keV) are combination of the intrinsic X-ray emission
(i.e. the corona emission) and the interaction of this emission with a photo-ionized absorbing
material, namely the "warm absorber". As a result, the observed X-ray variability is partly
due to the variations of the corona emission and partly due to the variations of the absorber’s
properties.
In this work I analyzed the observations of NGC 5548 obtained by the Swift observatory
from February, 17th to June, 22nd, 2014. I fitted the X-ray spectra with a proper theoretical
model and I managed to determine the intrinsic X-ray flux of the source. Then, I crosscorrelated
this flux with the ultraviolet lightcurve. I found that the correlation coefficient is
rather low and the time lag suggests that the X-rays lead the ultraviolet variations by ~6
days. The X-ray and ultraviolet correlation is very large (DCF ~0.85) during the first part
of the lightcurves, which show a clear, large-amplitude "flare". This is suggestive of accretion
rate (ṁ) variations in the inner part of the disk, which creates sound waves propagating
outwards. There is no observed correlations between the two fluxes in the second half of
the observations. Finally, the correlation analysis indicate that the w2 ultraviolet photons
(λeff = 2030Å) are not the soft input photons that produce the X-rays.
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Simultaneous Optical/Ultraviolet/X-ray variability study of NGC 5548
