The relationship between the x-ray and optical variability in Seyfert Galaxies

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Description/Abstract

The relationship between the X-ray and optical variability in Seyfert galaxies
by Elm´e Breedt Lategan
Seyfert galaxies have been known to vary since shortly after their discovery, but
the origin of the optical variability and its relationship to the X-rays, have not been
well established. Presented here is a comprehensive comparison between the long
term X-ray and optical emission in a sample of seven Seyfert galaxies, to determine
the relationship between the X-ray and optical variations. It is shown that the observed
optical variability is the sum of several independent mechanisms: the short
time-scale variability is dominated by X-ray reprocessing, but variations intrinsic
to the disc, such as propagating accretion rate fluctuations, add considerable
variability power on long time-scales.

Cross-correlation analysis reveal a statistically significant correlation between
the X-ray and optical emission of all the galaxies under investigation, with the
optical variations following the X-rays by 1–2 days. This time-scale is consistent
with the expected light travel time from the central X-ray source to the optically
emitting region of the accretion disc. Although this is a strong indicator of X-ray
reprocessing by the disc, it is also clear that reprocessing cannot account for all
the optical variations observed. At least one source displays clear long time-scale
(years) variations in the optical with no apparent counterpart in the X-rays, and
in others the long term optical amplitude exceed that of the X-rays. It is shown
that these long time-scale optical variations must originate from an independent
variability mechanism, possibly variations in the accretion rate or the geometry of
the system. There is also evidence that there may be a significant contribution to
the optical emission from as far out as the dust torus.

Finally it is shown that there may be a dependence of the strength of the
X-ray/optical correlation on the disc temperature and black hole mass.