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Title: The relationship between the X-ray and optical variability in Seyfert galaxies
Author: Breedt, Elme Brredt
ISNI:       0000 0004 2685 5939
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2009
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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.
Supervisor: Mchardy, Ian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QB Astronomy ; QC Physics