The relationship between the x-ray and optical variability in Seyfert Galaxies
The relationship between the X-ray and optical variability in Seyfert galaxies by Elmé 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.
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