Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 397, Issue 4, Pages 2004-2014Publisher
OXFORD UNIV PRESS
DOI: 10.1111/j.1365-2966.2009.15110.x
Keywords
galaxies: active
Categories
Funding
- Fellowship for International Young Researchers
- Chinese Academy of Sciences
- Max-Planck-Institut fur Astrophysik, Garching
- STFC Advanced Fellowship
- FONDECYT [1080603]
- Stobie-SALT Scholarship
- STFC [PP/D001013/1]
- Science and Technology Facilities Council [ST/G003084/1, PP/D00571X/1, PP/D001013/1] Funding Source: researchfish
- STFC [ST/G003084/1, PP/D001013/1, PP/D00571X/1] Funding Source: UKRI
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We present simultaneous X-ray and optical B- and V-band light curves of the Seyfert Galaxy NGC 3783 spanning 2 years. The flux in all bands is highly variable and the fluctuations are significantly correlated. As shown before by Stirpe et al. the optical bands vary simultaneously, with a delay of less than 1.5 d but both B and V bands lag the X-ray fluctuations by 3-9 d. This delay points at optical variability produced by X-ray reprocessing and the value of the lag places the reprocessor close to the broad-line region. A power spectrum analysis of the light curve, however, shows that the X-ray variability has a power-law shape bending to a steeper slope at a time-scale of similar to 2.9 d while the variability amplitude in the optical bands continues to grow towards the longest time-scale covered, similar to 300 d. We show that the power spectra together with the small value of the time delay are inconsistent with a picture where all the optical variability is produced by X-ray reprocessing, though the small amplitude, rapid optical fluctuations might be produced in this way. We detect larger variability amplitudes on long time-scales in the optical bands than in the X-rays. This behaviour adds to similar results recently obtained for at least three other active galactic nuclei and indicates a separate source of long-term optical variability, possibly accretion rate or thermal fluctuations, in the optically emitting accretion disc.
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