Correlation and time delays of the X-ray and optical emission of the Seyfert Galaxy NGC 3783 |
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| Authors: | P Arévalo P Uttley P Lira E Breedt I M McHardy E Churazov |
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| Institution: | Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, China;Departamento de Ciencias Fisicas, Universidad Andres Bello, Av. Republica 252, Santiago, Chile;School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ;Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile;Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85741 Garching, Germany;Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia |
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| Abstract: | 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 ～2.9 d while the variability amplitude in the optical bands continues to grow towards the longest time-scale covered, ～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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| Keywords: | galaxies: active |
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