Correlation and time delays of the X-ray and optical emission of the Seyfert Galaxy NGC 3783

Abstract

We present simultaneous X-ray and optical B and V band light curves of the\nSeyfert Galaxy NGC3783 spanning 2 years. The flux in all bands is highly\nvariable and the fluctuations are significantly correlated. As shown before by\nStirpe et al. the optical bands vary simultaneously, with a delay of less than\n1.5 days but both B and V bands lag the X-ray fluctuations by 3-9 days. This\ndelay points at optical variability produced by X-ray reprocessing and the\nvalue of the lag places the reprocessor close to the broad line region. A power\nspectrum analysis of the light curve, however, shows that the X-ray variability\nhas a power law shape bending to a steeper slope at a time-scale ~2.9 days\nwhile the variability amplitude in the optical bands continues to grow towards\nthe longest time-scale covered, ~ 300 days. We show that the power spectra\ntogether with the small value of the time delay is inconsistent with a picture\nwhere all the optical variability is produced by X-ray reprocessing, though the\nsmall amplitude, rapid optical fluctuations might be produced in this way. We\ndetect larger variability amplitudes on long time-scales in the optical bands\nthan in the X-rays. This behaviour adds to similar results recently obtained\nfor at least three other AGN and indicates a separate source of long term\noptical variability, possibly accretion rate or thermal fluctuations in the\noptically emitting accretion disc.\n