Abstract

Invited Talk - Splinter AGN   (MW-1250)

General properties about the continuum variability of radio-quiet AGN in optical/UV and X-ray band

Bozena Czerny
Center for Theoretical Physics

The continuum variability of active galactic nuclei is predominantly stochastic across all energy bands. This stochastic behaviour is frequently modelled as a Damped Random Walk (DRW), which provides only an approximate description on relatively short timescales, while the long-term variability usually exhibits a flatter (approximately −1) slope or a bending power spectrum. The characteristic bend frequencies in the optical and X-ray bands differ substantially. The origin of this variability is commonly attributed to local perturbations driven by magnetorotational instability (MRI) turbulence and their inward propagation through the accretion flow. However, both detailed modelling and observations indicate that the situation is more complex. Additional complications arise from non-local effects: on the shortest timescales, X-ray irradiation appears to dominate and to produce the observed interband delays in the optical/UV continuum. Such delays are sometimes measured after detrending the light curves to remove the long-term intrinsic variability, although this procedure may itself affect the inferred lags. Finally, dramatic transitions, now commonly referred to as Changing-Look or Changing-State phenomena, add another level of complexity and most likely reflect qualitative geometrical changes in the innermost regions of the accretion flow.