Thunderclouds and accretion discs: a model for the spectral and temporal variability of Seyfert 1 galaxies

X-ray observations of Seyfert 1 galaxies offer the unique possibility of observing spectral variability on time-scales comparable to the dynamical time of the inner accretion flow. They typically show highly variable light curves, on a wide range of time-scales, with power density spectra characterized by 'red noise' and a break at low frequencies. On the other hand, time-resolved spectral analyses have established that spectral variability on the shortest timescales is important in all these sources, with the spectra getting softer at high fluxes (in the 2-10 keV band, typically), while the reflection component and the iron line often exhibit a complex behaviour. Here we present a model that is able to explain a number of the above mentioned properties, in terms of magnetic flares shining above a standard accretion disc producing the X-ray spectrum via inverse Compton scattering of soft photons (both intrinsic and reprocessed thermal emission from the accretion disc and locally produced synchrotron radiation). We show that the fundamental heating event, likely caused by magnetic reconnection, must be compact, with typical size comparable to the accretion disc thickness, and must be triggered at a height at least an order of magnitude larger than its size. The fundamental property of our 'thundercloud' model is that the spatial and temporal distributions of flares are not random: the heating of the corona proceeds in correlated trains of events in an avalanche fashion. The amplitude of the avalanches obeys a power-law distribution and determines the size of the active regions where the spectrum is produced. As a result of the feedback effect of the X-ray radiation reprocessed in the disc, larger active regions produce softer spectra. With our model we simulate X-ray light curves that reproduce the main observational properties of the power density spectra and of the X-ray continuum short-term variability of Seyfert I galaxies. By comparing them with observations of MGC-6-30-15, we are able to infer that the accretion disc corona in this source must have a large optical depth (tau (T) greater than or similar to 1.5) and small average covering fraction.