Erratum: 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 ofobserving spectral variability on timescales comparable to the dynamical timeof the inner accretion flow. They typically show highly variable lightcurves,with Power Density Spectra characterized by `red noise' and a break at lowfrequencies. Time resolved spectral analysis have established that spectralvariability on the shortest timescales is important in all these sources, withthe spectra getting softer at high fluxes. Here we present a model that is ableto explain a number of the above mentioned properties in terms of magneticflares shining above a standard accretion disc and producing the X-ray spectrumvia inverse Compton scattering soft photons (both intrinsic and reprocessedthermal emission from the accretion disc and locally produced synchrotronradiation). We show that the fundamental heating event, likely caused bymagnetic reconnection, must be compact, with typical size comparable to theaccretion disc thickness and must be triggered at a height at least an order ofmagnitude larger than its size; the spatial and temporal distribution of flaresare not random: the heating of the corona proceeds in correlated trains ofevents in an avalanche fashion. The amplitude of the avalanches obeys apower-law distribution and determines the size of the active regions where thespectrum is produced. With our model we simulate X-ray lightcurves thatreproduce the main observational properties of the Power Density Spectra and ofthe X-ray continuum short-term variability of Seyfert 1 galaxies. By comparingthem with observations of MGC--6-30-15, we are able to infer that the corona inthis source must have a large optical depth (tau >1.5) and small averagecovering fraction.Comment: 12 pages, 8 figures, accepted for publication in MNRA