Long-term X-ray luminosity light curve of IC 3599. Error bars are at 1σ confidence level. ROSAT, Chandra and Swift count rates were converted into 0.01–10 keV unabsorbed luminosities by means of spectral fits assuming a source distance of 92 Mpc. The overall X-ray light curve has been fitted with a (t − t₀)^{− 5/3} function repeating over a P₀ time. Free parameters are the starting date, the power-law normalisation, the repetition time and a constant, setting the basic emission level of IC 3599. The quiescent level (AGN activity) has been evaluated as worsening the fit until a 10% null-hypothesis probability is attained. This is nicely consistent with the mean value and the observed variability in the power-law component used in the X-ray spectra fits.