Broadband X-ray/UV/optical time-resolved spectroscopy of NGC 5548: The origin of the UV/optical variability in active galactic nuclei

Recently, nearby active galactic nuclei (AGN) have been subject to long X-rays/UV/optical monitoring campaigns. These campaigns reveal a strong correlation between the various UV and optical bands, with time lags increasing with wavelength. In a series of papers, we demonstrated that a scenario in which a central X-ray source illuminates the accretion disc explains the observed correlations. However, some of the monitored AGN show low/moderate X-rays-UV correlations, which could challenge this scenario. In this paper, we study the broadband X-ray/UV/optical spectral energy distributions (SEDs) of NGC 5548, one of the most intensively monitored AGN. We aim to test if the X-ray illumination model explains the broadband spectral behaviour of the source, despite the moderate X-ray-UV/optical correlation. We model the broadband time-averaged SED, from the STORM monitoring campaign of the source, using the KYNSED model which assumes an X-ray illuminated disc. We assume that the accretion process powers the X-ray corona. We also model 15 time-resolved SEDs from the same campaign to check whether this scenario can account for the observed spectral variability. The proposed model describes well the time-averaged and the time-resolved SEDs of NGC 5548. In this scenario, the corona height, the X-ray photon index, and the power transferred to the corona all vary. This explains the variability behaviour at different wavelengths. The best-fit model is obtained for a non-rotating black hole accreting at a constant rate of 5 affects the observed flux in a particular way, the combined variability of all parameters explains the moderate X-ray-UV/optical correlation. The X-ray illuminated disc model provides a complete description of the behaviour of NGC 5548, explaining its broadband SEDs, time-lag spectrum, and its power spectral distribution.