Unveiling slim accretion disc in AGN through X-ray and Infrared observations

In this work, which is a continuation of Castelló-Mor et al., we present new X-ray and infrared (IR) data for a sample of active galactic nuclei (AGN) covering a wide range in Eddington ratio over a small luminosity range. In particular, we rigorously explore the dependence of the optical-to-X-ray spectral index α_OX and the IR-to-optical spectral index on the dimensionless accretion rate, \dot{M} = \dot{m}/η, where \dot{m} = L_AGN/L_Edd and η is the mass-to-radiation conversion efficiency, in low- and high-accretion rate sources. We find that the spectral energy distribution (SED) of the faster accreting sources is surprisingly similar to those from the comparison sample of sources with lower accretion rate. In particular: (I) The optical-to-UV AGN SED of slow and fast accreting AGN can be fitted with thin accretion disc (AD) models. (II) The value of α_OX is very similar in slow and fast accreting systems up to a dimensionless accretion rate \dot{M}_c ∼ 10. We only find a correlation between α_OX and \dot{M} for sources with \dot{M} > \dot{M}_c. In such cases, the faster accreting sources appear to have systematically larger α_OX values. (III) We also find that the torus in the faster accreting systems seems to be less efficient in reprocessing the primary AGN radiation having lower IR-to-optical spectral slopes. These findings, failing to recover the predicted differences between the SEDs of slim and thin ADs within the observed spectral window, suggest that additional physical processes or very special geometry act to reduce the extreme-UV radiation in fast accreting AGN. This may be related to photon trapping, strong winds and perhaps other yet unknown physical processes.