Modelling accretion disc emission with generalized temperature profile and its effect on AGN spectral energy distribution

Abstract

The broad-band spectral energy distribution (SED) of active galactic nuclei (AGNs) is investigated for a well-selected sample composed of 23 Seyfert 1 galaxies observed simultaneously in the optical/ultraviolet (UV) and X-ray bands with the Neil Gehrels Swift Observatory. The optical to UV continuum spectra are modelled, for the first time, with emission from an accretion disc with a generalized radial temperature profile, in order to account for the intrinsic spectra which are found to be generally redder than the model prediction of the standard Shakura-Sunyaev disc (SSD, F_ν ∝ ν^+1/3). The power-law indices of the radial temperature profile (T_eff(R) ∝ R^-p, R is the radius of the accretion disc) are inferred to be p = 0.5-0.75 (a median of 0.63), deviating from the canonical p = 0.75 for the SSD model as widely adopted in previous studies. A marginal correlation of a flatter radial temperature profile (a smaller p-value) with increasing the Eddington ratio is suggested. Such a model produces generally a lower peak of accretion disc emission and thus a smaller bolometric luminosity in some of the AGN, particularly those with high Eddington ratios, than that based on the SSD model by a factor of several. The broad-band SED, the bolometric correction factors, and their dependence on some of the AGN parameters are revisited. We suggest that such non-standard SSD discs may operate in AGN and are at least partly responsible for the reddened optical/UV spectra as observed. One possible explanation for these flattened temperature profiles is the mass-loss process in form of disc winds/outflows.