http://arxiv.org/abs/1804.00171
We develop a new spectral model for the broadband spectral energy distribution of active galactic nuclei. This includes an outer standard disc, an inner warm Comptonising region to produce the soft X-ray excess and a hot corona. For the first time, we tie these together into a physically based geometry as well as energetically by assuming Novikov-Thorne emissivity. This defines a size scale for the hard X-ray corona from the radius where the remaining accretion energy down to the black hole can power the observed X-ray emission. The data show that the average hard X-ray luminosity is remarkably constant in Eddington units, at 0.02Ledd for all bolometric luminosities, so the radial size scale required for gravity to power the corona decreases with increasing Eddington fraction. The hard X-ray spectral index is set self consistently from the ratio of this coronal luminosity to intercepted seed photon luminosity from the disc. This matches the observed correlation of steeper spectral index with increasing Eddington ratio. The model fits the overall spectra of representative, low intrinsic absorption objects: NGC 5548, Mrk 509 and PG1115+407, with Eddington ratios of 0.03, 0.1 and 0.3,as well as reproducing the observed tight UV/X relation of quasars. We also include the reprocessed emission produced by the hot inner flow illuminating the warm Comptonisation and standard disc regions and show that this predicts a decreasing amount of optical variability with Eddington ratio, as observed, though additional processes may also be important to explain all of the observed optical variability.
A. Kubota and C. Done
Tue, 3 Apr 18
25/57
Comments: 15 pages, 24 figures submitted to MNRAS