Modelling the spectral energy distribution of super-Eddington quasars
  We develop a broadband spectral model, agnsli}, to describe super-Eddington
black hole accretion disc spectra. This is based on the slim disc emissivity,
where radial advection keeps the surface luminosity at the local Eddington
limit, resulting in L(r)~ r^{-2} rather than the r^{-3} expected from the
Novikov-Thorne (standard, sub-Eddington) disc emissivity. Wind losses should
also be important but these are expected to produce a similar radiative
emissivity. We assume that the flow is radially stratified, with an outer
standard disc, an inner hot Comptonising region and an intermediate warm
Comptonising region to produce the soft X-ray excess. This gives the model
enough flexibility to fit the observed data, but with the additional
requirement of energy conservation to give physical constraints. We use this to
fit the broadband spectrum of one of the most extreme Active Galactic Nuclei,
the Narrow Line Seyfert 1 RX J0439.6-5311, which has a black hole mass of (6~9)
times 10^6 solar mass as derived from the H_beta line width. This cannot be fit
with the standard disc emissivity at this mass, as even zero spin models
overproduce the observed luminosity. Instead, we show that the spectrum is well
reproduced by the slim disc model, giving mass accretion rates around (5~10)
times Eddington limit. There is no constraint on black hole spin as the
efficiency is reduced by advection. Such extreme accretion rates should be
characteristic of the first Quasars, and we demonstrate this by fitting to the
spectrum of a recently discovered super-Eddington Quasar, PSO J006+39, at
z=6.6.
