Spectral Energy Distribution in Supercritical Disk Accretion Flows through Photon-trapping Effects

Abstract

We investigate the spectral energy distribution (SED) of the supercritical disk accretion flows around black holes by solving the multi-frequency zeroth moment equation of the radiation transfer equation under the flux-limited diffusion approximation as well as the energy equation of gas. Special attention is paid to the photon-trapping effects, the effects that photons are trapped within accretion flow and are swallowed by a black hole, with little being radiated away. It is found that when flow luminosity is below about twice the Eddington luminosity (L<2L_E) photon trapping is ineffective and the SED shifts to the higher-energy side as L increases. The peak frequency at which the SED reaches its peak becomes at maximum three times higher than that given by the standard-disk model, which might resolve the so-called 'too hot accretion disk problem.' When L>2L_E, in contrast the peak frequency of the SED tends to decrease with increase of mass-accretion rate. This is due to enhanced photon-trapping. Since high-energy photons are generated near the equatorial plane, they can be more effectively trapped in flow than low-energy ones and, hence, the high energy part of radiation is suppressed. Finally, the observed time variation of IC 342 S1, which is an ultraluminous X-ray source, in the X-ray HR diagram can be explained by the modulation of the mass accretion rate. In comparison with the observational data, we estimate the mass of black hole in IC 342 S1 is around 100 solar mass.