Radiation hydrodynamic simulations of a super-Eddington accretor as a model for ultra-luminous sources

Abstract

We perform two-dimensional radiation hydrodynamic (RHD) simulations of super-Eddington accretion flow and the accompanying outflow to investigate how they will be observed from various viewing directions. We consider gas flow around a 10 M⊙ black hole for mass injection rates of Ṁinj/ṀEdd = 102, 103, and 104 (in units of ṀEdd ≡ LEdd/c2, with LEdd and c being the Eddington luminosity and the speed of light, respectively), and solve gas dynamics and radiation transfer around the black hole, taking into account inverse Compton scattering. We confirm the tendency that the higher the mass accretion rate is, the larger the relative importance of outflow over accretion flow becomes. The observational appearance of the super-Eddington flow is distinct, depending on whether it is viewed from the edge-on direction or from the face-on direction. This is because nearly edge-on observers can only see the outer, cooler (∼106 K) surface of the inner, vertically inflated part of the flow. Observational properties are briefly discussed in the context of the ultra-luminous X-ray sources (ULXs), the extreme ULXs (E-ULXs), and the ultra-luminous supersoft sources (ULSs). We find that the extremely high luminosities of E-ULXs (L ∼ 1041 erg s-1) can be explained when the flow on to the black hole with ≳20 M⊙ with a very high accretion rate, ṁacc (≡ Ṁacc/ṀEdd) ≳ 103, is observed from the nearly face-on direction. The high luminosity (∼1039 erg s-1) and the very soft blackbody-like spectra with temperatures around 0.1 keV, which are observed in the ULSs, can be explained if the super-Eddington flow with ṁacc ∼ 102-103 is viewed from large viewing angles, θ ≳ 30°.