Comptonized photon spectra of supercritical black hole accretion flows with application to ultraluminous X-ray sources

Radiation spectra of supercritical black hole accretion flows are computed using a Monte Carlo method by post-processing the results of axisymmetric radiation hydrodynamic simulations. We take into account thermal/bulk Comptonization, free-free absorption, and photon trapping. We found that a shock-heated region (10⁸ K) appears at the funnel wall near the black hole where the supersonic inflow is reflected by the centrifugal barrier of the potential. Both thermal and bulk Comptonization significantly harden photon spectra although most of the photons upscattered above 40keV are swallowed by the black hole due to the photon trapping. When the accretion rate onto the black hole is 200L _E/c ², where L _E is the Eddington luminosity, the spectrum has a power-law component which extends up to 10keV by upscattering of photons in the shock-heated region. In higher mass accretion rates, the spectra roll over around 5keV due to downscattering of the photons by cool electrons in the dense outflow surrounding the jet. Our results are consistent with the spectral features of ultraluminous X-ray sources, which typically show either a hard power-law component extending up to 10keV or a rollover around 5keV. We found that the spectrum of NGC1313 X-2 is quite similar to the spectrum numerically obtained for high accretion rate () source observed with low viewing angle (i = 10°-20°). Our numerical results also demonstrate that the face-on luminosity of supercritically accreting stellar mass black holes (10 M) can significantly exceed 10⁴⁰ erg s ^-1.