Theoretical modeling of Comptonized X-ray spectra of super-Eddington accretion flow: origin of hard excess in Ultraluminous X-Ray Sources

TL;DR

This study models super-Eddington accretion flows around black holes to explain the origin of hard X-ray excesses in ULXs, revealing that hard spectra are produced by Compton scattering in overheated regions, consistent with observations.

Contribution

It provides a theoretical framework for the origin of hard X-ray excess in ULXs using Monte Carlo simulations of super-Eddington accretion flows, incorporating thermal and bulk Compton scattering.

Findings

Hard X-ray spectra are similar across different black hole masses when normalized by luminosity.

Hard X-ray component can be modeled by a Wien spectrum at ~3 keV with a photon index of ~3.

The model spectra agree with NuSTAR observations of ULXs.

Abstract

X-ray continuum spectra of super-Eddington accretion flow are studied by means of Monte Carlo radiative transfer simulations based on the radiation hydrodynamic simulation data, in which both of thermal and bulk Compton scatterings are taken into account. We compare the calculated spectra of accretion flow around black holes with masses of $M_{\rm BH} = 10, 10^2, 10^3$, and $10^4 M_\odot$ for a fixed mass injection rate (from the computational boundary at $10^3 r_{\rm s}$) of $10^3 L_{\rm Edd}/c^2$ (with $r_{\rm s}$, $L_{\rm Edd}$, and $c$ being the Schwarzschild radius, the Eddington luminosity, and the speed of light, respectively). The soft X-ray spectra exhibit mass dependence in accordance with the standard-disk relation; the maximum surface temperature is scaled as $T \propto M_{\rm BH}^{-1/4}$. The spectra in the hard X-ray bands, by contrast, look quite similar among different models, if we normalize the radiation luminosity by $M_{\rm BH}$. This reflects that the hard component is created by thermal and bulk Compton scattering of soft photons originating from an accretion flow in the over-heated and/or funnel regions, the temperatures of which have no mass dependence. The hard X-ray spectra can be reproduced by a Wien spectrum with temperature of $T\sim 3$ keV accompanied by a hard excess at photon energy above several keV. The excess spectrum can be well fitted with a power law with a photon index of $\Gamma \sim 3$. This feature is in good agreement with that of the recent NuSTAR observations of ULX (Ultra-Luminous X-ray sources).