Characteristic temperatures and spectral appearance of ULX disks

TL;DR

This paper investigates the spectral characteristics of ULX disks, showing that high accretion rates can cause the inner disk to become optically thin and scattering dominated, resulting in spectra similar to observed ULX emissions.

Contribution

It demonstrates how optically-thin, scattering-dominated inner disks produce spectral features consistent with ULX observations, highlighting the impact of high accretion rates on disk emission.

Findings

Inner disk becomes optically-thin at high accretion rates

Spectral shape resembles observed ULX spectra with power-law and soft excess

Inner disk temperature exceeds effective temperature by an order of magnitude

Abstract

A standard disk around an accreting black hole may become effectively optically-thin and scattering dominated in the inner region, for high accretion rates (as already predicted by the Shakura-Sunyaev model). Radiative emission from that region is less efficient than blackbody emission, leading to an increase of the colour temperature in the inner region, by an order of magnitude above the effective temperature. We show that the integrated spectrum has a power-law-like shape in the ~ 1-5 keV band, with a soft excess at lower energies and a downward curvature or break at higher energies, in agreement with the observed spectra of many ultraluminous X-ray sources.