New Spectral State of Supercritical Accretion Flow with Comptonizing Outflow

TL;DR

This paper introduces a new spectral state in supercritical accretion flows caused by Comptonizing outflows, explaining observed spectral hardening in ULX NGC1313 X-2 during brightening.

Contribution

The study performs radiation hydrodynamic simulations incorporating Compton scattering, revealing a novel hard spectral state at high luminosities in supercritical accretion flows.

Findings

A new hard spectral state appears at luminosities above 30L_E.

Photon index decreases as luminosity increases in this state.

The Compton y-parameter is around unity, consistent with observed spectral hardening.

Abstract

Supercritical accretion flows inevitably produce radiation-pressure driven outflows, which will Compton up-scatter soft photons from the underlying accretion flow, thereby making hard emission. We perform two dimensional radiation hydrodynamic simulations of supercritical accretion flows and outflows, incorporating such Compton scattering effects, and demonstrate that there appears a new hard spectral state at higher photon luminosities than that of the slim-disk state. In this state, as the photon luminosity increases, the photon index decreases and the fraction of the hard emission increases. The Compton $y$-parameter is of the order of unity (and thus the photon index will be $\sim 2$) when the apparent photon luminosity is ${\sim}30L_{\rm E}$ (with $L_{\rm E}$ being the Eddington luminosity) for nearly face-on sources. This explains the observed spectral hardening of the ULX NGC1313 X-2 in its brightening phase and thus supports the model of supercritical accretion onto stellar mass black holes in this ULX.