Spectra and Structure of Accretion Disks with Nonzero Inner Torque

TL;DR

This study models accretion disks around stellar-mass black holes at near-Eddington luminosities, incorporating advanced physics to analyze their spectra and structure, revealing how inner stresses and spin influence observable X-ray features.

Contribution

It introduces a comprehensive numerical model of accretion disks with nonzero inner torque, based on first-principles MHD simulations, extending previous models to include high-spin effects and detailed spectral predictions.

Findings

Spectra become more saturated with Comptonization closer to the black hole.

Higher black hole spin results in harder spectra but no broad power-law tail.

Inner stresses significantly increase disk temperature and alter spectral features.

Abstract

We present numerical spectral and vertical structure calculations appropriate for near-Eddington-luminosity, radiation-pressure-dominated accretion disks around stellar-mass black holes. We cover a wide range of black hole spins and incorporate dissipation profiles based on first-principles three-dimensional MHD disk interior simulations. We also include nonzero stresses at the innermost stable circular orbit, which results in the disk effective temperature increasing rapidly toward the black hole and gives rise to rather extreme conditions with high temperatures and low surface densities. We found that local annulus spectra become increasingly characteristic of saturated Comptonization with decreasing distance to the black hole. While the spectra become harder with increasing black hole spin, they do not give rise to a broad power-law tail even at maximum spin. We discuss the implications of our results in the context of the steep power-law state and the associated high-frequency quasi- periodic oscillations observed in some X-ray binary systems.