Observational properties of puffy disks: radiative GRMHD spectra of mildly sub-Eddington accretion

TL;DR

This paper uses GRMHD simulations to study puffy accretion disks around black holes, revealing their structure, observational signatures, and spectral properties, which may explain super-Eddington luminosities in X-ray binaries.

Contribution

It introduces the concept of puffy disks as a stable, stratified accretion disk structure at high luminosities, and analyzes their observational and spectral characteristics.

Findings

Puffy disks have a dense thin core with a thick, low-density layer above.

Obscuration and collimation effects influence observed luminosity and spectra.

Synthetic spectra resemble Comptonized thin disks, but standard models cannot recover black hole spin accurately.

Abstract

Numerical general relativistic radiative magnetohydrodynamic simulations of accretion disks around a stellar mass black hole with a luminosity above 0.5 of the Eddington value reveal their stratified, elevated vertical structure. We refer to these thermally stable numerical solutions as puffy disks. Above a dense and geometrically thin core of dimensionless thickness $h/r \sim 0.1$, crudely resembling a classic thin accretion disk, a puffed-up, geometrically thick layer of lower density and $h/r \sim 1.0$ is formed. We discuss the observational properties of puffy disks, in particular the geometrical obscuration of the inner disk by the elevated puffy region at higher observing inclinations, and collimation of the radiation along the accretion disk spin axis, which may explain the apparent super-Eddington luminosity of some X-ray objects. We also present synthetic spectra of puffy disks, and show that they are qualitatively similar to those of a Comptonized thin disk. We demonstrate that the existing xspec spectral fitting models provide good fits to synthetic observations of puffy disks, but cannot correctly recover the input black hole spin. The puffy region remains optically thick to scattering; in its spectral properties the puffy disk roughly resembles that of a warm corona sandwiching the disk core. We suggest that puffy disks may correspond to X-ray binary systems of luminosities above 0.3 of the Eddington luminosity in the intermediate spectral states.