The Eye of the Storm: Light from the Inner Plunging Region of Black Hole Accretion Discs

TL;DR

This paper investigates the contribution of light from the inner plunging region of black hole accretion discs, revealing that it can produce observable high-energy tails and influence interpretations of X-ray spectra.

Contribution

It introduces radiative transfer calculations based on 3D GRMHD simulations to assess the observational impact of inner plunging region emission, challenging the assumption of a perfectly dark inner disc.

Findings

Inner plunging region emits a small but detectable fraction of luminosity.

Neglected inner light causes less than systematic error in black hole spin estimates.

Infalling matter contributes to high-energy tail and coronal emission in X-ray binaries.

Abstract

It is generally thought that the light coming from the inner plunging region of black hole accretion discs contributes negligibly to the disc's overall spectrum, i.e. the plunging fluid is swallowed by the black hole before it has time to radiate. In the standard disc model used to fit X-ray observations of accretion discs, the plunging region is assumed to be perfectly dark. However, numerical simulations that include the full physics of the magnetized flow predict that a small fraction of the disc's total luminosity emanates from the plunging region. We investigate the observational consequences of this neglected inner light. We compute radiative transfer based disc spectra that correspond to 3D general relativistic magnetohydrodynamic simulated discs (which produce light inside their plunging regions). In the context of black hole spin estimation, we find that the neglected inner light only has a modest effect (this bias is less than typical observational systematic errors). For rapidly spinning black holes, we find that the combined emission from the plunging region produces a weak power-law tail at high energies. This indicates that infalling matter is the origin for some of the `coronal' emission observed in the thermal dominant and steep power-law states of X-ray binaries.