A self-shadowing black hole accretion disk at sub-Eddington luminosities

Abstract

Accretion onto black holes powers some of the most energetic sources in the Universe, yet its behaviour across the luminosity range remains poorly understood. Specifically, as the luminosity approaches the Eddington limit ($L_{\mathrm{Edd}}$), the inner accretion disk is expected to depart from the standard thin-disk solution, becoming vertically extended. The luminosity threshold and observational signatures of this transition remain, however, uncertain. We present multiwavelength spectroscopy obtained during the decay of a black hole X-ray binary outburst. At the peak ($\sim 0.2L_{\mathrm{Edd}}$), a cool optical and near-infrared spectrum dominated by strong recombination lines of hydrogen and He I is observed. However, as the luminosity decreases, it switches to a hot, highly irradiated, nearly featureless spectrum. This puzzling behavior reveals the emergence of a vertically extended inner disk at the outburst peak, shielding the outer regions from high-energy radiation produced near the black hole; a scenario supported by contemporaneous X-ray spectra. Observations of similar systems suggest that this phenomenon may be common among black hole accretion disks. Self-shadowing structures at luminosities well below $L_{\mathrm{Edd}}$ have major implications for black hole accretion flows and their observational properties, affecting inferred black hole spins, X-ray polarization measurements, and the physical conditions of the outer disk and its associated outflows.

arXiv comment24 pages, 10 figures. Published in Nature Astronomy

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