A Black Hole Nova Obscured by an Inner Disk Torus

TL;DR

This paper presents evidence that the black hole candidate Swift J1357.2-0933 is obscured by an inner disk torus, explaining its unique optical dips and high-inclination geometry, which impacts models of accretion and jet formation.

Contribution

It provides spectroscopic and high-time resolution optical observations revealing a toroidal obscuring structure in a black hole binary, a novel insight into inner accretion flow geometry.

Findings

Black hole candidate in a 2.8-hour orbit

Presence of an obscuring inner disk torus

Optical dips without X-ray counterparts

Abstract

Stellar-mass black holes (BHs) are mostly found in X-ray transients, a subclass of X-ray binaries that exhibit violent outbursts. None of the 50 Galactic BHs known show eclipses, which is surprising for a random distribution of inclinations. Swift J1357.2-0933 is a very faint X-ray transient detected in 2011. On the basis of spectroscopic evidence, we show that it contains a BH in a 2.8 h orbital period. Further, high-time resolution optical light curves display profound dips without X-ray counterparts. The observed properties are best explained by the presence of an obscuring toroidal structure moving outwards in the inner disk seen at very high inclination. This observational feature should play a key role in models of inner accretion flows and jet collimation mechanisms in stellar-mass BHs.