The black hole binary V404 Cygni: an obscured AGN analogue

TL;DR

This study shows that the black hole binary V404 Cygni exhibits spectral states during outbursts similar to obscured active galactic nuclei, with heavy absorption and reflection features indicating complex accretion and outflow processes.

Contribution

It provides the first detailed spectral analysis of V404 Cyg in low-flux states, revealing similarities to obscured AGN and suggesting a thick accretion flow with clumpy outflows as the cause.

Findings

Spectral features resemble those of obscured AGN.

High column density absorption and reflection observed.

Low-flux states may be due to obscuration, not just intrinsic dimming.

Abstract

Typical black hole binaries in outburst show spectral states and transitions, characterized by a clear connection between the inflow onto the black hole and outflow from its vicinity. The transient stellar mass black hole binary V404 Cyg apparently does not fit in this picture. Its outbursts are characterized by intense flares and intermittent low-flux states, with a dynamical range of several orders of magnitude on timescales of hours. During the 2015 June-July X-ray outburst a joint Swift and INTEGRAL observing campaign captured V404 Cyg in one of these low-flux states. The simultaneous Swift/XRT and INTEGRAL/JEM-X/ISGRI spectrum is reminiscent of that of obscured/absorbed AGN. It can be modeled as a Comptonization spectrum, heavily absorbed by a partial covering, high-column density material ($N_\textrm{H} \approx 1.4\times10^{24}\,\textrm{cm}^{-2}$), and a dominant reflection component, including a narrow Iron-K$\alpha$ line. Such spectral distribution can be produced by a geometrically thick accretion flow able to launch a clumpy mass outflow, likely responsible for both the high intrinsic absorption and the intense reflection emission observed. Similarly to what happens in certain obscured AGN, the low-flux states might not be solely related to a decrease in the intrinsic luminosity, but could instead be caused by an almost complete obscuration of the inner accretion flow.