Near-infrared emission lines trace the state-independent accretion disc wind of the black hole transient MAXI J1820+070

TL;DR

This study uses near-infrared spectroscopy to reveal a steady, state-independent accretion disc wind in the black hole transient MAXI J1820+070, demonstrating the wind's detectability across different spectral states.

Contribution

It provides the first evidence of a persistent accretion disc wind in a black hole soft state using near-infrared observations, expanding understanding of wind behavior beyond X-ray detection.

Findings

nIR signatures of the wind are present in both hard and soft states.

The wind properties are similar across different accretion states.

nIR spectroscopy is effective for studying accretion disc winds.

Abstract

The black hole transient MAXI J1820+070 displayed optical P-Cyg profiles and other wind-related emission line features during the hard state of its discovery outburst. We present near-infrared (nIR) spectroscopy covering the different accretion states of the system during this event. Our 8-epoch data set (VLT/X-shooter) reveals strong variability in the properties of the nIR emission lines. This includes the presence of absorption troughs and extended emission line wings with kinetic properties that are remarkably similar to those inferred from the wind signatures observed in optical emission lines, indicating that they are most likely tracing the same accretion disc wind. Unlike the optical features, these nIR signatures are not exclusive of the hard state, as they are also witnessed across the soft state with similar observational properties. This supports the presence of a relatively steady outflow during the entire outburst of the system, and represents the first detection of an accretion disc wind in a black hole soft state at energies other than X-rays. We discuss the visibility of the wind as a function of the spectral band and the potential of nIR spectroscopy for wind studies, in particular during luminous accretion phases.