Optical and near-infrared spectroscopy of the black hole GX 339-4 II. The spectrocopic content in the low/hard and high/soft states

TL;DR

This study analyzes optical and near-infrared spectra of the black hole GX 339-4 in different states, revealing changes in emission lines and the presence of an accretion disc wind, enhancing understanding of accretion processes.

Contribution

It provides detailed spectral line analysis across different states, identifying the formation of a geometrically thick envelope and potential disc wind in the hard state, which is a novel insight.

Findings

Emission lines originate from the irradiated accretion disc in the soft state.

Detection of a broad Pabeta emission line indicating an extended envelope.

Evidence of a thermally-driven disc wind during the second hard state.

Abstract

As a complement to our optical and near-infrared study of the continuum properties of GX 339-4 in the two hard and one soft state observations made by the ESO/VLT FORS2 and ISAAC in early 2010, we report here on the results of our spectral line analysis for the same observations. In the soft state, the presence of strong Balmer, Paschen and Brackett emission lines points to the optical and near-infrared spectra stemming from the irradiated chromosphere of the optically thick and geometrically thin accretion disc. Most of these HI features are still detected in emission in both hard states but are veiled by the compact jets continuum. We also confirm the presence of a broad Hbeta absorption feature, prominent in the soft state and shallower in the first hard state, which we argue forms in the deep layers of the optically thick accretion disc. However, this trough is absent in the second hard state, a likely consequence of the formation of a geometrically thick extended envelope that arises above the disc plane and eventually enshrouds the region where the Hbeta absorption feature forms. We detect this envelope through the presence of a broad Pabeta emission line, which is constant during the first hard state but correlates with the underlying continuum during the second hard state, pointing to changing physical properties. We consider that this behaviour may be consistent with the launch of a thermally-driven accretion disc wind during the second hard state.