State-dependent signatures of jets and winds in the optical and infrared spectrum of the black hole transient GX 339–4

¹ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
² Departamento de astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³ Center for Astro, Particle, and Planetary Physics, New York University Abu Dhabi, PO Box 129188 Abu Dhabi, UAE
⁴ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate (LC), Italy
⁵ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁸ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany
¹⁰ Department of Physics, Ehime University, 2-5, Bunkyocho, Matsuyama, Ehime 790-8577, Japan
¹¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹² Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan
¹³ Department of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, UK

^⋆ Corresponding author; ambrifi.astronomy@gmail.com

Received: 7 June 2024
Accepted: 19 December 2024

Abstract

GX 339–4 is one of the prototypical black hole X-ray transients, exhibiting recurrent outbursts that allow detailed studies of black hole accretion and ejection phenomena. In this work we present four epochs of optical and near-infrared spectroscopy obtained with X-shooter at the Very Large Telescope. The dataset includes two hard state spectra, collected during the 2013 and 2015 outbursts, and two soft state spectra observed during the 2021 outburst. Strong Balmer, Paschen, He I and He II emission lines are consistently observed in all spectra, while Brackett transitions and the Bowen blend are only prominent in the soft state. Although P-Cygni profiles are not identified, the presence of wind signatures, such as extended emission wings, flat-top and asymmetric red-skewed profiles, is consistently observed through most emission lines, suggesting the presence of wind-type ejecta. These features are particularly evident in the hard state, but they are also observed in the soft state, especially in the near-infrared. This strengthens the case for state-independent winds in black hole transients and increases the evidence for wind signatures in low-to-intermediate orbital inclination systems. We also study the spectral energy distribution, which provides evidence for the presence of synchrotron emission during the hard state. The jet significantly affects the near-infrared continuum, greatly diluting the emission features produced in the accretion flow. The simultaneous identification of both jet and wind signatures during the hard state reinforces the idea of a complex outflow scenario, in which different types of ejecta coexist.