Accretion onto compact objects, such as black holes and neutron stars, is one of the most powerful processes in the Universe, involving large exchanges of mass and energy. Among the astrophysical systems where this phenomenon is observed, X-ray binaries are particularly relevant due to their relatively short timescales of evolution, which are accessible to human observation. In these systems, mass is transferred from a donor star to a stellar-mass black hole or neutron star via an accretion disc. In particular, the subclass of low-mass X-ray binaries has donors less massive than the Sun and exhibits sporadic episodes of enhanced accretion known as outbursts. During these events, their accretion flows are strongly coupled to mass ejections or outflows, which can be studied from a multi-wavelength perspective. In this thesis, we investigate outflow-related processes in a sample of three low-mass X-ray binaries.
The first two studies consist of spectroscopic searches for a particular type of outflows, namely low ionisation accretion disc winds, in two sources with low-tointermediate inclinations (small angles between the perpendicular axis to the orbital plane and our line of sight). The classic neutron star system Aquila X-1 is analysed in the first work, using optical spectra obtained with the Gran Telescopio Canarias during the bright outburst of 2016. During the final decay of the episode, we find spectral signatures indicative of the existence of an accretion disc wind in the system. This represents one of the initial efforts to detect low ionisation winds in a neutron star low-mass X-ray binary.
Similarly, the second study analyses optical and near-infrared spectra of the recently discovered black hole system MAXI J1348-630, acquired with the Very Large Telescope. We detect spectral signatures of an accretion disc wind in both optical and near-infrared wavelengths throughout the event, which had been scarcely studied in low inclination systems before.
Lastly, the third work presents a photometric analysis of the optical dips observed in the high inclination black hole system Swift J1357.2-0933. These quasi-periodic dips had previously shown increasing periodicity as the outbursts decayed and seem to be correlated with outflows. We conduct a comprehensive investigation of their recurrence in all known outbursts of the source, and discover a correlation with their luminosity in optical and X-rays.
Overall, this thesis focuses on the study of low ionisation winds in X-ray binaries, with particular emphasis on exploring the dependence of the observational properties of these outflows with the inclination of the system. The three studies were published in Astronomy & Astrophysics between 2021 and 2024.