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Description
Gamma-ray binaries are a rare class of high-mass binary systems in which the non-thermal energy spectrum peaks in the gamma-ray regime. PSR B1259-63/LS2883 falls under this rare class and is one of the only three sources where the compact object has been identified as a young pulsar. This system has a 3.4 year orbital period and consists of a 48 ms pulsar and an O9.5 Ve star. The non-thermal emission in the system originates from the shock that forms between the pulsar and stellar winds, preventing accretion onto the system. The orbit of the pulsar around the optical companion is highly eccentric (e ∼ 0.87), resulting in the pulsar moving through the inclined decreation disc of the Oe star around periastron. This phenomenon is observed as an eclipse in the pulsed radio emission, as well as two peaks in the unpulsed radio and X-ray light curves, coinciding with the disc crossings. The system shows variability from one periastron to the next that is proposed to be due to changes in the size and/or structure of the circumstellar disc, which results in a change in the shock structure. In this work SALT optical spectra taken between 2018 and 2021, as well as around the 2024 periastron passage, are used to investigate how the circumstellar disc changes over the orbital phase.