Speaker
Description
Be X-ray binaries (BeXBs) consist of a compact object, in nearly all known cases a neutron star, in orbit around a massive Oe/Be companion that hosts a quasi-Keplerian equatorial decretion disc. This disc is the reservoir from which the compact object accretes, and its geometry, density and dynamical state set the conditions for X-ray outburst triggering, amplitude and recurrence. Despite decades of study, the physical conditions that produce Type II (giant) outbursts and long-term superorbital variability remain unresolved, largely because the disc and the X-ray behaviour are seldom tracked at the same time over the multi-year timescales on which the disc actually evolves.
We are carrying out long-term multi-wavelength monitoring of a sample of BeXBs spanning a wide range of orbital periods, eccentricities and inclinations. The programme combines MeerLICHT optical photometry in six bands (ugrizq), SALT spectroscopy of Hα and other disc diagnostics, and publicly available X-ray monitoring data. Together these trace the structural and thermal evolution of the disc from months to years, so that changes in disc size, density and state, including build-up, warping, precession and dissipation, can be linked causally to accretion episodes and to non-thermal emission from the compact object. The resulting dataset will be used to test theoretical models of disc-driven outbursts and superorbital periodicities. Here I present the study sample and first results.