Be/X-ray transients at low X-ray luminosity

Accretion is one of the most fundamental mechanisms found in our physical Universe. The study of this phenomenon helps us to understand the behaviour of matter under extreme environments present around compact objects, either neutron stars or black holes. Binary systems are normally comprised of Sun-like stars (called companions) gravitationally bound to core-collapsed objects (neutron stars or black holes). These systems are perfect laboratories to test the accretion mechanism as the compact objects accrete matter from their stellar companions. During the accretion process, these systems become bright in X-rays, giving rise to their name – X-ray binaries. In neutron star X-ray binaries, the neutron star gets heated up during accretion, and even after this process halts, the system may still be visible in X-rays as the neutron star cools down by emitting radiation. The study of such radiation provides us with a unique opportunity to test the properties of ultra-dense matter in neutron stars, more specifically in their crusts.
In this thesis, I present the results of my research focused on the behaviour of neutron star high-mass X-ray binaries, in particular Be/X-ray transients, at low X-ray luminosities, that is, when accretion only occurs at very low levels or has even halted. These systems harbour highly magnetised neutron stars that orbit around their massive Be-type stellar companions from which these compact objects accrete matter. The main aim of my research has been to investigate the physical processes at work during accretion of matter at very low level onto magnetised neutron stars. In addition, I have studied how the presence of strong magnetic fields alters the heating and cooling crust properties of accreting neutron stars.