A walk through the different spectral states of low-mass X-ray binaries

PhD ceremony: Ms. B. Hiemstra, 14.30 uur, Aula Academiegebouw, Broerstraat 5, Groningen

Dissertation: A walk through the different spectral states of low-mass X-ray binaries

Promotor(s): prof. R.M. Méndez

Faculty: Mathematics and Natural Sciences

Low-mass X-ray binaries are systems in which a compact object, a neutron star or a black hole, accretes matter from a normal star that has a mass equal to, or less than that of our Sun. Mass accretion proceeds via a so-called accretion disc in which matter spirals inwards, sometimes down to a few kilometers from the compact object. Internal friction heats up the disc such that it radiates in X-rays. The disc and compact object are surrounded by a hot plasma, the so-called corona, which also radiates in X-rays and gamma-rays. While there is observational evidence (and theoretical expectations) of such a corona, the geometry and origin of the corona remains unclear. X-ray satellites allow us to study the radiation coming from the disc and corona, and hence to explore the compact objects and their close environment. In this thesis I study specific properties of the accretion process in low-mass X-ray binaries. Accretion proceeds in different ways and change with time, giving rise to the so-called spectral states. The geometry and temperature of the accretion disc and corona depend upon (and in fact define) these different states. By walking through various spectral states, this thesis highlights from an observational perspective a few particular aspects of the individual states, which are key for the general understanding of the accretion processes in low-mass X-ray binaries.