Accretion disc boundary layers with white dwarfs

Abstract

The boundary layer between the accretion disc and white dwarf in non-magnetic cataclysmic variables is the major source of X-rays from these systems. Two modes of X-ray emission exist; at low accretion rates a thermal instability develops in the accreting matter and a hard X-ray emitting corona is formed around the white dwarf; at high accretion rates a cool, very soft X-ray emitting boundary layer is formed. Hard and soft X-ray observations of cataclysmic variables with a boundary layer are described and compared with theory. Emphasis is put on the new insight we have gained from the observations with the EXOSAT observatory.