Accretion Disks in Low Mass X-Ray Binaries

Abstract

The observed properties of accretion disks in LMXRBs (low mass X-ray binaries) are reviewed, at the outside where the disk interacts with the gas stream from the companion, and in the inner region close to the compact object. X-ray orbital modulations from the low mass X-ray binaries, LMXRBs, include eclipses by the companion and/or periodic dipping behaviour from structure at the edge of the disk. The dipping behaviour gives a direct probe into the gas stream/disk interaction. In some high inclination systems the central X-ray emitting compact object is shadowed by the rim of the disk, but X-rays are scattered to the observer via an ADC (accretion disk corona). A sinusoidal-like orbital modulation can be modelled by structure at the disk rim and places tight constraints on how the gas stream merges into the disk. Moving into the central emission region, the observations of the spectra of the LMXRB can be tested against models for the spectra from accretion disks and the interaction with a neutron star. Blackbody disk models do not give a good fit to the spectra. Comptonisation models seem to better represent the data, but currently are rather idealized.