Compton modelling of spectral variations observed in Z sources.

Abstract

We present a simple model to fit the spectra of Z sources: a neutron star radiating at the centre of a hot scattering cloud. It is shown that the EXOSAT data of GX 17+2, GX 5-1, GX 340+0, and Sco X-2 (GX 349+2) in all branches of their Z profiles are represented well by this model. The neutron star temperature increases from the Horizontal Branch to the Flaring Branch - which is thought to be the direction of increasing mass accretion rate - and is approximately constant on the Normal Branch. The properties of the scattering cloud change along the Z profile in a systematic manner, but the direction and magnitude of these variations are not easy to interpret.

We find that the source flux varies along the Normal Branch by about 20%, which somewhat limits the possibility of using Z sources on this branch as standard candles.

Some properties of the accretion flow are discussed in the framework of recent models of quasi-periodic oscillations on the Normal Branch and of cold, near-critical accretion flows.