On the neutron star-disc interaction in Be/X-ray binaries

We have investigated the long-term X-ray variability, defined as the root-mean-square (rms) of the All Sky Monitor Rossi X-ray Timing Explorer (ASM RXTE) light curves, of a set of galactic Be/X-ray binaries and searched for correlations with system parameters, such as the spin period of the neutron star and the orbital period and eccentricity of the binary. We find that systems with larger rms are those harbouring fast-rotating neutron stars, low eccentric and narrow orbits. These relationships can be explained as the result of the truncation of the circumstellar disc. We also present an updated version of the H alpha equivalent width-orbital period diagram, including sources in the Small Magellanic Cloud (SMC). This diagram provides strong observational evidence of the interaction of neutron star with the circumstellar envelope of its massive companion.