2006MNRAS.372.1140H

We numerically study the long-term evolution of the accretion disc around the neutron star in a coplanar Be/X-ray binary with a short period and a moderate eccentricity. From three-dimensional smoothed particle hydrodynamics simulations, we find that the disc evolves through three distinct phases, each characterized by different mass-accretion patterns. In the first `developing phase', the disc is formed and develops towards a nearly Keplerian disc. It has a relatively large, double-peaked mass accretion rate with the higher peak caused by the direct accretion at periastron, which is followed by the lower peak that is caused by the accretion induced by a one-armed spiral wave. In the second `transition phase', the disc is approximately Keplerian and grows with time. The mass-accretion rate increases as the disc grows. In the second phase, there is a transition in the mass-accretion rate from a double-peaked to a single-peaked pattern. In the final quasi-steady state, the mass-accretion rate is on average balanced with the mass-transfer rate from the Be disc and exhibits a regular orbital modulation. In the quasi-steady state, the mass accretion rate has a single peak that is caused by the wave-induced accretion as in a later stage of the transition phase. The orbital modulation of X-ray maxima could provide not only circumstantial evidence for the persistent disc but also an observational diagnosis of the disc evolutionary state.