We study the accretion on to the neutron star in Be/X-ray binaries , using a 3D SPH code and the data imported from a simulation by Okazaki et al .
( 16 ) for a coplanar system with a short period ( P _ { orb } = 24.3 d ) and a moderate eccentricity ( e = 0.34 ) , which targeted the Be/X-ray binary 4U 0115+63 .
For simplicity , we adopt the polytropic equation of state .
We find that a time-dependent accretion disc is formed around the neutron star regardless of the simulation parameters .
In the long term , the disc evolves via a two-stage process , which consists of the initial developing stage and the later developed stage .
The developed disc is nearly Keplerian .
In the short term , the disc structure modulates with the orbital phase .
The disc shrinks at the periastron passage of the Be star and restores its radius afterwards .
The accretion rate on to the neutron star is also phase dependent , but its peak is broader and much lower than that of the mass-transfer rate from the Be disc , unless the polytropic exponent is as large as 5/3 .
Our simulations show that the truncated Be disk model for Be/X-ray binaries is consistent with the observed X-ray behaviour of 4U 0115+63 .