One-armed spiral structure of accretion discs induced by a phase-dependent mass transfer in Be/X-ray binaries

We study the non-axisymmetric structure of accretion discs in Be/X-ray binaries, performing three-dimensional smoothed particle hydrodynamics simulations for a coplanar system with a short period and a moderate eccentricity. We find that ram pressure due to the phase-dependent mass transfer from the Be-star disc excites a one-armed, trailing spiral structure in the accretion disc around the neutron star. The spiral wave is transient; it is excited around the periastron passage, when the material is transferred from the Be disc, and is gradually damped afterwards. The disc changes its morphology from circular to eccentric with the development of the spiral wave, and then from eccentric to circular with the decay of the wave during one orbital period. It turns out that the inward propagation of the spiral wave significantly enhances the mass-accretion rate on to the neutron star. Thus, the detection of an X-ray luminosity peak corresponding to the peak in enhanced mass-accretion rate provides circumstantial evidence that an accretion disc is present in Be/X-ray binaries.