Structure and Evolution of the Accretion Flow in Be/X-ray Binaries

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We have studied the accretion flow in Be/X-ray binaries, by performing three dimensional Smoothed Particle Hydrodynamics simulations. To optimize the resolution and computational efficiency of our simulations, we have elected to break the decretion and accretion portions of the computation into two separate, but linked parts. The first focused on the decretion of the Be disk under the combined gravitational influence of the B-star and neutron star. The second part of the simulation focused on the accretion disk around the neutron star, with the phase-dependent mass transfer from the Be disk simulation used as an outer boundary condition for the accretion process. We have obtained the following conclusions.1. A persistent accretion disk is formed around the neutron star, irrespective of the orbital parameters. The accretion disk shrinks at the periastron passage of the Be star and restores its radius afterwards.2. The accretion disk is smaller in the case of a higher orbital eccentricit … More y.3. The ram pressure due to the phase-dependent mass transfer from the Be-star disk excites a one-armed, trailing spiral structure in the accretion disk. The inward propagation of the spiral wave significantly enhances the mass-accretion rate onto the neutron star.4. The accretion disk evolves through three distinct phases. In the first "developing phase", the disk is formed and develops towards a nearly Keplerian disk. In the second "transition phase", the disk is approximately Keplerian and grows with time. The mass-accretion rate increases as the disc grows. In the final quasi-steady state, the mass-accretion rate is on average balanced with the mass-transfer rate from the Be disk and exhibits a regular orbital modulation.5. The mass-accretion rate has a broad peak at a phase significantly later than periastron. This peak stems from the inward-propagating, one-armed spiral wave, and the phase lag results from a combination of the time for wave excitation and the wave propagation time. Less