Numerical Simulations of Accretion Flows onto a Compact Object in a Close Binary System

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Numerical simulations on an accretion flow and a related problem were performed. There are two types of accretion flow in a close binary system depending on the mangitude of the angular momentum of the accreting gas: a Roche lobe overflow and a wind-fed accretion. In the former case we have pointed out the existence of spiral shock waves on an accretion disc. In the present work our numerical results show good agreement with the anlytic solution obtained by Spruit. According to his theory there is a close relation between the pitch angle of the spiral shocks and the specific heat ratio. The smaller the specific heat ratio, the smaller the pitch angle. We have shown that accretion can occur even without a viscosity in some cases.Relating to the problem of the accretion, we have investigated the problem of spiral arm formation in a disc galaxy. In our view, the cause of spiral arms are a deviatin of gravitational force from the exact axisymmetry. In the accretin disc very similar phenomenon occurs due to the tidal force.We have also investigated the wind-fed accretion and the angular momentum accretion on to a compact object in a close binary system. We found that the flow is unsteady and the sign of the angular momentum of the accreting gas changes randomly with time. Although this is an unexpected result, Taam & Fryxcell in USA recently confirmed our results by their extensive numerical simulations. We have very new picture on the spin-up/down phenomena of a compact object. Our results can explain observations well.