Spin-up and spin-down of an accreting compact object

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Abstract
Two-dimensional hydrodynamic calculations of an accretion flow on to a compact object fed by a stellar wind in a close binary system with a mass ratio of unity are performed. We consider the case in which a stellar wind is driven by a thermal pressure. The parameters specifying the problem are the sound speed of gas, c0, at the surface of the mass-losing star and the ratio of specific heats, γ. In the case of γ = 5/3, it is found that the flow is rather unstable when c0 = 1.0–1.4, and a unique solution is not found. In the case of c0 = 1.5, we found a retrograde rotating accretion disc separated by a bow shock from a wind. In cases of c0 > = 2.0, we have a conical accretion shock. In the cases of γ = 4/3, we have a conical accretion shock if c0 > = 1.2. In some models a flow changes its pattern sporadically, and we term it a flip-flop flow. Time histories of a mass accretion rate and an angular momentum accretion rate on to the compact object are monitored. Their behaviour is not steady generally. The accreted angular momentum flux becomes negative in some models. In the flip–flop flow, the angular momentum accreted by the compact object changes it sign sporadically. This phenomenon is consistent with the random-walk model of a period fluctuation of Vela X-1 and 4U1538–52.

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