Disk Instabilities in the X-Ray Novae

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Session 61 -- Novae and Stellar Accretion
Oral presentation, Tuesday, 10, 1995, 10:00am - 11:30am

[61.03] Disk Instabilities in the X-Ray Novae

J.K. Cannizzo, W. Chen (NASA/GSFC/USRA), M. Livio (STScI)

We perform a parameter study of the accretion disk limit cycle model for the outbursts in the X-ray novae. We delineate a range of allowed parameters that are required to reproduce the observed behavior, and we examine systematic effects involved with isolating one of the control parameters to the theory. We find that for models in which the viscosity parameter $\alpha$ varies in a step function between the low and high states of accretion, there is a tendency for a slower-than-exponential plateau in X-ray flux during the early part of the outburst, followed by a faster-than-exponential decay. The introduction of a radial scaling in $\alpha$ does not modify this picture significantly. Taking $\alpha$ to be a function of the local disk aspect ratio $h/r$ produces outbursts which have an exponential decay as observed, and also leads to a tendency for secondary maxima to occur. These are caused by reflections of the cooling wave during the decay from maximum light and are unrelated to irradiation. In fact, the lack of observed power law decay in the outbursts argues against irradiation as being an important factor. Finally, we require a mechanism extrinsic to the basic model to evacuate mass from the inner disk during quiescence. Without such an agent, the predicted mass accretion rates in quiescence are six to eight orders of magnitude less than observed.

J.K.C. acknowledges the support of a NRC associateship at Goddard Space Flight Center. J.K.C. and W.C. are supported through the Universities Space Research Association.

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