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Sergei Nayakshin (NASA/GSFC), Saul Rappaport (CSR, MIT), Fulvio Melia (Pysics Department and Steward Observatory, University of Arizona, Tucson)
During the past few years, the galactic microquasar GRS1915+105 has exhibited a bewildering diversity of large amplitude, chaotic variability in X-rays. We argue that observations clearly require a quasi-stable accretion disk solution at high accretion rates, which rules out the standard Shakura-Sunyaev viscosity prescription. We have therefore devised a modified viscosity law which has a quasi-stable upper branch. We then developed and applied a code to solve the time-dependent equations for an accretion disk. We show that the model does indeed account for a number of the gross observational features of GRS1915, including the overall cyclic behavior on timescales of ~ 100 - 1000 s. On the other hand, the rise/fall time scales are too long and no rapid oscillations on timescales \leq 10 s emerge naturally from the model.
We then consider several effects as a possible explanation for the disagreement between the theory and the observations. A hot corona above the disk, a radius dependent \alpha-parameter, and advection of energy into the hole do not appear to be promising in this respect. Further, we note that whatever the explanation for GRS1915 is, it should be strongly coupled with the spin of the black hole, since few other Galactic accreting black holes, while being close to the Eddington limit, did not show similar chaotic variability. In conclusion, we believe that observations clearly require an S-curve type of instability for GRS1915, which may be due to dependence of the \alpha-parameter on the state of the gas, but it also appears that other effects, probably connected with existence of a jet in this system are important.
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