SIMBAD references

During the past three years, the Galactic black hole microquasar GRS 1915+105 has exhibited a bewildering diversity of large-amplitude, chaotic variability in X-rays. Although it is generally accepted that the variability in this source results from an accretion disk instability, the exact nature of the instability remains unknown. Here we investigate different accretion disk models and viscosity prescriptions in order to provide a basic explanation for some of the exotic temporal behavior in GRS 1915+105. We discuss a range of possible accretion flow geometries. Any geometrically thick disk (e.g., an advection-dominated accretion flow [ADAF] or a ``slim'' accretion disk) has trouble explaining the very long cycle times unless the α-parameter is exceedingly small (∼10<sup>–4</sup>). In addition, the rise/fall timescales in GRS 1915+105 can be a factor of 100 shorter than the cycle times, whereas thick disks predict that these two timescales should be comparable. We thus concentrate on geometrically thin (though not necessarily standard) Shakura-Sunyaev type disks. We argue that X-ray observations clearly require a quasi-stable accretion disk solution at a high accretion rate at which radiation pressure begins to dominate, which excludes the standard α-viscosity prescription. We have therefore devised a simplified model of a disk with a corona and a modified viscosity law that has a quasi-stable upper branch, and we have developed a code to solve the time-dependent equations to study the evolution of this configuration. Via numerical simulations, we show that the model does account for several gross observational features of GRS 1915+105, including its overall cyclic behavior on timescales of ∼100-1000 s. On the other hand, the rise/fall timescales are not as short as those observed, no rapid oscillations on timescales ≲10 s emerge naturally from the model, and the computed cycle-time dependence on the average luminosity is stronger than is found in GRS 1915+105. We then consider, and numerically test, a more elaborate model that includes the ``cold'' disk, a corona, and plasma ejections from the inner disk region that occur when the luminosity of the source is near the Eddington luminosity. The inclusion of a jet allows us to reproduce several additional observed features of GRS 1915+105. We conclude that the most likely structure of the accretion flow in this source is that of a cold disk with a modified viscosity law, plus a corona that accounts for much of the X-ray emission and unsteady plasma ejections that occur when the luminosity of the source is high. The disk is geometrically thin (as required by the data) because most of the accretion power is drained by the corona and the jet.