Model for relaxation oscillations of a luminous accretion disk in GRS 1915+105: Variable inner edge

To understand the bursting behavior of the microquasar GRS 1915+105, we calculate time evolution of a luminous, optically thick accretion disk around a stellar-mass black hole undergoing limit-cycle oscillations between the high- and low-luminosity states. Specifically, we carefully solve the behavior of the innermost part of the disk, since it produces significant number of photons during the burst and fit the theoretical spectra with the multicolor disk model. The fitting parameters are T-in (the maximum disk temperature) and R-in (the innermost radius of the disk). We find an abrupt, transient increase in T-in and a temporary decrease in R-in during a burst, which are actually observed in GRS 1915+105. The precise behavior is subject to the viscosity prescription. We prescribe the radial-azimuthal component of viscosity stress tensor to be T-rphi = -alphaPi(p(gas)/p)(mu), with Pi being the height-integrated pressure, alpha and mu being the parameters, and p and p(gas) being the total pressure and gas pressure on the equatorial plane, respectively. The model with mu=0.1 can produce the overall time changes of T-in and R-in but cannot give an excellent fit to the observed amplitudes. The model with mu=0.2, on the other hand, gives the right amplitudes, but the changes of T-in and R-in are smaller. Although precise matching is left as future work, we may conclude that the basic properties of the bursts of GRS 1915+105 can be explained by our "limit-cycle oscillation'' model. It is then required that the spectral hardening factor at high luminosities should be about 3 at around the Eddington luminosity instead of less than 2 as is usually assumed.