Model for Relaxation Oscillations of a Luminous Accretion Disk in GRS 1915+105: Variable Inner Edge

Abstract

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 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 Tin (the maximum disk temperature) and Rin (the innermost radius of the disk). We find an abrupt, transient increase in Tin and a temporary decrease in Rin 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 Tr = -αΠ(pgas/p)μ, with Π being the height-integrated pressure, α and μ being the parameters, and p and pgas being the total pressure and gas pressure on the equatorial plane, respectively. The model with μ = 0.1 can produce the overall time changes of Tin and Rin but cannot give an excellent fit to the observed amplitudes. The model with μ = 0.2, on the other hand, gives the right amplitudes, but the changes of Tin and Rin 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.