The Pattern of Correlated X-Ray Timing and Spectral Behavior in GRS 1915+105

From data obtained from the PCA in the 2-11 keV and 11-30.5 keV energy range,GRS 1915+105 is seen during RXTE observations between 1996 May and October ontwo separate branches in a hardness intensity diagram. On the hard branch, GRS1915+105 exhibits narrow quasi-periodic oscillations ranging from 0.5 to 6 Hzwith ${\Delta \nu \over \nu} \sim 0.2$. The QPOs are observed over intensitiesranging from about 6,000 to 20,000 counts s$^{-1}$ in the 2 - 12.5 keV energyband, indicating a strong dependence on source intensity. Strong harmonics areseen, especially, at lower frequencies. As the QPO frequency increases, theharmonic feature weakens and disappears. On the soft branch, narrow QPOs areabsent and the low frequency component of the power density spectrum isapproximated by a power-law, with index $\sim -1.25$ for low count rates and$\sim -1.5$ for high count rates ($\gta 18000$ cts/s). Occasionally, a broadpeaked feature in the 1-6 Hz frequency range is also observed on this branch.The source was probably in the very high state similar to those of other blackhole candidates. Thermal-viscous instabilities in accretion disk models do notpredict the correlation of the narrow QPO frequency and luminosity unless thefraction of luminosity from the disk decreases with the total luminosity.Comment: ApJ Lett accepte