The pattern of correlated X-ray timing and spectral behavior in GRS 1915+105

From data obtained from the proportional counter array in the 2-11 keV and 11-30.5 keV energy ranges, GRS 1915+105 is seen during Rossi X-Ray Timing Explorer observations between 1996 May and October on two separate branches in a hardness-intensity diagram. On the hard branch, GRS 1915+105 exhibits narrow quasi-periodic oscillations (QPOs) ranging from 0.5 to 6 Hz with Delta nu/nu similar to 0.2. The QPOs are observed over intensities ranging from about 6000 to 20,000 counts s(-1) in the 2-12.5 keV energy band, indicating a strong dependence on source intensity. Strong harmonics are seen, especially at lower frequencies. As the QPO frequency increases, the harmonic feature weakens and disappears. On the soft branch, narrow QPOs are absent and the low-frequency component of the power density spectrum is approximated by a power law, with index of about -1.25 for low count rates and -1.5 for high count rates (greater than or similar to 18,000 counts s(-1)). Occasionally, a broad, peaked feature in the 1-6 Hz frequency range is also observed on this branch. The source was probably in a very high state similar to those of other black hole candidates. Thermal-viscous instabilities in accretion disk models do not predict the correlation of the narrow QPO frequency and luminosity unless the fraction of luminosity from the disk decreases with the total luminosity.