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-Ra y Timing Explorer observations between 1996 May and October on two sep arate branches in a hardness-intensity diagram. On the hard branch, GR S 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 observe d over intensities ranging from about 6000 to 20,000 counts s(-1) in t he 2-12.5 keV energy band, indicating a strong dependence on source in tensity. Strong harmonics are seen, especially at lower frequencies. A s the QPO frequency increases, the harmonic feature weakens and disapp ears. On the soft branch, narrow QPOs are absent and the low-frequency component of the power density spectrum is approximated by a power la w, with index of about -1.25 for low count rates and -1.5 for high cou nt rates (greater than or similar to 18,000 counts s(-1)). Occasionall y, a broad, peaked feature in the 1-6 Hz frequency range is also obser ved on this branch. The source was probably in a very high state simil ar to those of other black hole candidates. Thermal-viscous instabilit ies in accretion disk models do not predict the correlation of the nar row QPO frequency and luminosity unless the fraction of luminosity fro m the disk decreases with the total luminosity.