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Session 35 - Galactic X-Ray (XTE) Sources.
Display session, Tuesday, January 14
Metropolitan Ballroom,
From data obtained from the PCA in the 2-10 keV and 10-30 keV energy range, GRS 1915+105 is seen during RXTE observations between May and October 1996 on three separate branches in a hardness intensity diagram. On the hard branch, GRS 1915+105 exhibits narrow quasi-periodic oscillations ranging from 0.5 to 6 Hz with \Delta \nu øver \nu \sim 0.2. The QPOs are observed over intensities ranging from about 6,000 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, QPOs are absent and the low frequency component of the power density spectrum is approximated by a power-law (with index \sim -1.25). Occasionally, a broad peaked feature in the 1-6 Hz frequency range is also observed on this branch. On the intermediate branch, the power density spectrum is similar in kind to that on the soft branch, but differs in degree since the low frequency component is steeper with a power law index of \sim -1.5. The source was probably in the very high state similar to those of other black hole candidates. Thermal-viscous instabilities in accretion disk models do not predict the correlation of frequency and luminosity unless the fraction of luminosity from the disk decreases with the total luminosity.
