RXTE Spectral Observations of the 1996-97 Outburst of the Microquasar GRO J1655-40

Excellent coverage of the entire 16-month 1996-97 outburst cycle of GRO J1655-40 was provided by RXTE. We present a full spectral analysis of these data, which includes 52 PCA spectra from 2.5-20 keV and HEXTE spectra above 20 keV. The source exhibits two principal outburst states which we associate with the very high and the high/soft states. During the very high state, the spectrum is often dominated by a power-law component with photon index 2.3-2.7. The source exhibits intense hard flares which are correlated with changes in both the temperature and radius of the inner accretion disk on time scales of hours to days. During the high/soft state, the spectrum is dominated by the soft thermal emission from the accretion disk with approximately constant inner disk radius and temperature. During the last few observations, the source undergoes a transition to the low/hard state. The principal discovery of this work is the tight relationship between the inner radius of the disk and the flux in the power-law component. The correlation between the inner disk radius and power-law flux suggests that the physical mechanism responsible for producing the power-law emission is linked to the inner disk region. Other results include the following: The 300 Hz QPO (Remillard et al. 1998) is present over a range of inner disk radii; therefore, it cannot be due to the precession frequency of the inner disk at the last stable orbit. We confirm that GRO J1655-40 contains a rapidly rotating black hole with a_* > 0.9. We also find that the 14-28 Hz QPO frequency decreases as the inner disk moves inward, which is contrary to the behavior expected if the QPO were tied to the Keplerian frequency at the shrinking inner edge of the disk