SPECTRAL AND TEMPORAL VARIABILITY IN THE X-RAY FLUX OF GS 1124(683,

Correlated variations in timing and spectral properties were observed near the peak of the outburst of Nova Muscae 1991 (GS 1124(683). Two regions with di†erent spectral and timing characteristics can be distinguished in color-color and hardness-intensity diagrams. Two types of quasi-periodic oscillations (QPOs), each associated with regions in these diagrams, have been observed. Rapid Nip-Nop transitions sometimes occur between the regions. In one region the QPOsI centroid frequency is a strong, increasing function of X-ray intensity. Regions with similar characteristics and similar forms of QPOs were earlier found in GX 339(4 in its very high state. These two regions may turn out to be common and unique to black hole candidates in their very high state, even though they bear some similarities to the spectral branches in neutron star Z sources. Subject headings: black hole physics E novae, cataclysmic variables E stars: individual (Novae Muscae 1991) E stars: oscillations E X-rays: stars