The X-ray source GRS 1915+105: The low-luminosity state and transitions between the states during 1996-1997 (RXTE observations)

We present observations of GRS 1915+105 with the instruments of the RXTE (R ossi X-ray Timing Explorer) X-ray Observatory from October 1996 through Apr il 1997, when the source exhibited transitions between the states with high and low X-ray luminosities. With the exception of some individual features , the general temporal and spectral characteristics of the X-ray emission f rom GRS 1915+105 and the pattern of their evolution were similar to the pro perties of other Galactic black-hole candidates in the so-called "intermedi ate" state, which corresponds to a transition between the canonical "low" a nd "high" states. The source's 3-150-keV energy spectrum can be represented as the sum of a relatively weak soft component with a characteristic tempe rature of 1-2 keV and a hard component; the latter can be fitted by a power law with a photon index similar to 1.8-3.0 and an exponential cutoff at en ergies similar to 60-120 keV. The temporal evolution of the energy spectrum is characterized by a gradual increase in its hardness as the total X-ray luminosity decreases from similar to 10(39) to similar to 2 X 10(38) erg s( -1) (for the assumed distance of 12.5 kpc to the source). A distinctive fea ture of the power spectrum of GRS 1915+105 during the period under consider ation is the presence of a quasi-periodic-oscillation (QPO) peak at frequen cies similar to 2-10 Hz. The variations in QPO centroid frequency have been found to correlate with the variations in the temporal and spectral proper ties of the source's X-ray emission, particularly with the variations in so ft-component parameters. Based on our observational data and taking into ac count the similarity of the salient features in the behavior of GRS 1915+10 5 to the corresponding properties of Galactic black-hole candidates, we may assume that a further decrease in the source's luminosity (below the obser ved values during the period in question) would result in a transition of G RS 1915+105 to a state close to the canonical low state.