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

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

Nov 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999astl...25..718t&link_type=abstract

Astronomy Letters, Volume 25, Issue 11, November 1999, pp.718-738

Astronomy and Astrophysics

Astronomy

36

Scientific paper

We present observations of GRS1915+105 with the instruments of the RXTE (Rossi X-ray Timing Explorer) X-ray Observatory from October 1996 through April 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 from GRS1915+105 and the pattern of their evolution were similar to the properties of other Galactic black-hole candidates in the so-called ``intermediate'' state, which corresponds to a transition between the canonical ``low'' and ``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 temperature of 1-2 keV and a hard component; the latter can be fitted by a power law with a photon index of about 1.8-3.0 and an exponential cutoff at energies 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 1039 to 2 x 1038 erg/s (for the assumed distance of 12.5 kpc to the source). A distinctive feature of the power spectrum of GRS1915+105 during the period under consideration is the presence of a quasi-periodic-oscillation (QPO) peak at frequencies 2-10 Hz. The variations in QPO centroid frequency have been found to correlate with the variations in the temporal and spectral properties of the source's X-ray emission, particularly with the variations in soft-component parameters. Based on our observational data and taking into account the similarity of the salient features in the behavior of GRS1915+105 to the corresponding properties of Galactic black-hole candidates, we may assume that a further decrease in the source's luminosity (below the observed values during the period in question) would result in a transition of GRS1915+105 to a state close to the canonical low state.

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