RXTE observations of Galactic microquasar XTE J1748--288 during its 1998 outburst
Abstract
We present an analysis of the RXTE observations of recently discovered Galactic microquasar XTE J1748-288 during its 1998 outburst. The spectral evolution of the source during the outburst can be considered as a sequence of three qualitatively distinct states. During the first observations corresponding to the maximum of X-ray flux the spectrum of the source was formed by the sum of the dominating hard power law component and soft thermal component, which can be described by the model of multicolor disk emission. Hard component contributed >=80% to the X-ray luminosity in 3-25 keV energy band. This spectral shape is quite different from spectra of high and low spectral states typical for Galactic black hole systems. Later on, as the X-ray source faded, its energy spectrum qualitatively changed, and first high, then low standard states have been observed. In correlation with the spectral changes the character of fast variability have changed dramatically also. Initially the power density spectrum was formed by the dominating band-limited noise component, QPO features at 20-30 Hz and at ~0.5 Hz, and very low frequency noise component. After transition to high state the amplitude of the fractional variability decreased by order of magnitude and PDS spectrum became of power-law shape with broad QPO peak around 0.03 Hz. When the systems switched to low state, PDS shape changed again and QPOs have not been detected anymore. Clear correlation between QPO parameters and X-ray flux have been found, when the source have been in its initial bright hard state. Such a correlation is of special interest for understanding of the accretion process in X-ray black hole binaries.Keywords
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