RXTE observations of Galactic microquasar XTE J1748-288 during its 1998 outburst

Abstract

We present an analysis of the RXTE observations of the recently discovered Galactic microquasar XTE J1748-288 during its 1998 outburst. General spectral and temporal properties of the source and their evolution were very typical for the Galactic black hole candidates (BHC) and, in particular, black hole X-ray novae. The spectral evolution of the source during the outburst can be considered a sequence of qualitatively distinct states. During the first observations, corresponding to a maximum of the X-ray flux, the spectrum of the source consisted of a dominating hard power-law component and a soft thermal component, which can be described by a model for the multicolour-disc emission. The hard component contributed ≥80 per cent to the X-ray luminosity in the 3–25 keV energy band. Overall the two-component spectral shape is an attribute of the very high state observed previously in BHC, but the domination of the hard component is unusual. Later on, as the X-ray source faded, its energy spectrum qualitatively changed, showing high and then low states, both typical for black hole binaries. As the energy spectrum changed, the fast variability also evolved dramatically. Initially the power-density spectrum was formed by a dominating band-limited noise component, quasi periodical oscillation features at 20–30 Hz and at |0.5 Hz, and a very low-frequency noise component. After a significant decrease of the contribution of the hard spectral component, the amplitude of the fractional variability decreased by an order of magnitude and the power-density spectrum adopted a power-law shape with a broad quasi periodical oscillation peak around 0.03 Hz. When the system switched to the low state, the power-density spectrum shape changed again and the quasi periodical oscillations have not been detected since. When the source was observed in the very high state, a clear correlation between quasi periodical oscillation parameters and X-ray flux was seen. Such a correlation gives an insight into our understanding of the accretion process in X-ray black hole binaries.