RXTE broad band observations of X-ray Nova XTE J1755-324

Abstract

The properties of X-ray Nova XTE J1755-324, observed with RXTE and GRANAT observatories in 1997 are reported. The lightcurve of the source was typical for X-ray Novae, except for somewhat shorter decay time scale and the time elapsed between primary and subsequent secondary and tertiary outbursts. At the peak of the lightcurve the source had two-component spectrum, typical for the bright X-ray Novae, with disk blackbody temperature Tin~0.8 keV and photon index of the hard power law tail α~ 2.0. The peak luminosity can be estimated as L~10^{38} erg/s (0.1-100 keV, assuming 8.5 kpc distance). A notable and peculiar feature of the spectral evolution of the source was a short, ~10 days long, episode of increased hardness, occurred shortly before the tertiary maximum of the light curve. During the last pointed observations ~100 days after the primary outburst the source has been found in the hard spectral state with luminosity ~fewX10^{36} erg/s. The overall pattern of the temporal and spectral evolution of XTE J1755-324 resemble in general that of ``canonical'' X-ray Novae (e.g. Nova Muscae 1991) and suggests that the compact object in the binary system is a black hole