GRANAT and RXTE observations of the X-ray Nova XTE J1755-324

The SIGMA/GRANAT observations of the Galactic-center region in the fall of 1997 and the GRANAT and RXTE observations of the soft X-ray nova XTE J1755- 324 in 1997 are presented. The Light curve of XTE J1755-324 is typical of X -ray novae, with the exception of a slightly shorter e-folding decline time of the X-ray flux and the time between the primary and subsequent maxima o f the light curve. At maximum light, the source exhibited a two-component s pectrum, which is typical of X-ray novae, with a temperature of the inner e dge of the optically thick region in the accretion disk T-in similar to 0.8 keV and a photon index of the hard spectral component alpha similar to 2.0 . For an assumed distance of 8.5 kpc to XTE J1755-324, its maximum 0.1-100- keV luminosity is estimated to be L-x similar to 10(38) erg s(-1). An inter esting feature of the source's spectral evolution is a short period of incr ease in its hardness, similar to 10 days, before the tertiary maximum of th e X-ray light curve. During this event, the source was detected in hard X-r ays by the SIGMA/GRANAT telescope. During the last RXTE observations, the s ource was in a low/hard spectral state with a luminosity L similar to 2 x 1 0(36) erg s(-1) within similar to 100 days after the outburst onset. The te mporal and spectral evolution of XTE J1755-324 resembles in pattern the evo lution of "canonical" X-ray novae, such as the X-ray nova Muscae 1991. This suggests that the compact object in this system is a black hole.