Sigma and RXTE observations of the soft X-ray transient XTE J1755-324

We present observations of the X-ray transient XTE J1755-324 performed duri ng 1997 summer with the Rossi X-Ray Timing Explorer (RXTE) satellite and wi th the SIGMA hard X-ray telescope on board the GRANAT observatory. The sour ce was first detected in soft X-rays with RXTE on 1997 July 25 with a rathe r soft X-ray spectrum, and its outburst was monitored in soft X-rays up to 1997 November. On September 16 it was first detected in hard X-rays by the French soft gamma-ray telescope SIGMA during a Galactic center observation. The flux was stronger on September 16 and 17, reaching a level of similar to 110 mcrab in the 40-80 keV energy band. On the same days, the photon ind ex of the spectrum was determined to be alpha = -2.3 +/- 0.9 (1 sigma error ), while the 40-150 keV luminosity was similar to 8 x 10(36) ergs s(-1) for a distance of 8.5 kpc. SIGMA and RXTE results indicate that this source ha d an ultrasoft-like state during its main outburst and a harder secondary o utburst in September. These characteristics make the source similar to Nova Muscae 1991, a well known black hole candidate.