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 during 1997 summer with the Rossi X-Ray Timing Explorer (RXTE) satellite and with the SIGMA hard X-ray telescope on board the GRANAT observatory. The source was first detected in soft X-rays with RXTE on 1997 July 25 with a rather 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 γ-ray telescope SIGMA during a Galactic center observation. The flux was stronger on September 16 and 17, reaching a level of ~110 mcrab in the 40-80 keV energy band. On the same days, the photon index of the spectrum was determined to be α=-2.3+/-0.9 (1 σ error), while the 40-150 keV luminosity was ~8×10³⁶ ergs s^-1 for a distance of 8.5 kpc. SIGMA and RXTE results indicate that this source had an ultrasoft-like state during its main outburst and a harder secondary outburst in September. These characteristics make the source similar to Nova Muscae 1991, a well known black hole candidate.