M. Vargas

SIGMA/GRANAT Discovery of GRS 1739–278, A Hard X-Ray Transient in the Galactic Bulge

While the SIGMA telescope performed its twelfth observing campaign on the Galactic center region, a new hard X-ray transient source was discovered. The source, named GRS 1739-278, was bright during the whole campaign with a hard spectrum. Thanks to the accurate position provided by the coded-mask imaging technique, the GRS 1739-278 counterparts at other wavelengths have been promptly identified. The collected observations and similarities of the source behaviors with those of Nova Muscae 1991 suggest that GRS 1739-278 may contain a black hole located in the Galactic bulge. The fact that the hard X-ray peak luminosity of GRS 1739-278 is similar to that of GRS 1730-312 (another hard X-ray transient observed by SIGMA in the bulge) leads us to consider the hard X-ray transient peak luminosity as a possible distance indicator.

Properties of the hard X-ray radiation from Cygnus X-l and 1E1740.7 — 2942

The entire dataset of the GRANAT/SIGMA observations of Cyg X-1 and 1E1740.7-2942 in 1990-1994 was analyzed in order to search for correlations between primary observational characteristics of the hard X-ray (40-200 keV) emission - hard X-ray luminosity, hardness of the spectrum (quantified in terms of the best-fit thermal bremsstrahlung temperature kT) and the RMS of short-term flux variations. Although no strict point-to-point correlations were detected certain general tendencies are evident. It was found that for Cyg X-1 the spectral hardness is in general positively correlated with relative amplitude of short-term variability. The correlation of similar kind was found for X-ray transient GRO J0422+32 (X-ray Nova Persei 1992) and recently for GX339-4. For both sources approximate correlation between kT and L_X was found. At low hard X-ray luminosity - below 10E37 erg/sec - the kT increases with increase of L_X. At higher luminosity the spectral hardness depends weaker or does not depend at all on the hard X-ray luminosity. The low luminosity end of these approximate correlations (low kT and low RMS) corresponds to extended episodes of very low hard X-ray flux occurred for both sources during SIGMA observations.

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.

Hard X-ray observations of GRS 1009-45 with the SIGMA telescope

We report on hard X-ray observations of X-ray Nova Velorum 1993 (GRS 1009–45) performed with the SIGMA coded mask X-ray telescope in January 1994. The source was clearly detected with a flux of about 60 mCrab in the 40–150 keV energy band during the two observations with a hard spectrum (α∼−2.1) extending up to ∼150 keV. These observations confirm the duration of the activity of the source in hard X-rays over 100 days after the first maximum and suggest a spectral hardening which has already been observed in Nova Muscae 1991. These and other characteristics found in these observations strengthen the case for this Nova to be a Black Hole candidate similar to Nova Muscae.