A. Dyachkov

SIGMA/GRANAT soft gamma-ray observations of the X-ray nova in Musca : discovery of positron annihilation emission line

The day after its discovery by the Watch instrument, the X-ray nova GRS 1124-684 in Musca was detected by the soft γ-ray telescope SIGMA at the limit of its field of view. SIGMA pointed the source seven other times between 1991 January and February, and GRS 1124-684 has always been detected up to 300 keV, showing it was one of the hardest objects of the sky. After the flare of January 9 the average spectrum is well fitted by a power law of index 2.38, and the light curve shows a slower decrease than observed at low energy with superposed variability on time scales of several hours

Three years of monitoring GRS 1758-258 : an extremely hard X-ray source near GX 5-1

One of the hardest X-ray sources in the vicinity of the Galactic center, GRS 1758-258, has been monitored by the GRANAT/SIGMA above 35 keV since 1990 spring. The hardness of the observed spectrum (clearly detected up to (200 keV) suggests considering GRS 1758-258 to be a black hole candidate. The range of flux variations on a time scale of about a year, observed by SIGMA, corresponds to factor of more than 10. During all observations in 1990 it was one of the two brightest sources within a few degrees of the Galactic center (1E 1740.7-2942 was another one) with 40-150 keV flux at the level of ∼90 mCrab, while in 1991 fall its flux had declined below the SIGMA detection limit (<9 mCrab, 3 σ upper limit)

Discovery with the Sigma telescope of hard X-rays from the globular cluster Terzan 2

During the survey of the Galactic center performed with the Sigma hard X-ray/soft gamma-ray telescope, a new source has been discovered in the 35−200 keV energy range. The location of the source is consistent with the position of the globular cluster Terzan 2. Although we also discuss other possibilities, it is likely that the hard X-rays emission originates in X1724−308 (4U 1724−30), a source of X-ray bursts located in the globular cluster. Under this assumption, it would be the first detection of a low-mass X-ray binary in this energy range

Low-flux hard state of 1E 1740.7-2942

GRANAT observations of 1E 1740.7-2942 in 1991 October revealed the source at low 40-150 keV flux level, ∼20-30 mCrab (∼1/5 of the typical 1990 value). The source spectrum in the 150-600 keV energy domain exhibits notable excess above a power-law extrapolation of the lower energy part (4-150 keV) of the spectrum. Comparison of the 1991 October spectrum with the «standard» state spectrum observed in 1990 suggests some analogy with γ 2 -γ 1 states transition, observed for the well-known black bole candidate Cygnus X-1 by HEAO 3 (Ling et al.)

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.

On the hard X-ray variability of Centaurus A

The radio galaxy Centaurus A has been observed by the imaging telescope SIGMA on board the GRANAT satellite on three occasions during a 1 yr period. Hard X-ray emission was detected each time at a position compatible with the position of the galaxy and from no other source in the region. A comparison between the observations indicates a flux increase by a factor of 3 over a 1 yr time scale but even more interesting is a similar decrease which was observed in just 4 days; this is the first report of short-time scale variability at hard X-ray frequencies; it agrees well with soft X-ray measurements. If due to a flaring component, we estimate the duration of a typical event to be 8-10 days (rise and decay time) with a frequency of ∼45 events per year

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 observation of two quasars : 3C 273 and 0241+62

The quasar 3C 273 has been observed with the imaging telescope SIGMA on board the GRANAT satellite on four occasions during 1990 for a total exposure time of 62 hr. In the first two sessions in the summer of 1990 the source was detected, at the expected position, at 4 and 5.4 σ confidence levels in the 40-120 keV energy band, while a marginal detection (≤2.5 K) was obtained 4 months later. Comparison between observations indicates a marginally significant flux variation (3 σ) of a factor of ∼2 in a time scale as short as 41 days; this is the first suggestion of a short time variation at hard X-ray frequencies, and it agrees very well with similar measurements in soft X-ray

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.

A high-sensitivity search for gamma-ray bursts by the SIGMA telescope on board GRANAT (the Galactic center and all sky data)

During more than 2 years of the GRANAT mission the gamma-ray burst (GRB) cell of the narrow aperture coded mask telescope SIGMA was operating for ∼5300 hr. The coding system of SIGMA is capable to localize with arcmin accuracy the GRB events detected through the 18:1 by 16:8 coded vield of view. The sensitivity of the cell to the on-axis events is very high, ∼3×10 −8 and 8×10 −8 ergs cm −2 in the 40-90 keV band for 0.25 and 2.0 s of integration time. Nevertheless, no bursts were actually found within the coded field of view