B. S. Novikov

SIGMA detection of hard X-ray emission from the soft transient type I X-ray burster KS 1731-260

During the observations of the Galactic center region performed by the French hard X-ray/soft γ-ray telescope SIGMA, several transient sources were detected. One of them was the soft transient type I X-ray burster KS 1731-260 recently discovered by the TTM experiment aboard the Soviet MIR-KVANT observatory. The SIGMA observations of KS 1731-260 reveal the existence of a hard state for which the spectrum extends up to 150 keV, with a power-law shape having a photon spectral index of ≃2.9 (±0.8)

Sigma hard X-ray observations of GX 339-4 during the hard state and a transition to the soft state

The X-ray system GX 339-4 has been observed (more than 135 hr), between 1990 March and 1991 September, with the SIGMA hard X-ray/soft γ-ray (35 keV-1.3 MeV) imaging telescope on board the GRANAT observatory. In particular, the 1991 outburst has been well covered by SIGMA. The source stayed in a hard state during more than 3 weeks with an intensity greater than 380 mCrab up to 350 keV. Then, the last two observations performed on 1991 September 28 and October 5 revealed that the source had begun a transition from the hard to the soft state, as the intensity in the 35-230 keV band decreased below 100 mCrab. In the hard state, the spectra we observe are usually compatible with a power law of index ∼2.2. During the transition phase, this index rises to ∼3

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 hard X-ray observations of GX339-4

Abstract The X-ray system GX339-4 has been observed on several occasions between March 1990 and September 1991 with the SIGMA hard X-Ray/soft gamma-ray imaging telescope (35 keV–1.3 MeV) on board the GRANAT observatory /1/. This object was detected in hard and soft states, as well as during a transition between these two states. We analyse the spectral and temporal behaviour of this black hole candidate.