R. Kremnev

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)

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.)

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

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

The diffuse galactic continuum emission detected by SIGMA below 1 MeV

Abstract Two cylindrical interruptions of the SIGMA telescope passive shield act as collimators projecting arc-shaped images on the detector. The field of view (FOV) of these collimators produces significant sidelobes in the telescope sensitivity diagram. The imaging properties of SIGMA exploiting these sidelobes enable spatial analysis of the diffuse galactic continuum in the 35–600 keV energy range by means of model fitting. A model, based on the spatial distribution of atomic and molecular gas of the Galaxy, is considered. The obtained spectrum of the diffuse γ-ray emission is then compared to theoretical predictions of various contributions in which orthopositronium and cosmic-ray electrons are involved.

Sigma upper limits to the hard X-ray/soft gamma-ray emission from the millisecond pulsars of the nearby globular cluster 47 Tucanae

We report upper limits to the hard X-ray/soft γ-ray emission from the millisecond pulsars (MSPs) of the nearby globular cluster 47 Tucanae. The observations have been performed by the French coded-mask imaging telescope SIGMA aboard the Soviet GRANAT spacecraft. From observations accumulated from 1990 April to 1992 May, the 2 σ upper limits to the 40-100, 100-300, 300-600, 600-900, and 900-1200 keV fluxes are 8.8×10 −4 , 6.5×10 −4 , 13.2×10 −4 , 19.0×10 −4 , and 22.4×10 −4 photons cm −2 s −1 , respectively. These fluxes correspond to luminosities ranging from ≃2×10 3.5 ergs s −1 in the 40-100 keV band to ≃7.5×10 36 ergs s −1 in the band centered around 1 MeV

SIGMA/GRANAT observations of hard X-ray emission from type I X-ray bursters

Unlike black hole candidate systems, accreting neutron stars seem to encounter appreciable difficulties in emitting strong hard X-ray fluxes. However, in the catalogue of the hard X-ray sources detected by SIGMA, three sources are associated with type I X-ray bursters. In this paper, we review the present status of the SIGMA observations of these three X-ray burst sources, namely X 1724-308 in the globular cluster Terzan II, KS 1731-260, and GX 354+0.

SIGMA observations of two new hard X-ray and soft gamma-ray transients: GX 354-0 and Nova Persei

Abstract We report the first detection of two soft gamma ray transients GX 354-0 and Nova Persei (GRO J0422+32) by the coded-mask telescope SIGMA. Only preliminary results are presented with regard to Nova Persei while special emphasis is given to the data on GX 354-0 which has been monitored during the 1992 February–April survey of the Galactic Center. GX 354-0 underwent two flares of about two week duration and its average spectrum is well described by a thermal Bremsstrahlung model or a broken power law whereas the Nova Persei spectrum seems to be similar to the one observed for GRS 1124-684 in Musca.

GRANAT/SIGMA observations of X-ray Nova Persei 1992

Abstract We report on the results of the GRANAT/SIGMA observations of the X-ray transient GRO J0422+32 in the 35–1300 keV energy band in 1992–1993. In the maximum of the light curve, this black hole candidate was the brightest hard X-ray source detected by SIGMA so far. During the monitoring of the GRO J0422+32 outburst, SIGMA detected a gradual hardening of the source spectrum. In terms of thermal emission of optically thin plasma this hardening corresponds to an increase of the best-fit temperature from ∼ 110 keV to ∼ 140 keV. Source spectrum is well represented by two component Comptonized disk model /9/. The parameters of the model do not exhibit any statistically significant variation from the average value of (α; kT e ) of (0.737 ± 0.01; 25.4 ± 0.05) for the softer component and (0.39 ± 0.09; 62.6 ± 2.4) for the harder (with τ disk ∼ 1.8 for both components). Nevertheless relative normalizations of the two components are changing during our observations by a factor of two. The harder component is fluctuating on a day time-scale and a growth of its contribution to the total flux is accompanied by an increase in the fractional RMS [0.001–0.1 Hz].