V. Kovtunenko

LMXBs and black hole candidates in the Galactic Center region

Abstract The central part of the Galactic Plane has been intensively observed by GRANAT/SIGMA in 1990–1994. At least 11 sources are seen on the averaged 35–100 keV image of this region, most of them being low mass X-ray binaries. With GRANAT/SIGMA observations the X-ray bursters are recognized as sources emitting hard X-rays (up to ∼ 100 keV) during substantial fraction of time. This “bimodal” behavior (soft and hard spectral states) of X-ray bursters resembles that of black hole candidates (BHC), although unlike BHCs soft state for the bursters seems to be the dominant one. On average the spectra of bursters in the SIGMA energy band (above 35 keV) were found to be steeper than that of BHCs. This difference is discussed in terms of Comptonization model.

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 observations of the low mass X-ray binaries of the Galactic Bulge

Abstract The soft gamma-ray telescope (35–1300 keV) SIGMA aboard the high energy GRANAT space observatory has been monitoring the Galactic Bulge region for more than 2000 h of effective time since March 1990. In the resulting average 35–75 keV image we detected ten sources at a level of > 5 standard deviations, 6 of which can be identified with low mass X-ray binaries (LMXB). Among them, one is the 1993 X-ray nova in Ophiuchus (GRS 1726-249), one is an X-ray pulsar (GX 1+4), two are associated with X-ray bursters (GX 354-0 and A 1742-294) and two with bursting X-ray binaries in the globular clusters Terzan 2 and Terzan 1. Their spectral and long term variability behavior as measured by SIGMA are presented and discussed.

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].

Hard X-ray spectral properties and discovery of narrow annihilation line in the spectrum of Nova Muscae

Abstract The observations of X-ray Nova in Musca (GRS1124-684) by two coded mask telescopes on board GRANAT observatory provided spectral data in broad 3 – 1300 keV band. During these observations, spanned over a ∼year, the Nova was detected in a three apparently different spectral states, corresponding to different epochs of the soft X-ray light curve: (1) A spectrum with two distinct components (soft, below ∼ 8 keV and hard power law tail with slope ∼ 2.5, detected up to ∼ 300 keV). The soft emission changed gradually with characteristic decay time around 30 days, while power law component exhibited strong variability on the time scales of several hours and decreased much more slowly. (2) A soft spectrum (without hard power law tail), observed during the “kick” of the soft X-ray light curve. (3) A hard power law spectrum with slope ∼ 2.2. Thus, while the 3 – 300 keV luminosity decreased by more than order of magnitude, the source passed through all spectral states known for galactic black hole candidates (Cyg X-1, GX339-4, 1E1740.7-2942, GRS1758-258 etc.). On January 20–21 1991, the SIGMA telescope aboard GRANAT detected a relatively narrow variable emission line near 500 keV (Fig.1,2) with net flux ≈ 6 · 10 −3 phot/s/cm 2 , most probably related with electron-positron annihilation processes, occurring in the source /1–4/. Additional excess above power law continuum, centered around ∼ 200 keV, was found during this observation.

1E 1740.7-2942 revisited: Confirmation of its identification with the core of a double sided radio jet structure

Abstract After more than two years of successful in-orbit operations, the γ-ray coded aperture SIGMA telescope has accumulated 800 hours of live-time observations of the Galactic Center region, including the remarkable hard source identified with the X-ray source 1E 1740.7–2942. The long-term behavior of the soft γ-ray emission of 1E 1740.7–2942, as determined from the SIGMA survey, supplemented with previously available soft γ-ray data, leads to its identification with a singular radio source, which consists of a double sided radio jet emanating from a compact variable core whose variability is correlated with that of the soft γ-ray source. The compact radio core, which lies well inside the improved soft γ-ray (40–150 keV) error circle (27″ radius) derived from the high-resolution SIGMA survey, is also inside the ROSAT and TTM error circle derived respectively in the soft and hard X-ray bands.

SIGMA observations of hard X-ray and soft gamma-ray emission from X-ray binaries

Abstract After more than two years of operation, the imaging γ-ray SIGMA telescope has accumulated several days of observation toward well known X-ray binaries. Four bright sources falling in this category have been detected so far: The pulsar GX 1+4 near the center of our galaxy, the stellar wind accreting system 4U 1700-377, and the black hole candidates Cygnus X-1 and GX 339-4. Moreover, SIGMA have observed three transients sources, which turned out to be also hard X-ray sources : The burster KS 1731-260, Tra X-1, and the Musca Nova. The properties of these systems in the SIGMA domain will be reviewed and a spectral distinction between black holes and neutron stars will be sketched.

High angular resolution observations in the soft gamma-ray band with sigma

Abstract The launch in December 1989, of SIGMA, one of the main devices aboard the GRANAT spacecraft, has provided high-energy astronomers for the first time with a telescope whose imaging properties in the soft γ-ray regime match those of instruments operating in the hard X-ray band. Having examined ∼ 300 celestial fields during more than two years of successful in-orbit operations, this coded-aperture telescope, sensitive to radiation in the energy range 35 keV to 1.3 MeV, has succeeded both in disentangling severe source confusion problems and in providing firm identifications and reliable spectra of sources at soft γ-ray wavelengths. A selection of the salient scientific results obtained so far is presented, with intent to emphasize the unique contribution of a coded-mask telescope able to perform accurate images of the sky in the soft γ-ray domain.