J. Paul

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

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)

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.)

SIGMA observation of hard X-ray emission from the ultrasoft X-ray transient Triangulum Australe X-1 (A1524 - 62)

While monitoring the region of the sky containing Cir X-1, the SIGMA telescope detected a new hard X-ray sources whose position is compatible with that of TrA X-1 (A1524-62), an ultrasoft X-ray transient discovered 15 years ago by Ariel 5. The SIGMA result supports the idea that TrA X-1 may be powered by accretion onto a black hole, as previously proposed by White et al. (1984, AIP Conf. Proc., 115) on the basis of its soft X-ray spectral properties. Since none of the X-ray all-sky monitors operating at the time of the SIGMA observation has reported a bright state for TrA X-1, the high-energy emission detected by SIGMA seems more likely to be related to a low-intensity outburst, further characterized by a hard spectrum with a weak soft X-ray component

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

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.

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

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

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.