P. Mandrou

SIGMA: The hard X-ray and soft gamma-ray telescope on board the GRANAT space observatory

Abstract The SIGMA telescope, the largest French scientific space-payload ever launched, is one of the main devices aboard the Soviet astronomy satellite GRANAT, successfully launched on December 1, 1989 from Baikonour, USSR. This high-energy space-telescope of unprecedented size, has been designed to produce high-resolution images of the hard X-ray and soft gamma-ray sky, in the energy range from 35 KeV to 1.3 MeV. After a comprehensive description of the instrument, a report is given on the most relevant characteristics of the telescope, including preliminary results from in-flight calibrations performed in the course of bright source observations.

Three spectral states of 1E 1740.7-2942 : from standard Cygnus X-1 type spectrum to the evidence of electron-positron annihilation feature

The source 1E 1740.7-2942 is known to be the brightest hard X-ray source close to the dynamic center of our Galaxy. Three apparently different spectral states of this source were detected by the GRANAT observatory during 1990-1991 observations of the Galactic Center (GC) region. In almost all 1990 observations the source had Cyg X-1-like spectrum with nearly constant flux. The hardest of the states (observed on 1990 October 13-14) exhibits a prominent high-energy bump on the spectrum at 300-600 keV, probably related to the annihilation processes in relatively cold electron-positron plasma

Gamma-ray continuum emission from the inner galactic region as observed with INTEGRAL/SPI

The diffuse continuum emission from the Galactic plane in the energy range 18–1000 keV has been studied using 16 Ms of data from the SPI instrument on INTEGRAL. With such an exposure we can exploit the imaging properties of SPI to achieve a good separation of point sources from the various diffuse components. Using a candidate-source catalogue derived with IBIS on INTEGRAL and a number of sky distribution models we obtained spectra resolved in Galactic longitude. We can identify spectral components of a diffuse continuum of power law shape with index about 1.7, a positron annihilation component with a continuum from positronium and the line at 511 keV, and a second, roughly power-law component from detected point sources. Our analysis confirms the concentration of positron annihilation emission in the inner region (|l| < 10 ◦ ), the disk (10 ◦ < |l| < 30 ◦ ) being at least a factor 7 weaker in this emission. The power-law component in contrast drops by only a factor 2, showing a quite different longitude distribution and spatial origin. Detectable sources constitute about 90% of the total Galactic emission between 20 and 60 keV, but have a steeper spectrum than the diffuse emission, their contribution to the total emission dropping rapidly to a small fraction at higher energies. The spectrum of diffuse emission is compatible with RXTE and COMPTEL at lower and higher energies respectively. In the SPI energy range the flux is lower than found by OSSE, probably due to the more complete accounting for sources by SPI. The power-law emission is difficult to explain as of interstellar origin, inverse Compton giving at most 10%, and instead a population of unresolved point sources is proposed as a possible origin, AXPs with their spectral hardening above 100 keV being plausible candidates. We present a broadband spectrum of the Galactic emission from 10 keV to 100 GeV.

INTEGRAL/SPI ground calibration

Three calibration campaigns of the spectrometer SPI have been performed before launch in order to determine the instrument characteristics, such as the effective detection area, the spectral resolution and the angular resolution. Absolute determination of the effective area has been obtained from simulations and measurements. At 1 MeV, the effective area is 65 cm^2 for a point source on the optical axis, the spectral resolution ~2.3 keV. The angular resolution is better than 2.5 deg and the source separation capability about 1 deg. Some temperature dependant parameters will require permanent in-flight calibration.

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)

The SIGMA mission on the GRANAT satellite

Abstract The soviet spacecraft GRANAT, to be launched in july 1989, will carry seven experiments dedicated to the hard X-ray/Low energy gamma ray astronomy. After a brief description of the mission and of the overall flight hardware, we shall focus on the SIGMA experiment. This french telescope designed to obtain high sensitivity images of the sky in the 30 – 1300 keV energy range will be extensively described. The main scientific objectives of the instrument will also be discuss.

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.)