G. Vedrenne

A Catalog of Cosmic Gamma-Ray Bursts Detected by the PHEBUS Instrument on the Granat Observatory: October 1994-December 1996

We present the final part of the catalog of cosmic gamma-ray bursts (GRBs) observed in the PHEBUS experiment on the Granat orbiting astrophysical observatory. The first three parts of the catalog were published by Terekhov et al. (1994, 1995a) and Tkachenko et al. (1998). The fourth part contains information on 32 events recorded from October 1994 until December 1996. We give burst light curves in the energy range 100 keV to 1.6 MeV, integrated energy spectra, and information on the fluence and energy flux at the luminosity peak for energies above 100 keV. Over the entire period of its operation, the PHEBUS instrument detected 206 cosmic GRBs. The mean ‹V/Vmax› was 0.336±0.007. The mean hardness corresponding to the ratio of count numbers in the energy ranges 400–1000 and 100–400 keV is 0.428±0.018 for events with a duration shorter than 2 s and 0.231±0.004 for events with a duration longer than 2 s.

First observations with the sigma telescope

Abstract The SIGMA telescope has been launched on December 1st 1989. After a calibration period, January and February 1990, the scientific observations started. In this paper, we review the first measurements obtained with SIGMA, and we give the preliminary highlights results obtained in the field of hard X-Ray/Gamma-Ray astronomy.

A space bourne crystal diffraction telescope for the energy range of nuclear transitions

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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.

Experimental results obtained with the positron-annihilation- radiation telescope of the Toulouse-Argonne collaboration

We present laboratory measurements obtained with a ground-based prototype of a focusing positron-annihilation-radiation telescope developed by the Toulouse-Argonne collaboration. This balloon-borne telescope has been designed to collect 511-keV photons with an extremely low instrumental background. The telescope features a Laue diffraction lens and a detector module containing a small array of germanium detectors. It will provide a combination of high spatial and energy resolution (15 arc sec and 2 keV, respectively) with a sensitivity of {approximately}3{times}10{sup {minus}5} photons cm{sup {minus}2}s{sup {minus}1}. These features will allow us to resolve a possible narrow 511-keV line both energetically and spatially within a Galactic center ``microquasar`` or in other broad-class annihilators. The ground-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3{times}3 germanium array that detects the concentrated beam in the focal plane. Measured performances of the instrument at different line energies (511 keV and 662 keV) are presented and compared with Monte-Carlo simulations. The advantages of a 3{times}3 Ge-detector array with respect to a standard-monoblock detector have been confirmed. The results obtained in the laboratory have strengthened interest in a crystal-diffraction telescope, offering new perspectives for die future of experimental gamma-ray astronomy.

Observation of the galactic 1809 keV gamma‐ray line with the HEXAGONE spectrometer

We report an observation of the galactic 1809 keV gamma-ray line produced by radioactive {sup 26}Al in the interstellar medium. The measurement was performed with our high resolution germanium spectrometer HEXAGONE on a balloon flight in May 1989 from Alice Springs, Australia. Our differential spectrum of the Galactic Center region shows a narrow line at 1809 keV corresponding to a flux of (1.9 +/{minus} 0.9) {center dot} 10{sup {minus}4} photons {center dot} cm{sup {minus}2} s{sup {minus}1} assuming a source at the Galactic Center. We discuss the available observations of the 1809 keV line in the context of models that have been proposed for the origin of the galactic {sup 26}Al.

An observation of the Galactic center region with the HEXAGONE high resolution gamma‐ray spectrometer

The galactic center region was observed for 6 hours on 22 May 1989 from a high altitude balloon with the HEXAGONE high resolution gamma-ray spectrometer. The instrument had a 285 cm{sup 2} array of cooled germanium detectors with an energy resolution of 2.2 keV at 511 keV and an 18{degree} FWHM field of view. 511 keV gamma-rays from electron-positron annihilation and 1809 keV gamma-rays from the radioactive decay of {sup 26}Al were observed to have fluxes of 8.9{times}10{sup {minus}4} and 1.9{times}10{sup {minus}4} ph/cm{sup 2}-s, respectively. Continuum emission was detected from 20 to 800 keV and preliminary results have been obtained for the spectrum. Below 120 keV this is well described by power law with a slope of {minus}2.6. In the 120--250 keV band the spectrum contains a broad line-like feature with a flux of (2 to 6){times}10{sup {minus}3} ph/cm{sup 2}-s, depending on the assumed underlying continuum. This is interpreted as the result of Compton backscattering of {similar to}511 keV photons from a compact source of electron-positron annihilation radiation.

Observation of the 511 keV annihilation line in the direction of the galactic center with HEXAGONE

The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4 γcm−2u2009s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104 K for the temperature of the annihilation medium of the positrons.The HEXAGONE balloon‐borne spectrometer has flown on 22 May 1989. HEXAGONE is a high resolution gamma‐ray spectrometer and consists of an array of twelve cooled germanium detectors. One of the observed targets was the Galactic Center and its vicinity (field of view 19° at 511 keV) and it was seen during 6.3 hours. The 511 keV annihilation line was observed with a flux of (8.88±2.67)×10−4 γcm−2u2009s−1, a width 1.09+1.38, −1.09 keV and its centroid at 511.54±0.38 keV. The results are consistent with an upper limit of 8.3×104 K for the temperature of the annihilation medium of the positrons.

A statistical study of energy spectra of cosmic gamma‐ray bursts detected by the APEX experiment

Results are presented of a statistical study of spectra of cosmic gamma‐ray bursts detected in the Soviet‐French experiment APEX on the Phobos‐2 spacecraft. Intense events with V/Vmax<0.3 are shown to have harder spectra than the weak ones with V/Vmax≳0.3. The time histories of intense and weak events are also different: the rise fronts of strong bursts are longer and correspond to more energetic photons than for the weaker ones. On the other hand, both for strong and weak events the mean durations and energy spectra of back slopes are similar. The astrophysical consequences of those results are considered for models of sources of gamma‐ray bursts.

Can INTEGRAL detect 0.511 MeV radiation from giant molecular clouds

One of the interesting prospects raised by CGRO but never properly investigated due to the lack of instrumental sensitivity and resolution is the gamma-ray line emission from giant molecular clouds. In this paper, we study the 0.511 MeV annihilation line that is emitted when cosmic rays (CRs) bombarding a cloud produce radioisotopes that decay and produce positrons, which subsequently annihilate either directly or after the formation of Positronium. Several interesting factors come into play to decide whether the flux of this emission is strong enough to be detected by INTEGRAL, among them the intensity of the cosmic rays and their nuclear composition, as well as the mass, distance, and other characteristics of the molecular cloud. We have computed both the flux and the line width of the radiation for four nearby clouds. It is found that the line width can be very small (0.1 keV or less) in the case of a partially ionized cold core. This preliminary result, however, does not take into account Doppler shif...