M. Niel

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.

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.

Duration versus brightness of gamma‐ray bursts: Comparisons between SIGNE and BATSE

We analyze duration and brightness distributions of both the SIGNE Venera 13 & 14 and BATSE gamma‐ray burst databases. We search for correlations between duration, peak brightness, and the variability measure V, proposed in Lamb, Graziani, and Smith. The duration histogram for SIGNE is consistent with the BATSE histogram. Estimating the instantaneous brightness by C64, we find that SIGNE confirms the BATSE result that the long and short bursts have similar maximum instantaneous brightnesses. Scatterplots between duration, brightness, and V are consistent for both databases; we show that SIGNE confirms the BATSE observation that there is a lack of bursts that are both bright over 1024 ms and contain a short, bright spike.

Statistics of cosmic gamma‐ray bursts: Evolution of different population of galactic sources

Below new results are presented of statistical studies of cosmic Gamma‐Ray Bursts (GRBs) detected with the Russian‐French APEX experiment onboard the PHOBOS‐2 spacecraft in 1988–89. For 48 APEX events, the averaged time history and hardness ratio profiles are provided with one second time resolution. For the two subsets of events (19 ‘strong’ GRBs with 0<V/Vmax<0.3, and 29 ‘weak’ ones with 0.3<V/Vmax<1), the above‐mentioned profiles compare as follows: (i) the time histories differ before the burst reach their maxima, but their tails are almost identical; (ii) the hardness ratio profiles differ significantly close to the bursts’ maxima. Several implications of these observations are drawn for cosmological and galactic models of GRBs.