Astroparticle Physics

120 6 Primary Cosmic RaysFig. 6.47Light curve of SN 1987A. Thesolid line corresponds to completeconversion of 56 Co γ rays into theinfrared, optical, and ultravioletspectral rangeγ line emissionlight curve of SN 1987Acoming from this object. It has to be noted, however, that ahigh-energy gamma outburst of this source has never beenseen again.Apart from the investigation of cosmic sources in thelight of high-energy γ rays the sky is also searched forγ quanta of certain fixed energy. This γ -ray line emissionhints at radioactive isotopes, which are formed in the processof nucleosynthesis in supernova explosions. It could beshown beyond any doubt that the positron emitter 56 Ni wasproduced in the supernova explosion 1987A in the LargeMagellanic Cloud. This radioisotope decays into 56 Co witha half-life of 6.1 days. The light curve of this source showeda luminosity maximum followed by an exponential brightnessdecay. This could be traced back to the radioactive decayof the daughter 56 Co to the stable isotope 56 Fe with ahalf-life of 77.1 days (see Fig. 6.47).Interesting results are also expected from an all-sky surveyin the light of the 511 keV line from e + e − annihilation.This γ -ray line emission could indicate the presence of antimatterin our galaxy. The observation of the distribution ofcosmic antimatter could throw some light on the problemwhy our universe seems to be matter dominated.6.3.5 γ Bursterdiscovery of γ -ray burstsCosmic objects which emit sudden single short outbursts ofγ rays have been discovered in the early seventies by Americanreconnaissance satellites. The purpose of these satelliteswas to check the agreement on the stop of nuclear weapontests in the atmosphere. The recorded γ rays, however, didnot come from the surface of the Earth or the atmosphere,but rather from outside sources and, therefore, were not relatedto explosions of nuclear weapons which also are asource of γ rays.