"Hypernovae and Gamma Ray Bursts"
Gamma ray bursts were discovered in 1973 with the Vela
military satellites. The strongest events are so powerful that they damage
electronic instruments in space, and for a few seconds they out shine
all cosmic objects which are beyond our solar system. In 1991 the
BATSE instrument on the Compton Gamma-Ray Observatory established that
the bursts were distributed randomly all over the sky, but non-uniformly
in the distance, as there were very few weak bursts. This indicated
that we are located at the center of a spherical distribution of exploding
sources, and suggested that gamma ray bursts are at cosmological distances.
In the spring of 1997 the Italian/Dutch satellite BeppoSAX
provided accurate positions of several bursts within several hours of their
explosion, and the X-ray, optical and radio follow-up observations discovered
so called afterglows. Optical spectra firmly established that the
bursts are indeed at cosmological distances, about as far away as quasars.
As a rule the afterglows are found to be located in very faint, distant
galaxies, in which new stars are forming rapidly. It appears that
bursts are related to very powerful stellar explosions, possibly exceptional
and very rare type of supernovae, which generate ultra-relativistic jets.
We can detect only those which are beaming towards us. We still do
not understand what is the physical mechanism responsible for this spectacular
phenomenon.