Prologue

Almost every day energetic explosions illuminate across the sky. The powerful explosions come from random directions, i.e., at an unpredictable time from unpredictable directions in the sky.
We don't know what causes them and you can't see them with the naked eye. They are called "GRB s", short for Gamma-Ray Bursts. GRBs were discovered by accident in the 1960's, and
for the past 30 years they've been the target of intense research and speculation by astronomers, astrophysicists, and nowadays by High-Energy Physicists as well!
Until recently, we couldn't know if they came from our own solar system or perhaps as far as
the edge of the universe. They've been called the greatest mystery of modern astronomy.

History

1967: GRBs were first detected by the Vela satellites.

In the early 1960's, the USA and the USSR signed a treaty banning nuclear tests. However, some Americans suspected that the USSR would carry out nuke tests in the atmosphere or in space where they might be able to conceal them from the USA. Consequently, in October 1963, the US Air Force launched the first in a series of satellites which are called "Vela" (from the Spanish verb `vela' meaning `to watch'). Their goal was to develop the technology to monitor nuke tests from space and give the US a means of verifying the conditions of the treaty. The satellites were launched and operated in pairs with two identical satellites on opposite sides of a circular orbit of 2.5 x 10⁵km in diameter (about a 4-day orbit), so that no part of orbit was shielded from direct observation.

16 bursts were detected between 1969 and 1972.
Observations had sufficient spatial resolution to show that bursts were:

Non-terrestrial in origin
Non-solar origin

1991: The Compton Gamma-ray Observatory (CGRO) was launched.

It detects ~ 1 burst / day

April 1, 1996:
Super-Kamiokande began taking neutrino data which could detect neutrino bursts
from GRBs.

Since their discovery (just by accident!) in the late 1960's, several thousand bursts have been detected, most of them with BATSE (the Burst and Transient Source Experiment) on board the Compton Gamma-Ray Observatory (CGRO in short). Their distribution on the sky is completely uniform. In particular, they do not appear to come from the Milky Way.

Characteristics of GRBs Known So Far

Completely uniform distribution on the sky.
not appear to come from the Milky Way.
Bright sources in the sky where on
Pure gamma-rays (only ~ 2% show x-ray emmision)
Short duration (5 ms ~500 s)

What we thought

Between the time of the discovery of bursts in the late 1960's, and the launch of the BASTE experiment in the 1991, most researchers were convinced that bursts originated in our own Galaxy, on or near objects called "Neutron Stars". Our Milky Way Galaxy contains neutron stars as massive as the Sun (about 3x10⁵ times the mass of the Earth), but no bigger about 10 km in diameter. Their tremendous gravitational field and magnetic field made them an ideal source for Gamma-Ray Bursts. This appealing idea was confronted with reality in 1992, when it became clear from BATSE observations that GRBs are distributed uniformly on the sky, without a concentration to the plane of the Milky Way, or towards its center or in other clumps of or concentrations. In fact, the distribution could lead one to argue that perhaps we see the bursts coming from quite nearby, from distances small compared to the thickness of the disc of the Milky Way. However, this would imply, as we saw above, that in no direction do we start to see the edge of the GRB distribution, or in this case, the edge of the MilkyWay disc. Such an edge would reveal itself in the distribution of brightness of bursts, whichwould show a deficit of faint bursts. But this is contradicted by the observed brightness distribution of GRBs, which shows a distinct dearth of very weak GRBs: It is as though in all directions we do see that edge.

What we should do?

The combined angular and brightness distributions of bursts eliminates the possibility that GRBs come from the disc of the Milky Way, and left us with a choice between one of two possibilities.

GRBs originate either in a very large spherical halo (or corona) around the Galaxy, or
They come from the far depths of the Universe, billions of LY (light years) away from us.

The galactic halo would have to be very big, about a million of LY across, which is much bigger than the diameter of the known Milky Way system (which is less than 10⁵ LY), much less the halo that Milky Way is known to have. The very large halo size is required to avoid an asymmetry
in the GRB sky distribution caused by the fact that the Earth is offset from the center of the Galaxy, by about 25 000 LY. In the center of the other distance scale (referred to as "Cosmological") it is the smoothness of the GRB sky distributions that tell us that their distances must exceed the length scales over which the distribution of matter appears clumpy, i.e., larger than clusters and super clusters of galaxies, and the voids between them.

Epilogue

The evidence probably suggests that gamma ray bursts originate a long way off. From our view, they must radiate about as much energy as a star's rest mass. As a matter of fact, some theorists hypothesize that a Neutron Star-Neutron Star (NS-NS) binary pair eventually inspirals (because
of gravitational radiation), resulting in such a heavy body that it becomes a Black Hole;
Its gravitational binding energy is somewhat converted to the gamma rays we can observe.

For the Super-Kamiokande Experiments

The neutrinos in the universe all come from the weak interactions (like in beta decays).
But, there are many types of neutrino sources, such as:

The Solar Neutrinos
The Neutrinos from Cosmic Rays (Atmospheric Neutrinos)
The Neutrinos from Supernovae Explosions
The Neutrinos from the BB (Big-Bang)
The Neutrinos from the GRBs, etc.

GLOSSARY

More Information

BATSE | EGRET | BLAST

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Originally created by Sung Yong Yoon.