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Gamma-ray Astronomy |
History of Gamma-ray Astronomy
Long before experiments could detect
gamma-rays
emitted by cosmic sources,
scientists had known that the Universe should be producing these
photons. Work
by Feenberg and Primakoff in 1948, Hayakawa and Hutchinson in 1952, and,
especially, Morrison in 1958 had led scientists to believe that a number
of
different processes which were occurring in the Universe would result in
gamma-ray emission. These processes included cosmic ray interactions
with
interstellar gas,
supernova
explosions, and interactions of energetic electrons with
magnetic
fields. However, it was not until the 1960s that our ability to
actually detect these emissions came to pass.
Gamma-rays coming from space are mostly absorbed by the Earth's
atmosphere.
So gamma-ray
astronomy
could
not develop until it was possible to get our detectors above all or most
of
the atmosphere, using balloons or spacecraft. The first gamma-ray
telescope
carried into orbit,
on the Explorer-XI satellite in 1961, picked up fewer than 100 cosmic
gamma-ray photons. These appeared to come from all directions in the
Universe, implying some sort of uniform "gamma-ray
background".
Such a background would be expected from the interaction of cosmic rays
(very
energetic charged particles in space) with gas found between the
stars.
Significant gamma-ray emission from our
Galaxy was first
detected in 1967 by the the gamma-ray detector aboard the
OSO-3 satellite.
It
detected 621 events attributable to cosmic gamma-rays. However, the
field of
gamma-ray astronomy took great leaps forward with the
SAS-2 (1972) and
the
COS-B (1975-1982)
satellites. These two satellites provided an exciting
view into the high-energy universe (sometimes called the 'violent'
universe,
because the kinds of events in space that produce gamma-rays tend to be
explosions, high-speed collisions, and such!). They confirmed
the earlier findings of the gamma-ray background, produced the first
detailed
map of the sky at gamma-ray
wavelengths,
and detected a number of point sources. However, the poor
resolution
of the instruments made it impossible to identify most of these
point
sources with individual stars or stellar systems.
Perhaps the most spectacular discovery in gamma-ray astronomy came in
the
late 1960s and early 1970s from a constellation of defense satellites
which
were put into orbit for a completely different reason. Detectors on
board the
Vela satellite
series, designed to detect flashes of gamma-rays from nuclear
bomb blasts, began to record bursts of gamma-rays -- not from the
vicinity of
the Earth, but from deep space! Today, these gamma-ray bursts are seen
to last
for fractions of a second to minutes, popping off like cosmic flashbulbs
from
unexpected directions, flickering, and then fading after briefly
dominating
the gamma-ray sky. Studied for over 25 years now with instruments
on board a
variety of satellites and space probes, including
Soviet Venera
spacecraft and the
Pioneer Venus
Orbiter,
the sources of these enigmatic high-energy
flashes remain a mystery. In one of the most intense debates in modern
astrophysics, some scientists claim that the bursts originate in a
halo of
neutron stars which surround our Galaxy while others argue that
their
origins are far beyond the Galaxy, at
cosmological distances. In either setting, these
gamma-ray bursts are among the most powerful events in the Universe.
In 1977, NASA announced plans to build a "great
observatory" for
gamma-ray astronomy. The
Compton Gamma-Ray
Observatory (CGRO) was designed to take advantage of the major
advances in
detector technology during the 1980s, and was launched in 1991. The
satellite
carried
four major experiments which have greatly improved the spatial and
temporal resolution of gamma-ray observations. The CGRO provided large
amounts of data which are being used to improve our understanding of
the high-energy processes in our
Universe. CGRO was de-orbited in June 2000 as a result of the failure of
one of its stabilizing gyroscopes.
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