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X-RAY ASTRONOMY

When were X-rays first discovered?

X-rays were first discovered in 1895 by the
German physicist Wilhelm Roentgen. Roentgen discovered a new form of
radiation which could pass through many different materials, and he
called it X-radiation due to its mysterious nature.

How are X-rays produced?
We now know that X-rays are a form of light,
that is, photons. The are produced in the laboratory (or in the
hospital) when beams of fairly high energy electrons strike a target,
but in nature they are usually produced by a hot gas. The light
emitted by a hot object depends on its temperature. As an iron poker
in a fire gets hotter, it first emits heat, then red, orange and white
light in succession. When the temperature gets up to millions of
degrees (not possible for the poker!), X-rays are
emitted.



When were X-rays first detected in space?
X-rays were first detected in 1949, when
rockets carrying radiation detectors emerged from the Earth's
atmosphere and detected X-rays from the Sun's outer atmosphere, the
corona, which is at several million degrees in
temperature.

How are X-rays produced in space?
As explained above, hot matter at a temperature
greater than a million degrees is a strong source of X-ray
radiation. Very hot material like this is usually found in the regions
of strong gravitational field near a neutron star or a black
hole.

Where are regions of such hot matter found?

Regions of hot matter are most usually found
in the discs accretion discs which form around bodies such as
neutron stars, Black Holes and Quasars, or in the hot balls of gas
around clusters of galaxies. More information about black holes and
quasars can be found on the Black Holes and
Quasars page, and about the telescope XMM will use to look at them
on the mirrors page.

THE HISTORY OF X-RAY ASTRONOMY
Below is a list of previous X-ray satellites, together with a brief
description of each mission:
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Uhuru was launched by NASA in the early 1970's, and carried a simple
x-ray detector similar to a Geiger counter together with an optical
monitor to identify sources. The mission returned evidence of Black
Holes and Neutron Stars.
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Ariel 5 was the fifth of the series of UK/US collaborative
satellites. It was launched in 1975 and carried a sophisticated
payload of X-ray instruments. It showed that galaxy clusters contain a
huge ball of hot gas, investigated transient X-ray sources and carried
out an improved sky survey.
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Einstein, launched in 1978 by NASA, was the first large x-ray telescope to use mirrors. It returned images of shock waves from exploded stars and regions of hot gas in galaxies and galaxy clusters. Einstein also accurately located over 7000 X-Ray sources.
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ROSAT was launched in 1990 by Germany, the UK and the USA, and carried
an even larger telescope than Einstein. The mission increased the
number of known X-ray sources to over 60,000 and became important in
the study of hot gas present in the upper atmosphere of stars.
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ASCA is the Advanced Satellite for Cosmology and Astrophysics and was
the result of a collaboration between Japan and the US. Launched in
1993, it was designed to study the distribution of X-rays with energy.
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RXTE is the Rossi X-ray Timing Explorer and was launched by NASA in
1995. It does not include X-ray focusing mirrors, but was designed
instead to study the variation with time of the emission from X-ray
sources.
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BeppoSAX is a Dutch-Italian collaboration and was launched in 1996. It
was designed to study a wide range of X-ray energies from 0.1 to 300
keV and is used to monitor the X-ray afterglow of Gamma Ray Bursts.
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Chandra was launched in 1999 by NASA and contains four sets of nested
mirrors. It will detect sources further away and in greater detail
using highly polished mirrors. The mirrors are so smooth that if the
surface of the Earth were as smooth as Chandra's mirrors, the largest
mountain would only be 2 m tall!
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