History of X-ray Astronomy
History of astronomy goes back to several hundred years, but until the 20th
century, astronomers learned virtually all they knew about sources in the
sky from only the tiny fraction of
electromagnetic radiation
which is
visible
to the eye. However, with the advances in technology during last few decads,
it has become possible to collect
radiation
outside this part of the
spectrum
, allowing us to 'see' the
Universe
in all its glory. Many objects reveal different aspects of their behavior
at different
wavelengths
. Other objects are completely invisible at one wavelength yet show up
clearly at another. What determines the type of electromagnetic radiation
emitted by astronomical objects is mainly their TEMPERATURE! Following chart
shows what kinds of objects typically emit what kinds of radiation?
| Type Of Radiation |
Radiated By Objects At This Temperature |
Typical Sources |
| Gamma-rays |
more than 108
Kelvin
(K) |
accretion disks around black holes |
| X-rays |
106-108 K |
gas in clusters of
galaxies
;
supernova
remnants; stellar
corona
|
| Ultraviolet |
104-106 K |
supernova remnants; very hot
stars
|
| Visible |
103-104 K |
planets, stars, some
satellites
|
| Infrared |
10-103 K |
cool clouds of
dust
and gas; planets |
| Radio |
less than 10 K |
radio emission produced by electrons moving in
magnetic fields
|
The study of astronomical objects at the high energies of
X-rays
and gamma-rays
really began only in the early 1960's. The Earth's atmosphere absorbs
most of the X-rays, so the advances in space technology, by which various
scientific payloads can be lifted above the Earth's atmoshpere were necessary.
The first X-ray source, "Scorpius X-1" ("Sco X-1" for short), was detected
in 1962 by a rocket flight launched by a group at American Science and Engineering
(AS&E). However, the rocket flights were not suitable for detailed studies
mainly because of their very short duration (typically just few
minutes above the atmosphere before the rocket falls back to Earth). This
led to the use of balloons for observing the X-ray sources. Balloon flights
can carry instruments to altitudes of 40 kilometers above sea level, where
they are above the bulk of the Earth's atmosphere and are of much longer
duration (typically few hours). But even at 40 kilometer altitude, X-rays
with energy less than 20 keV are still absorbed. Thus balloon flights could
be used for study in only higher energy X-rays (also known as "hard X-rays").
The breakthrough came with use of satellites.
Many dedicated X-ray astronomy satellites, such as Uhuru
, Ariel 5
, SAS-3
, OSO-8
were launched during 1970s. Much higher sensitivity X-ray satellites,
such as HEAO
series
, EXOSAT
, Ginga
launched during 1980s developed this field of science at an astounding pace.
Today, the study of high-energy
astrophysics
continues to be carried out using data from many satellites of past and
present: the
CGRO
, RXTE
, ROSAT
, ASCA
and most recent Chandra
and XMM Newton
. Data from these satellites aid our further understanding of the nature
of these sources and the mechanisms
by which the X-rays and gamma-rays are emitted. Understanding these mechanisms
can in turn shed light on the fundamental physics of our
universe
. By looking at the sky with X-ray and gamma-ray instruments, we gain unique,
important information in our attempt to address questions such as "How did
the Universe Begin, How does it Evolve, and What is its Fate?"
Various X-ray sources
Typical targetes for the X-ray observations include
It is believed that most of the galactic X-ray sources are X-ray binaries,
in which a normal star and a compact object rotate around a common center
of mass. In these "X-ray Binaries", the X-rays originate from material
falling from the normal star onto the compact object in a process called
accretion
. The compact object can be a white dwarf, a neutron star or a black hole.
Some of these galactic X-ray sources are highly variable; in fact, some
sources appear in the sky, remain bright for a few weeks, and then fade
again from view. Such sources are called "X-ray Transients". The inner regions
of some galaxies are also found to emit X-rays. The X-ray emission from
these "Active Galactic Nuclei" is believed to originate from ultra-relativistic
gas near a very massive black hole at the galaxy's center. Lastly, a diffuse
X-ray emission is found to exist all over the sky.
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