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X-ray Astronomy |
History of X-ray Astronomy
The study of astronomical objects at the highest energies of
X-rays and
gamma-rays
really began only in the early 1960's. Before then, we knew only that the Sun
was an intense source in these wavebands. The Earth's atmosphere absorbs most
X-rays and gamma-rays and, so, rocket flights which could lift scientific
payloads above the Earth's atmosphere were necessary. The first rocket flight
which successfully detected a cosmic source of X-ray emission was launched in
1962 by a group at American Science and Engineering (AS&E). The team of
scientists on this project included Riccardo Giacconi, Herb Gursky, Frank
Paolini, and Bruno Rossi. This rocket flight detected a very bright source
they named "Scorpius X-1" ("Sco X-1" for short), because
it was the first X-ray source found in the constellation Scorpius.
In the 1970s, dedicated X-ray
astronomy
satellites, such as
Uhuru,
Ariel 5,
SAS-3,
OSO-8, and
HEAO-1,
developed this field of science at an astounding pace.
Scientists began to believe that X-rays from stellar sources in our
Galaxy were
primarily from a
neutron star
in a binary
system with a
normal star. In
these "X-ray Binaries", the X-rays originate from material falling
from the normal star to the neutron star in a process called
accretion. The
binary nature of the system allowed for measurements of
mass of the neutron star. For other systems, the inferred mass of the
degenerate object supported the idea of the existence of
black holes, as
they were too massive to be neutron stars. Some of the systems displayed a
characteristic X-ray pulse, just as
pulsars had been
found to do in the
radio regime, which
allowed a determination of the spin rate of the neutron
star. Finally, some of these galactic X-ray sources were found to be highly
variable; in fact, some sources would 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 were 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 was found to
exist all over the sky.
Today, the study of high-energy
astrophysics
continues to be carried out using data from a host of satellites past and
present: the
HEAO series,
EXOSAT,
Ginga,
CGRO,
RXTE,
ROSAT,
ASCA. 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?"
Targets of X-ray Astronomy Observations
Below are listed a number of topics which can be explored in greater
detail. Some of these topics include links to science groups that are actively
pursuing research in these fields.
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