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X-rays were first observed and
documented in 1895 by Wilhelm Conrad Röntgen, a German scientist who
found them quite by accident when experimenting with vacuum tubes. A
week later, he took an X-ray photograph of his wife's hand which clearly
revealed her wedding ring and her bones. The photograph electrified the
general public and aroused great scientific interest in the new form of
radiation. Röntgen called it "X" to indicate it was an
unknown type of radiation. The name stuck, although (over Röntgen's
objections), many of his colleagues suggested calling them Röntgen
rays. They are still occasionally referred to as Röntgen rays in
German-speaking countries. |
Mrs. Röntgen's hand, the first X-ray
picture of the human body ever taken.
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In June 1990, the United States
launched a new German-built satellite to record X-rays from the sky.
This joint U.S./German/U.K. program was named Röntgen Satellite in his
honor (though it is almost always referred to as ROSAT).
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How Astronomers Observe X-rays Emitted by Cosmic
Sources
Although the more energetic X-rays (E > 30 keV) can
penetrate the air at least for distances of a few meters (Röntgen would never
have observed them if they could not, and medical X-ray machines would not
work), the Earth's atmosphere is thick enough that virtually none are able to
penetrate from outer space all the way to the Earth's surface. X-rays in the 0.5
- 5 keV range, where most celestial sources give off the bulk of their energy,
can be stopped by a few sheets of paper; ninety percent of the photons in a beam
of 3 keV X-rays are absorbed by traveling through just 10 cm of air!
To observe X-rays from the sky, the X-ray detectors
must be flown above most of the Earth's atmosphere. There are at present three
methods of doing so:
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Rocket flights
A detector is placed in the nose cone section of the
rocket and launched above the atmosphere. This was first done at White
Sands missile range in New Mexico with a V2 rocket in 1949. X-rays from
the Sun were detected by the Navy's experiment on board. An Aerobee 150
rocket launched in June of 1962 detected the first X-rays from other
celestial sources. The experiment package contained in this rocket is
pictured at left. The largest drawback to rocket flights is their very
short duration (just a few minutes above the atmosphere before the rocket
falls back to Earth) and their limited field of view. A rocket launched
from the United States will not be able to see sources in the southern
sky; a rocket launched from Australia will not be able to see sources in
the northern sky.
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Balloons
Balloon flights can carry instruments to altitudes
of 35 kilometers above sea level, where they are above the bulk of the
Earth's atmosphere. Unlike a rocket where data are collected during a
brief few minutes, balloons are able to stay aloft for much longer.
However, even at such altitudes, much of the X-ray spectrum is still
absorbed. X-rays with energies less than 35 keV cannot reach balloons.
One of the recent balloon-borne experiments was called the High
Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS). It was
launched from the Antarctic where steady winds carried the balloon on a
circumpolar flight lasting for almost two months! A picture of the
launch of HIREGS can be seen at right.
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Satellites
A detector is placed on a satellite which is taken
up to an orbit well above the Earth's atmosphere. Unlike balloons,
instruments on satellites are able to observe the full range of the
X-ray spectrum. Unlike rockets, they can collect data for as long as the
instruments continue to operate. In one instance, the Vela 5B satellite,
the X-ray detector remained functional for over ten years! |
The Kinds of Objects in the Universe that X-ray
Astronomers Observe
There are a variety of different kinds of astronomical
sources which emit electromagnetic radiation in the X-ray regime. These include:
Active Galaxies, Binary Star Systems, Black Holes, Cataclysmic Variables, Dark Matter,
Diffuse Background, Gamma-Ray Bursts, Neutron Stars, Pulsars, Stellar Coronae, The Sun,
Supernovae and their Remnants, White Dwarfs, and X-ray Transients.
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?"
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X-RAY