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X-ray Emitting Astronomical Objects
The Suzaku Science Working Group has divided
Suzaku targets into 5 broad categories. These are:
Stars and Star Forming Regions
The corona of our Sun was the first celestial X-ray source to be
detected. It's not surprising, since the Sun is very close, in
astronomical terms. The corona is a region above the visible
surface (called the photosphere) of the Sun with very tenuous and
very hot gas. It turns out that some stars have far more active
coronae than does our Sun (which is a good thing, particularly for
the astronauts who will spend many months in the International Space
Station!). Some stars, particularly young stars, rotate more rapidly,
which leads to stronger, and more twisted magnetic fields, which makes
the corona more active. Scientists still do not understand all the
details of how the coronal gas is heated to such high temperatures,
though, or why some elements appear to be more common in the corona
than in the photosphere.
X-ray Binaries and Cataclysmic Variables
Binary star systems contain two stars that orbit around their common
center of mass. Many of the stars in our Galaxy are part of a binary system.
A special class of binary stars is X-ray binaries, so-called because they
were first discovered as very strong X-ray sources. X-ray binaries are made
up of a normal star and a collapsed star
(neutron star or black hole). These pairs of stars produce
X-rays if the stars are close enough together that material is pulled off the
normal star by the gravity of the dense, collapsed star. The X-rays come from
the area around the collapsed star where the material that is falling toward
it is heated to very high temperatures (over a million degrees!). This
area is known as the accretion disk.
Cataclysmic variables are like X-ray binaries, except with a white
dwarf instead of a black hole or a neutron star. A white dwarf has
mass comparable to the Sun but is closer to the Earth in size
(neutron stars and black holes are much smaller
still, with radii about 1/1000th of that of the Earth).
Because the gravitational
potential well of a white dwarf is not as deep as for a neutron star
or a black hole, a cataclysmic variable is not as X-ray bright as the
X-ray binaries. But there are many more cataclysmic variables,
some of them relatively close to the Sun. So we can often study
the details of the accretion process better in a cataclysmic variable
than in an X-ray binary.
Galactic Diffuse Emission
When a massive star explodes in a supernova, it expels a large amount of
material (often many times
more than the mass of the Sun) at thousands of kilometers per second.
This high speed gas then collides with the interstellar medium --- that's
the very tenuous gas and dust in between the stars --- and heats it up
to millions of degrees. For the next 20,000 years or so, a hot ball of
gas is left, glowing in X-rays: this is called a supernova remnant.
This material is then used to create the next generation of stars and
planets; Earth and the elements that our bodies are made of originated from
this same material.
Scientists are naturally interested in how elements like carbon, oxygen,
and iron are distributed in supernovae, and so they
study the supernova remnants.
X-ray energies turn out to be a good region of the electromagnetic
spectrum in which to do this.
Extragalactic Compact Source
Many galaxies appear to contain a massive black hole --- perhaps a million
times the mass of the Sun --- at their centers, or the nuclei. When they
accrete matter (like in X-ray binaries), they become some of the most
luminous objects in the universe, with names like quasars and blazars and
Seyfert galaxies. Collectively, they are known as 'Active Galactic Nuclei',
or AGN. With X-ray spectroscopy, it is possible to study the
motion of gas very close to the central black hole, even to the point of
verifying the predictions of Einstein's General Relativity Theory. Also
of interest:
what is the connection between the different types of AGN, and between AGN
and normal galaxies?
Extragalactic Diffuse Emission
Clusters of galaxies were first detected as a collection of galaxies in a
small patch of sky. When the X-ray telescopes are pointed at the clusters
of galaxies, scientists found out that they are strong sources of X-rays.
They are seen to originate from gases that fill the entire clusters of
galaxies, in and in-between the component galaxies. It turns out that the
mass of this gas exceeds that of the stars in the visible galaxies, and that
they are held in place by an even larger amount of dark matter. This finding
is very important in cosmology --- the question of whether the Universe
will keep on expanding forever hinges on the amount of matter. The hot gas
itself comes from supernova explosion in the early Universe, and can be
used to trace the star formation history of the Universe, another
important topic in cosmology.
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