A Plethora of X-ray Telescopes
What observatories will we use in the coming years to explore the structure
and evolution of the Universe? What observatories are we currently using?
Chandra was launched from the space shuttle in 1999,
ASTRO-E was attempted to be launched in Feb., 2000, and
Constellation X-Ray Observatory is still being designed. Current X-ray
observatories include RXTE and ASCA.
Chandra X-ray Observatory
NASA's Chandra X-ray Observatory, which was launched and deployed
by Space Shuttle Columbia on July 23, 1999, is
a very sophisticated X-ray observatory.
Chandra is designed to observe X-rays from high energy regions of the
universe, such as hot gas in the remnants of exploded stars. The two
images of the Tycho supernova remnant shown below illustrate how higher
resolution improves the quality of an image:
Low resolution and high resolution images
of the Tycho supernova remnant
The image on the left is from a low-resolution detector on
the Einstein Observatory. The image on the right, taken by
the High Resolution Imager on the Einstein Observatory,
has ten times better resolution, or finer detail (pixel area
ten times smaller), than the one on the left. Chandra images will be
fifty times better than the image on the right.
Chandra detects and images X-ray sources that are
billions of light years away.
The imaging mirrors on Chandra are some of
the largest, most precisely shaped and
aligned, and smoothest mirrors ever constructed.
If the surface of Earth was as smooth as the
Chandra mirrors, the highest mountain would be less than six feet tall!
The images Chandra makes are twenty-five times sharper than the best
previous X-ray telescope. This focusing power is equivalent to the
ability to read a newspaper at a distance of half a mile. Chandra's
improved sensitivity is making possible more detailed studies of black holes, supernovae, and dark matter. Chandra will increase our understanding of the
origin, evolution, and destiny of the Universe.
The Chandra telescope system consists of four pairs of mirrors and their
support structure. The mirrors have to be exquisitely shaped and aligned
nearly parallel to incoming X-rays. Thus they look more like nested glass
barrels than the familiar dish shape of optical telescopes.
The function of the science instruments on Chandra is to record as
accurately as possible the number, position and energy of the incoming
X-rays. This information can be used to make an X-ray image and study
other properties of the source, such as its temperature.
Chandra resides in an orbit approximately 6,214 by
86,992 miles in altitude.
For more information, see http://chandra.harvard.edu/pub.html.
The Constellation-X Observatory will assist in putting together the
missing pieces to understanding the X-ray Universe. The observatory
consists of four X-ray telescopes or satellites that will detect a broader
range of X-ray wavelengths than any current technology, especially X-rays
at higher frequencies. Combining the observing power of four telescopes
means that the total X-ray effective collecting area is much larger than
that of previous telescopes. Constellation-X's total light collecting area
is 3 square meters, a hundred times greater than the finest current
instruments. The increased light gathering ability will allow
Constellation-X to observe extremely faint X-ray emitting sources within
our Galaxy and far beyond. Useful data from these faint sources will be
collected in hours instead of days or weeks.
Constellation-X will be launched near the end of the coming decade. Its
four satellites will orbit together in space about a few hundred miles
from each other, and will detect and collect X-ray photons (instead of
generating these photons like a medical X-ray machine). It will require
several rocket missions to launch the entire observatory. The point at
which the satellites will orbit is 1.5 million miles away from Earth where
both the Sun's and Earth's gravitational pull are equal.
What will Constellation-X Observe?
Constellation-X will obtain spectra of distant sources, including
supermassive black holes, X-ray binaries, galaxy clusters, supernova
remnants, and stellar coronae. (See our Introduction to Spectroscopy
for more information on spectra.) With a larger
number of collected light photons, the resolution of spectroscopy
increases tremendously. Higher resolution means that the collected data
will be more quantitative. A high resolving power, for example, is
necessary to distinguish the lithium satellite lines from the overlapping
helium-like lines or transitions. Therefore, scientists will know exactly
what elements are in X-ray sources such as supernova remnants, as well as
their abundance, their density, and how fast they are moving. Spectra from
Constellation-X are like "the fingerprint of elements in far-away stars
and clouds of gas." High spectral resolution is essential to making unique
identifications (from emission lines).
Constellation-X will be able to focus on smaller areas, which will
automatically exclude picking up X-ray signals from the external medium of
hot gas or other nearby sources. Its ability to discriminate among
different X-ray wavelengths will be far better than any other X-ray
What questions will Constellation-X answer?
"Constellation-X will be the next best thing to reaching out and touching
supernova remnants, black holes, clusters of galaxies, and dark matter."
What happens close to a black hole?
The observatory will be able to
measure the extreme gravitational force around a black hole. A black hole
is defined by a surface called the event horizon, where gravity is so
intense that nothing, not even light, can escape. Stellar matter is
crushed into a single point behind the event horizon. Around black holes,
interstellar gases move, heat up, and emit light energy in the form of
X-rays. Constellation-X will be able to zoom to within a few miles of the
event horizons of supermassive black holes in active galaxies outside our
own Milky Way and obtain spectra of the gas found there. The spectra will
be utilized to see the effects of how extreme gravity around a black hole
affects the composition, pressure, density, temperature, and velocity of
nearby gas. Scientists will eventually be able to collect quantitative
data regarding the formation and evolution of these black holes residing
in the centers of many (if not most) galaxies. For more information, see:
Recycling: The law of the Universe?
From individual stars to clusters of
galaxies, the Universe is one big recycling machine. Constellation-X will
produce detailed measurements of the formation of elements between carbon
and zinc in stars, by observing supernova remnants. Galaxy Clusters are
the largest objects in the Universe. They are complex, multi-component
systems with hundreds of galaxies, hot gaseous intracluster medium, and
dark matter, all evolving together. Constellation-X will study the
chemical abundance of the intergalactic medium, and will also be able to
measure the mass and motion of gas in the cores of galaxies. The motion of
gases will be examined to determine if this gaseous motion is the cause of
galactic mergers. Once it is understood how galaxies evolve and merge, a
basis for understanding the structures of the Universe will perhaps
develop. For more information, see:
Is there any Matter missing from the Universe?
One of the biggest
mysteries in modern astronomy
is "What holds clusters of galaxies together?". While the earth holds
the moon in place, what prevents galaxy
clusters from spreading apart? The gravitational pull from the gases
between the clusters is not strong enough. One major discovery made by
scientists is the fact that most of the mass of galaxies, clusters, and
the Universe is in the form of dark matter. Dark matter is in a form
whereby it is not directly detectable. Scientists, however, know that dark
matter exists by its strong gravitational effects. Even though dark matter
cannot be directly observed, the Constellation Observatory will be able to
map out its location. Perhaps the mystery of dark matter will begin to
unfold. For more information, see: http://constellation.gsfc.nasa.gov/public/science/dark_matter.html
ASCA (formerly named Astro-D) is Japan's fourth cosmic X-ray astronomy
mission, and the second for which the United States is providing part of the
scientific payload. The satellite was successfully
launched on February 20, 1993.
ASCA has played an important role in the astrophysicists' never-ending
quest for better X-ray spectra. This has been achieved by a combination
of light-weight telescopes with imaging detectors.
In designing ASCA's 4 X-Ray Telescopes (XRTs), Dr. Serlemitsos at GSFC
and his team deliberately chose not to pursue the best (sharpest) X-ray
images. Rather, they optimized the design for high collecting area (i.e.,
how much X-rays an XRT can collect from a given celestial source) within
a tight weight constraint. They achieved this by using an innovative
design of 'conical foil mirrors', which they had previously demonstrated
on the Shuttle-based BBXRT mission in 1990. ASCA became the first satellite
with XRTs that can operate up to 10 keV (previously, Einstein observatory's
telescope was useful up to 4 keV). All 4 XRTs on ASCA point to the same
region of the sky, further increasing the collecting power.
There are two types of detectors on board ASCA --- 2 Gas Imaging Spectrometers
(GIS) and 2 Solid-state Imaging Spectrometers (SIS),
each behind its own XRT. Although the GIS's are excellent instruments
which have produced many important results, the SIS's are what astrophysicists
were most excited about. At the heart of the SIS's are X-ray sensitive
CCDs developed at MIT's Lincoln Laboratory. Each SIS consists of 4 CCD
chips; each CCD consists of about 420 by 420 picture elements (or pixels).
The energy of each X-ray photon striking a CCD is converted into electric
charge, which is then measured by the on-board electronics. This gives
X-ray CCDs a good spectral resolution that had not been available for
routine use on faint X-ray sources. ASCA was the pioneer in the use of
X-ray CCDs. More than 5 years later, the use of X-ray CCDs are becoming
routine in newer X-ray astronomy satellites.
The Rossi X-ray Timing Explorer (RXTE), named after astronomer Bruno Rossi, probes the physics of cosmic X-ray sources by making sensitive
measurements of their variability over time scales ranging from milliseconds to years. How these sources behave over time is a source of important
information about processes and structures in white-dwarf stars, X-ray binaries, neutron stars, pulsars, and black holes.
With instruments sensitive to a wide range of X-ray energies (from 2-200 keV),
RXTE is designed for studying known sources, detecting transient
events, X-ray bursts, and periodic fluctuations in X-ray emissions.
The objectives of RXTE are to investigate:
- periodic, transient, and burst phenomena in the X-ray
emission from a wide variety of objects,
- the characteristics of X-ray binaries, including the masses
of the stars, their orbital properties, and the exchange of
matter between them,
- the inner structure of neutron stars, and properties
of their magnetic fields,
- the behavior of matter just before it falls into a black
- effects of general relativity which can be seen only near
a black hole,
- properties and effects of supermassive black holes in the
centers of active galaxies,
- and the mechanisms which cause the emission of X-rays in all these
RXTE has three instruments. The Proportional Counter Array (PCA) has five
xenon gas proportional counter detectors (the X-rays interact with the
electrons in the xenon gas) that are sensitive to X-rays with energies
from 2-60 keV. The PCA has a large collecting area (6250 cm2).
The PCA's pointing area overlaps that of the HEXTE instrument, increasing the
collecting area by another 1600 cm2.
The High Energy X-ray Timing Experiment (HEXTE)
extends the X-ray sensitivity of RXTE up to 200 keV, so that with the PCA,
the two together form an excellent high resolution, sensitive X-ray detector.
The All Sky Monitor (ASM) rotates in such a
way as to scan most of the sky every 1.5 hours, at 2-10 keV,
monitoring the long-term behavior of a number of the brightest X-ray sources,
and giving observers an opportunity to spot any new phenomenon
ASTRO-E, launched in Feb, 2000,
was to be the 5th in a series of Japanese astronomy satellites
devoted to observations of celestial X-ray sources. Unfortunately,
the first stage of the M-V launch vehicle
had a burn through that caused loss of attitude control. By the time the second
and third stages finished (successfully), there was not enough velocity to
reach orbit. Losing ASTRO-E was a huge blow to the
astronomical community. But sometimes this is the unfortunate consequence of
launching a satellite on a rocket. ASTRO-E was not the first, and will not
be the last satellite lost during its launch.
ASTRO-E was a joint
Japanese-US mission, with the US contributing significantly to two of
the three types of instruments on-board. It was developed at
Japan's Institute of Space and Astronautical Science (ISAS) in
collaboration with other Japanese institutions, as well as NASA's
Goddard Space Flight Center and the Massachusetts Institute of
ASTRO-E was designed for "broad-band, high-sensitivity, high-resolution"
It consisted of 5 X-ray telescopes and a high energy x-ray detector.
Four of the telescopes focused x-rays onto
imaging CCD detectors. The fifth telescope focused x-rays onto the
microcalormeter. Thus, Astro-E was
not only sensitive to both low and high energy X-rays, but
could distinguish very small differences in the energy of the X-ray
photons (http://heasarc.gsfc.nasa.gov/docs/astroe_lc/glossary.html#photons) that are being detected.
Some of the key themes that astronomers hoped that ASTRO-E would be
able to advance are: When and where are the chemical elements created?
What happens when matter falls onto a black hole?
How do you heat gas to X-ray emitting temperatures?
See the ASTRO-E Learning Center (http://heasarc.gsfc.nasa.gov/docs/astroe_lc/) for more
information about the design of ASTRO-E and what ASTRO-E hoped to accomplish.
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