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.

About XMM-Newton About Chandra About RXTE About ASCA
About ASTRO-E About Constellation-X

About XMM-Newton

XMM-Newton XMM-Newton was launched on Dec 10, 1999 at 14:32 GMT from Kuru (in French Guyana) by a Ariane-5 rocket. Less than half an hour after lift-off, the satellite was released into space. The satellite has a highly eccentric orbit and travels out to nearly one third of the distance to the Moon every 48 hours.

XMM-Newton is an international collaboration between the European Space Agency (and its 14 member countries) and the USA.

The key instruments on-board XMM-Newton are the 3 EPIC cameras and the 2 RGS which were designed to give high resolution spectra in the 0.35-2.5 keV energy band.

Another innovation of the XMM-Newton satellite is the presence on-board of a UV/Optical Monitor which was included to provide multiwavelength information (both in optical and ultraviolet) to the X-ray observations.

About Chandra

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:

Image from Low-Resolution 
                                Detector on the Einstein Observatory Image from High Resolution 
                                Imager on the Einstein Observatory

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.

About ASCA

ASCA 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 carries four large-area X-ray telescopes. At the focus of two of the telescopes is a Gas Imaging Spectrometer (GIS), while a Solid-state Imaging Spectrometer (SIS) is at the focus of the other two. The GIS is a gas imaging scintillation proportional counter and is based on the GSPC that flew on the second Japanese X-ray astronomy mission TENMA. The two SIS are identical Charge Coupled Device (CCD) cameras were provided by a hardware team from MIT, Osaka University and ISAS.

ASCA is the first X-ray astronomy mission to combine imaging capability with a broad pass band, good spectral resolution, and a large effective area. The mission also is the first satellite to use CCDs for X-ray astronomy. With these properties, the primary scientific purpose of ASCA is the X-ray spectroscopy of astrophysical plasmas-especially the analysis of discrete features such as emission lines and absorption edges.

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About RXTE

RXTE 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 hole,
  • 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 objects.

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 quickly.

About ASTRO-E

ASTRO-E 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, something happened with the first stage of its launch vehicle and the satellite was placed in an unrecoverable orbit. Losing ASTRO-E was a huge blow to the astronomical community.

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 Technology (MIT).

ASTRO-E was designed for "broad-band, high-sensitivity, high-resolution" spectroscopy. This means that not only were its instruments sensitive to both low and high energy X-rays, but they can distinguish very small differences in the energy of the X-ray photons that are being detected. ASTRO-E used brand-new microcalorimeter technology.

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 for more information about the design of ASTRO-E and what ASTRO-E hoped to accomplish.

About Constellation-X

The Con-X Observatory 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. A spectrum (the plural of spectra) is a plot of the intensity of light at different frequencies (or equivalently, at different wavelengths or energies - see our Introduction to Spectroscopy). 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 telescope.

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: http://constellation.gsfc.nasa.gov/public/science/black_holes.html

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: http://constellation.gsfc.nasa.gov/public/science/life_cycles.html

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

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