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.

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Chandra X-ray Observatory

Artists rendering of the Chandra satellite

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 image of the Tycho supernova remnantHigh resolution image of the Tycho supernova remnant

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.


Artists rendering of the Constellation-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. (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 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.

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.

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.


Artists rendering of the ASCA satellite

ASCA 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.


Artists rendering of the RXTE satellite

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.


Artists rendering of the Astro-E satellite

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 Technology (MIT).

Astro-E was designed for “broad-band, high-sensitivity, high-resolution” spectroscopy. 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 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?