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X-Ray Instrument Design

X-ray astronomy is a relatively new science; it originated in the 1960s with a rocket flight which revealed the existence of X-ray emitters in the cosmos. X-ray instruments on satellites since then have discovered that X-ray emission is found from a wide variety of objects in the sky: single stars, binary star systems, supernova remnants, galaxies, clusters of galaxies, and active galactic nuclei.

Though new on the scene, there's a lot to be learned from X-rays. From the earliest missions, it was clear that X-ray spectra of celestial objects are complex and diverse, and carry a tremendous amount of information. X-rays come from matter that is highly energized: usually from gas that is very hot (millions of degrees Kelvin) or very fast electrons losing their kinetic energy. Both the shape of X-ray spectra and individual features, can tell much about a source, making X-ray spectra useful scientific tools in X-ray astrophysics.

What can an X-ray spectrum tell me?

What is resolution, and what effect does it have on what we can learn from X-ray data?

What Makes Observing X-rays Hard?

The first problem with observing X-rays from Earth is that they are absorbed by Earth's atmosphere. Because of this, X-ray detectors have to be above all or most of the atmosphere, which has only been possible since the invention of rockets.

Another problem is caused by the fact that there are much much fewer X-ray photons than, say, optical photons. Optical photons are thousands of times less energetic than X-ray photons, so if two sources emit the same amount of energy, one in X-rays and one in optical light, there will be about a thousand times more optical photons than X-ray photons (This is like comparing the number of $100 bills that make up a million dollars compared to the number of $1 bills it would take to make a million dollars). With so few X-ray photons, every single one can show up on an image, and it is crucial to capture as many as possible (see the image of the Moon at right, for example).

Even if X-ray photons were as plentiful as optical photons, however, their very nature makes them more difficult to observe. You already know, for example, that X-rays tend to pass through many things rather than being absorbed or reflected as optical light is. Special techniques have to be developed to observe the Universe in the X-rays and learn its X-ray secrets.

ROSAT image of moon
ROSAT image of the Moon

Designing X-ray Instruments

X-rays are like visible light, but their high energy means that they behave more like particles than like optical light (which behaves like a wave). They are few in number, they do not penetrate Earth's atmosphere, and they cannot be focused by a lens or single mirror. With these characteristics in mind, scientists have developed a number of effective ways to conduct X-ray observations. Until very recently, the primary X-ray detector instruments were proportional counters and semiconductor or sold-state detectors. Proportional counters rely on the conversion of an X-ray energy to charge pairs, and their energy resolution is fundamentally limited to no better than ~15%. With this technique, only the overall spectral shape of the source of X-rays can be obtained. That is enough to make general statements about the temperature of the gas radiating in clusters of galaxies, or to distinguish between a thermal and non-thermal process in supernova remnants. It is not, however, enough to determine what elements and how much are present in such systems. Solid state X-ray detectors use a specially prepared volume of semiconducting material that absorbs X-rays. The absorption creates electron-hole pairs that can be counted, and the number of pairs created is proportional to the energy of the incoming X-ray.

Tell me more about proportional counters

Tell me more about solid state detectors

The quantum X-ray microcalorimeter, the primary instrument for Astro-E, is a new approach to the problems of X-ray detection that seems best able to maximize both the energy resolution of the instrument, and the number of photons per energy resolution element. This detector was invented and developed at the Laboratory for High-Energy Astrophysics at NASA/Goddard jointly with the University of Wisconsin (other groups, at Lawrence Livermore National Labs, for example, are developing similar instruments). This detector can measure the energies of incoming X-ray photons over a broad range of energies all at once, and with an unprecedented spectral resolution (~0.4 to 10 keV with a 12 eV energy resolution).

Tell me more about microcalorimeters

New X-ray Science

With the next generation of X-ray detectors aboard X-ray telescopes, scientists will be better able to determine what elements are present in supernova remnants and galaxy clusters, measurements that may help us to complete our understanding of how stars evolve and how galaxies form. Another example where the improvements of the new instruments is essential to further our understanding is the structure of Active Galactic Nuclei, and of the black holes that lurk within. Definite proof that black holes are present in AGNs requires a more direct measurement of the conditions and kinematics in the AGN, measurements that are only possible with greatly improved X-ray spectral resolution. The current generation of X-ray telescopes has allowed us to infer much about the physical conditions near black holes, AGN, neutron stars, and other objects. The next generation will allow us to probe ever closer to the ultimateenvironmental limits of each of these objects.

Thanks to Greg Madejski for contributions to this article

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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