9. X-RAY TELESCOPES:

X-rays span the spectral range from 70 A down to 0.1 A, corresponding to photon energies in the range 0.18 - 100 keV. The X-rays with longer wavelengths (and lower photon energies) are called soft X-rays and are not very penetrating, while the X-rays with the shorter wavelengths are called hard X-rays. Hard X-rays are used to take X-ray images of your body and to scan your baggage in airports. X-rays are only emitted by astronomical objects hotter than 10⁶ K, i.e., hotter than the surfaces of normal stars. Yet when astronomers first began to observe the sky in X-rays, beginning with rocket flights in the early 1960s, they saw many different kinds of objects. The first cosmic X-ray source to be observed was the Sun. The Sun's surface (more precisely, its photosphere, a term that we will define later) has a temperature of about 6000 K, too cool to emit X-rays. But the Sun also has an extended hot atmosphere above the photosphere, called the corona. The corona has temperatures exceeding 10⁶ K and can be seen easily with X-ray telescopes, as we shall discuss in Lesson 3. Since then, we have seen X-rays from all sorts of astronomical objects: stars, exploding stars, interstellar gas, galaxies. Perhaps the most interesting types of cosmic X-ray sources are collapsed stars, including neutron stars and black holes, and the active nuclei of galaxies (which, as you will see, may contain black holes).

In 1970, NASA launched the first X-ray space observatory, called UHURU. UHURU mapped the entire sky and found a few hundred cosmic X-ray sources. UHURU was followed by several X-ray observatories, launched by NASA, European Nations, and Japan. Three of them are operating currently: NASA's Rossi X-ray Timing Explorer (RXTE) and Chandra X-ray Observatory, and the European Space Agency's X-ray Multi-Mirror (XMM) mission.

The main technical problem in building X-ray telescopes is that X-ray photons won't reflect off a mirror if they strike it at nearly perpendicular angles, the way optical photons bounce off the mirror of an optical telescope. The X-rays will simply penetrate into the mirror and be absorbed. But X-rays will reflect off a mirror if they strike the mirror at grazing incidence, i.e., at a very small angle. The situation is analogous to skipping rocks off water. If you throw the rock straight down at the water, it will surely sink. But if you throw it at grazing incidence, you can make the rock skip. So, astronomers design X-ray telescopes with mirrors that are nearly cylindrical so that the X-rays strike the mirrors at grazing incidence. To increase the collecting area of the telescope, they build telescopes with several of these cylindrical mirrors nested inside each other.

On July 23, 1999, NASA took a giant step forward in our ability to observe the X-ray sky with the launch of the Chandra X-ray Observatory. With a focal length of 10 meters and a cost of more than a billion dollars, Chandra is the X-ray counterpart of the Hubble Space Telescope. The Chandra mirrors are more precise than those on previous X-ray telescopes and so can provide much sharper X-ray images. The angular resolution of Chandra is 0.3 arcsec, better than ground-based optical telescopes but not quite as good as HST (0.1 arcsec).

Our ability to observe the X-ray sky continues to advance rapidly. On December 10, 1999, the European Space Agency launched the X-ray Multi-Mirror (XMM) mission. XMM has greater sensitivity than Chandra and will be more powerful for observing faint sources and measuring spectra, but its angular resolution (10") is not as good. XMM appears to be working fine. Check out the XMM web site to see the first images and spectra from this wonderful observatory.

In the 35 years since it began, X-ray astronomy has yielded fascinating new insights into the cosmos, as you will see throughout this course. We can be confident that these powerful new X-ray telescopes will produce many tremendous new discoveries during the next few years.

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Last modified October 11, 2002
Copyright by Richard McCray