Advanced XRay Astronomy Facility AXAFChandra

Launched on July 23, 1999, the third of NASA's "great observatories," the AXAF/Chandra is designed to study our universe in X-rays. The working name of the observatory was the Advanced X-Ray Astronomy Facility, but the telescope was renamed (after an open naming contest) in honor of the famous Indian American astronomer Subrah-manyan Chandrasekhar. Chandrasekhar provided a great deal of the theoretical underpinnings that describe the emission of X-rays from astrophysical objects; he won the 1983 Nobel Prize for his work.

Because of the high energies of X-ray photons, X-ray telescopes cannot be designed like optical telescopes. If X-ray photons strike a mirror surface at too great an angle, as measured from the mirror, they pass right through, depositing some of their energy in the mirror material. However, if the X-ray photons strike the mirror at a small (grazing) angle, they can be redirected, or focused, in a detector. For this reason, X-rays are focused in Chandra by a series of grazing reflections (Figure 2.10).

Figure 2.10. A schematic diagram of the Chandra/AXAF spacecraft. The Chandra spacecraft is typical in providing a support structure for the imaging equipment on board (optical bench), a power generation system (solar array), and a way to protect the sensitive equipment from solar radiation (sunshade door). Energetic X-ray photons strike a nested set of mirrors at small angles so that they are reflected into the focal plane. Courtesy of the Chandra Science Center.

Figure 2.10. A schematic diagram of the Chandra/AXAF spacecraft. The Chandra spacecraft is typical in providing a support structure for the imaging equipment on board (optical bench), a power generation system (solar array), and a way to protect the sensitive equipment from solar radiation (sunshade door). Energetic X-ray photons strike a nested set of mirrors at small angles so that they are reflected into the focal plane. Courtesy of the Chandra Science Center.

The instrumentation on Chandra includes the High Resolution Camera (HRC), the Advanced CCD Imaging Spectrometer (ACIS), and two high-resolution spectrometers: the High Energy Transmission Grating Spectrometer (HETGS) and the Low Energy Transmission Grating Spectrometer (LETGS). The HRC is the instrument that produces high-resolution "pictures" of sources as seen in X-rays, and the three spectrometers divide up the electromagnetic spectrum into smaller pieces so that astronomers can determine the presence, abundance, and motions of particular atoms and molecules.

The goal of the Chandra mission has been to increase both the angular resolution and spectral resolution available to X-ray astronomers. The bulk of Chandra's results so far have been related to stars and galaxies (Figure 2.11). Supernovae, supernova remnants, neutron stars, black holes, and certain magnetic dwarf stars all produce X-rays. Within just the first month of launch, Chandra produced images of the neutron star associated with the Cassiopeia A supernova remnant. Studies of supernovae are important to an understanding of the chemistry of the interstellar medium because all of the heavy elements found on Earth, such as iron, silicon, and gold, were created in supernova explosions.

Chandra has confirmed that the X-ray emission coming from the region surrounding the black hole at the center of the Andromeda galaxy (our closest galactic neighbor) originates from material far cooler than predicted by current models for black holes of this type. Astronomers believe that the mechanism by which material is funneled from the surrounding interstellar medium into the black hole must now be reworked, showing once again that observing even nearby, familiar sources with new instruments can lead to striking discoveries.

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