Chandra X‐ray Observatory

Abstract

The Chandra X‐ray Observatory (originally called the Advanced X‐ray Astrophysics Facility—AXAF) is the X‐ray component of NASA's “Great Observatory” Program. Chandra is a NASA facility that provides scientific data to the international astronomical community in response to scientific proposals for using it. The Observatory is the product of the efforts of many organizations in the United States and Europe. The Great Observatories also include the Hubble Space Telescope for space‐based observations of astronomical objects primarily in the visible portion of the electromagnetic spectrum, the now defunct Compton Gamma‐Ray Observatory that was designed to observe gamma‐ray emission from astronomical objects, and the soon‐to‐be‐launched Space Infrared Telescope Facility (SIRTF). The Chandra X‐ray Observatory (hereafter CXO) is sensitive to X rays in the energy range from below 0.1 to above 10.0 keV, corresponding to wavelengths from 12 to 0.12 nanometers. The relationships among the various parts of the electromagnetic spectrum, sorted by characteristic temperature and the corresponding wavelength are illustrated in this article. X‐ray astronomy is an extremely important field because it has been found that all categories of astronomical objects (or a subset thereof), from comets to quasars, emit X‐rays. Thus learning how and why these objects produce X‐rays are fundamental to our understanding of the way astronomical systems work. This, together with the large amounts of energy required to produce X‐rays, makes their study interesting and exciting. A second reason that the field is so important is that the vast bulk of the matter in the Universe that we can directly observe through the electromagnetic radiation that it emits is in the very hot (temperatures of millions of degrees) X‐ray emitting gas that fills the space between galaxies in clusters of galaxies, the largest collections of matter in the Universe. The CXO is the prime method of gaining new information about the X‐ray emission seen in the Universe. The CXO was named in honor of the late Indian‐American Nobel laureate, Subrahmanyan Chandrasekhar, nicknamed “Chandra” which means “moon” or “luminous” in Sanskrit. He was one of the foremost astrophysicists of the twentieth century and, in 1983, was awarded the Nobel prize for his studies of the physical processes important to the structure and evolution of stars. The observatory and its instrumentation are discussed. Scientific results are detailed CXO's role in solving the mystery of gamma ray bursts is also discussed.

Radiation Hydrodynamics

“…However, Hummer adopted a model from Heitler [114] for the redistribution in the frame of the atom for this case, which turns out to be in error. The correct formula was in fact given by Weisskopf [260] and by Wooley and Stibbs [268]. This was clarified by Omont, Smith and Cooper [200].…”