The web pages for High-Energy Astrophysics are not, in general, copies or summaries of the lectures. Rather, they are links to supplemental resources that may or may not be mentioned in class, and illustrations that could not otherwise be distributed easily. These are intended to complement the lecture, not to replace it, and I will describe their relevance in class. These web pages may continue to be modified before or after each class if additional material suggests itself, or in response to student questions and needs. The date of the latest modification will be given at the bottom of each page.
The discovery of the subatomic particles in the late 19th and early 20th centuries laid much the foundation for high-energy astrophysics. In 1895, Wilhelm Conrad Roentgen's discovery of X-rays using a Crookes tube was an instant sensation, largely because of its quick adoption for medical diagnosis and novelty entertainment. Several years later, X-rays were shown to be short wavelength electromagnetic radiation rather than a neutral particle. The mysterious neutron was at first thought to be a gamma-ray photon, so its "discovery" was not made until 1932 when James Chadwick showed that it was a neutral particle with approximately the same mass as the charged proton.
Baade and Zwicky in 1934 surmised that extremely compact and stable stars made of neutrons could exist, and that they were produced in the collapse of an ordinary star, releasing enough gravitational energy to account for the supenovae explosions. Furthermore, Baade and Minkowski in 1942 proposed that a star at the center of the Crab Nebula, the remnant of the supernova in 1054, was such a neutron star. These prescient inferences were, of course proven correct by the discovery of radio pulsars in 1967.
Natural radioactivity from trace elements in rocks ionizes molecules of air (and everything) around us. The discovery of cosmic rays came about after it was noticed in 1910 by Theodore Wulf that the rate of ionization at the top of the Eiffel Tower at a height of 300 m did not decline as much as expected if all of the ionizing radiation was coming from the ground. (Built in 1889, the Eiffel Tower was the world's tallest building until 1930). In a series of dangerous balloon ascents to heights of 5-9 km from 1912-1914, Victor Hess determined that the ionization rate actually increased with altitude at a rate that indicates that enormously energetic particles must be entering the atmosphere from above. We know know that cosmic rays are atomic nuclei, mostly protons, but with representation of all elements. It is still not known for sure what is(are) the source(s) of cosmic rays, in part because they seem to come from everywhere (Why is that?). However, the leading candiates for most of the cosmic rays are just what Baade and Zwicky first proposed in 1934, namely, supernovae.
While we will study some aspects of cosmic rays, for the most part high-energy astrophysics is concerned with X-rays and gamma-rays, i.e., high-energy photons. Shortly after WWII, the first use of rockets for astrophysics enabled the detection of X-rays from the Sun in 1948. As seen today, the hot solar corona emits X-rays from plamsa of temeperature of several million degrees Kelvin, although any other star is much too far away to be detected by those early experiments if it has similar properties as the Sun. The future development of the field X-ray astronomy was then far from obvious.
The birth of "modern" high-energy astrophysics is generally considered to be a 1962 sounding rocket flight, the results of which were reported in Giacconi et al., Physical Review Letters, vol. 9, p. 439 (1962). In 1962, Riccardo Giacconi was a scientist at American Science & Engineering (AS&E) in Cambridge, MA. A joint AS&E-MIT experiment flew proportional counters on an Air Force Aerobee rocket. This was the beginning of cosmic X-ray astronomy, as it was the first experiment to detect both the cosmic X-ray background and point celestial X-ray sources from outside the solar system; the latter were eventually determined to be accreting neutron stars. (Can you guess what else AS&E does?) Giacconi then created the first small astronomy satellite (SAS-1), also named UHURU, which was the first satellite devoted entirely to X-ray astronomy. It operated from 1970 to 1973, scanning the entire sky and detecting 339 X-ray sources, both galactic and extragalactic. HEAO-2, or the Einstein Observatory (1979-1981), was also developed by Giacconi when he and his group moved to the Harvard-Smithsonian Center for Astrophysics. Einstein was the the first satellite to use an imaging X-ray telescope. Giacconi and Harvey Tananbaum submitted a proposal for an Advanced X-ray Astrophysics Facility (AXAF), which eventually became the Chandra X-ray Observatory (see below). Giacconi went on to become the first director of the Space Telescope Science Institute, and other prominent astronomical organizations. In 2002, he was awarded a Nobel Prize in Physics for pioneering contributions to astrophysics that led to the discovery of cosmic X-ray sources.
2. High-Energy Astrophysics in the Modern Epoch
A Brief History of High-Energy Astronomy is a comprehensive history of high-energy astrophysics that is organized primarily as a summary of X-ray and gamma-ray space missions, with links to more detailed web pages on individual satellites. It is located on the web page of NASA's HEASARC (High Energy Astrophysics Science Archive Research Center). Descriptions of defunct, active, and future High-Energy Astrophysics Observatories are available. On the HEASARC page you can also view the daily entry as well as archives of the Astronomy Picture of the Day, which is a striking and/or newsworthy image, often selected to be of interest to current research. Also displayed is the less familar but more relevant to this class HEASARC Picture of the Week, which is specialized to X-ray and gamma-ray astronomy.
The currently operating Chandra X-ray Observatory is one of NASA's four Great Observatories. Originally called AXAF, the Advanced X-ray Astrophysics Facility, it was renamed after launch in 1999 in memory of the renowned astrophysicist and Nobel laureate Subrahmanyan Chandrasekhar. Focusing of X-rays in the range 0.1-10 keV is technically challenging, and Chandra is the ultimate achievement in high-quality X-ray imaging with resolution of 0.5 arcseconds - similar to that of ground-based optical telescopes. Click here to see how an X-ray telescope is made and many other details about Chandra. Columbia astronomers make extensive use of Chandra, as well as a complementary European X-ray satellite known as XMM-Newton, which was also launched in 1999 and is is even larger than Chandra. Although similar in design, the image quality of XMM-Newton was not intended to be as good as Chandra's. However, the larger area of XMM-Newton's X-ray mirrors is advantageous for X-ray spectroscopy.
The most recent addition to the the suite of imaging X-ray telescopes is NuSTAR, which is the first X-ray satellite to make images at energies greater than 10 keV. Launched in 2012, its optics were built at Columbia.
Instruments with even larger X-ray collecting area can be constructed if they are not required to produce focussed images. Historically, simple collimated proportional counters were first used to study the spectra and variability properties of bright, compact x-ray sources. The currently operating and most sophisticated mission of this type is the Rossi X-ray Timing Explorer (RTXE), named after the late Bruno Rossi, one of the pioneers of X-ray Astronomy.
2.2. Recent Gamma-ray Missions
The former Compton Gamma-ray Observatory (CGRO) was another one of NASA's four Great Observatories. It operated from 1991-2000. It had four instruments that, among them, covered the energy range 30 keV to 30 GeV, and discovered many new sources. It was not possible to focus at these energies, so source positions were determined by a variety of indirect techniques. Although the location and nature of most of the persistent gamma-ray sources emitting above 100 MeV remained uncertain, CGRO played an important role in resolving the long-standing mystery of the celestial gamma-ray bursts. In 2004, the Swift mission began operating. It autonomously slews to the position of gamma-ray busrts and studies them with a multiwavelength suite of instruments. In 2009, the successor to CGRO, the Fermi Gamma-ray Space Telescope, was launched. With unprecedented sensitivity, it has discovered GeV emission from more than 3000 Galactic and extragalactic sources, including new classes.
At very high energy (VHE), greater than 300 GeV, it surprisingly becomes possible (and necessary) to use telescopes on the ground. Such high-energy gamma-rays are not detected directly, but when the gamma-ray strikes the upper atmosphere, it makes an extensive shower of particles moving at greater than the speed of light in air, and these emit a flash of blue light (Cherenkov radiation). The newest arrays of ground-based telescopes to detect sources of TeV (1012 eV) gamma-rays are H.E.S.S. (High Energy Stereoscopic System) in Namibia and VERITAS (Very Energetic Radiation Imaging Telescope Array System), in Arizona. Approximately 176 TeV sources of various types have been discovered by these telescopes in recent years.
Although radio waves are certainly not high-energy radiation, they are often emitted by high-energy electrons that are produced by the same cosmic accelerators that that are responsible for the X-ray and gamma-ray sources that we will be studying. Indeed, radio telescopes were responsible for the discovery of many of these classes of objects. So radio astronomy is naturally a part of high-energy astrophysics. The birth of radio astronomy is generally attributed to Karl Jansky's 1933 discovery of radio emission from the Milky Way. The first systematic survey of the radio sky was made by Grote Reber.Most radio astronomy in the US is done using the facilities of the National Radio Astronomy Observatory (NRAO), which operates a variety of single-dish telescopes and interferometer arrays. Single dishes observe one position at a time and do not make images, whereas interferometer arrays can make high-resolution maps of a small region of the sky.
The largest fully steerable radio telescope is the 100-meter diameter Green Bank Telescope (GBT) in Green Bank, West Virginia. First used for scientific observations in 2001, the GBT has already detected interesting new radio pulsars, for example. Other important radio telescopes are the Parkes Observatory in Australia and the Arecibo Telescope in Puerto Rico, which at 305 meters diameter is the largest (stationary) reflector in the world, but is soon to be exceeded by the Five hundred meter Aperture Spherical Telescope (FAST) in China.
NRAO also operates the Very Large Array, which is one of the world's most productive radio observatories. It consists of 27 movable radio dishes of 25 meter diameter in a Y-shaped configuration that can be spread over 22 miles. By combining the signals in phase, images of the radio sky can be made that have the resolution of a 22 mile diameter telescope. The headquarters of the VLA is in Socorro, New Mexico. Also operated from Socorro is the Very Long Baseline Array (VLBA), which consists of 10 remotely controlled telescopes that are spread across the entire United States, from the Virgin Islands to Hawaii.
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