The X-Ray Universe by WALLACE H. TUCKER X-ray images of the Universe are strikingly different from the usual visible-light images. ILHELM ROENTGEN’S WINITIAL DISCOVERY of X-radiation in 1895 led immediately to practical applications in medicine. Over the next few decades X rays proved to be an invaluable tool for the investigation of the micro-world of the atom and the development of the quantum theory of matter. Almost a century later, telescopes designed to detect X-radiation are indispensable for understanding the structure and evolution of the macro-world of stars, galaxies, and the Universe as a whole. BEAM LINE 45 THE BIRTH OF THE FIELD did not think: I investigated.” these results were verified by the Undeterred by NASA’s rejection AS&E group and confirmed by Fried- X-ray astronomy emerged with of a proposal to search for cosmic man’s group at the Naval Research the space age, because X-ray obser- X-radiation, Giacconi persuaded the Laboratory. A new field of astrono- vatories could now be positioned Air Force to fund the project with the my had been born. As a historical above earth’s X-ray-absorbing atmo- understanding that its primary goal footnote, X-ray emission from the sphere. This may seem strange, since was to look for X rays from the moon was not detected until 1990 by X rays pass right through our flesh, moon. the Roentgen Satellite X-ray obser- which is much denser than the The AS&E team’s first two X-ray vatory (ROSAT). atmosphere. Even though the atoms astronomy rocket flights failed. The in the atmosphere are widely spaced, third try, in June 1962, was a success. the total thickness of the atmosphere THE X-RAY SATELLITES During an observation period that is so great that an X ray has a neg-ligi- lasted just over five minutes, Geiger By 1967 there were a dozen or ble chance of getting to the ground. counters a hundred times more sen- more groups involved in X-ray (The lower-energy visible-light sitive than any used before detect- astronomy, and more than thirty photons interact weakly with the at- ed a strong source in the constella- sources had been found. Major mospheric atoms and pass through tion of Scorpius, as well as a smooth advances in the field began in the with little absorption.) background glow. Within a year 1970s with the use of satellites In September 1949 a team led by Herbert Friedman of the Naval Research Laboratory was the first (0.1 keV) (200 keV) to detect X-ray emission from the 1m 1mm 7000Å 100Å 0.05Å Gamma solar corona, the hot outer layers of Radio and Microwave IR UV X-Rays the sun’s atmosphere. Their exper- Vis Rays iment consisted of a collection 800 of small Geiger counters aboard a Satellites captured German V-2 rocket. The 400 observed X-ray emission was weak, Rockets amounting to only one millionth 200 of the total energy radiated at all wavelengths by the sun. 100 The low X-ray output from the Balloons sun led many astronomers and 50 astrophysicists in the late 1950s and early 1960s to think that efforts to Altitude (km) 25 Aircraft build larger detectors and telescopes 12 to observe X rays from sources out- side the solar system would be fruit- 6 Mountain Top less. Fortunately, a group led by Observatories Riccardo Giacconi at American 3 Science & Engineering (AS&E) did not listen to the pundits. Instead Sea Level they followed the example set by Roentgen, who when asked what The absorption of X rays by the earth’s atmosphere restricts ground-based he thought when he first observed observations to radio, near infrared, and visible wavelengths. X rays are absorbed X rays in his laboratory, replied, “I high above the earth. 46 SUMMER 1995 X-ray astronomy missions, from United States Future Missions: 1960–2000. Missions Explorers HEAO-2 (Einstein) “Observer” Series AXAF Successor Skylab OAO-3 (Copernicus) AXAF SAS-1 (Uhuru) XTE Vela Series Spartan BBXRT DXS Early Rocket Flights Suborbital Program 1960 1970 1980 1990 2000 2010 Scorpius X-1 40 X-Ray First X-Ray 2000 X-Ray ROSAT Discovered Sources Known Images Sources Known Launched 60,000 Sources Known Cosmic X-Ray X-Ray Pulsars First X-Rays From Background Confirmed Discovered Space Shuttle SN 1987 A Flight equipped with X-ray detectors. The vast spaces between galaxies to the in the violent final seconds of a mas- first of these, Uhuru, was launched bizarre warped space around neutron sive star’s normal existence. The in 1970. In 1978, NASA’s Einstein stars and black holes. X-ray emission from these collapsed X-ray observatory was the first large The ROSAT mission, an interna- stars is billions of times greater than focusing X-ray telescope to be placed tional collaboration involving Ger- that from the sun. in orbit. The Einstein X-ray tele- many, the United Kingdom, and the The launch in 1970 of the Uhuru scope produced high-resolution United States, launched in 1990, has X-ray satellite by NASA made it images and accurate locations for the most sensitive detector so far for possible to monitor X-ray stars for thousands of cosmic X-ray sources. low-energy X rays of the type emit- prolonged periods of time. It was This and later missions have ob- ted by stars similar to the sun. The discovered that the X-ray emission served X rays from ordinary stars, source of X rays from these stars is from these stars undergoes rapid, white dwarf stars, neutron stars, a hot gaseous upper atmosphere, or intense, and sometimes periodic black holes, interstellar shock waves corona, that has been heated to variations. Combined observations produced by stellar explosions, the temperatures of millions of degrees with optical and X-ray telescopes nuclei of galaxies, and hot gas in Celsius. have demonstrated that these X-ray intergalactic space. Young stars less than a hundred sources are members of binary sys- The X rays detected by X-ray million years old are observed to tems in which matter streams from astronomers, like those put to use have an X-ray output a thousand a normal star onto a nearby collapsed in industry, medicine, and labora- times more than that of the sun. This star with an intense gravitational tory research, must be produced by suggests that the X-radiation from field. high-energy particles. It is not sur- the young sun could have been much In most X-ray binary star systems, prising, then, that an X-ray image stronger than it is today. How did the collapsed star is a neutron star. of the sky can look markedly differ- this enhanced radiation affect the Neutron stars are the end products ent from an optical image. In essence, evolution and chemistry of the of the evolution of stars approx- X-ray images reveal hot spots in the primordial atmosphere of the earth? imately ten times more massive universe: regions where particles X-ray observations should help to than the sun. These stars undergo a have been energized or raised to very answer this vital question. supernova explosion in which most high temperatures by phenomena of the star is expelled into space at such as strong magnetic fields, very high speeds. A shock wave anal- THE STRONGEST violent explosions, or intense grav- ogous to a immense sonic boom X-RAY SOURCES itational forces. The temperatures spreads through space, heating inferred are typically several orders The brightest X-ray sources in the interstellar gas to temperatures of of magnitude higher than those on sky are associated with the end phas- millions of degrees. X-ray observa- the surfaces of stars. Where do such es of stellar evolution: the remnants tions study these shock waves for conditions exist? In an astonishing of supernova explosions as well as clues about the origin of all of the variety of places, ranging from the neutron stars and black holes formed heavy elements from carbon on up. BEAM LINE 47 X Rays Mass Accretes influence. The critical distance from a black hole is called the gravitational Normal Star horizon. Anything that falls within Accretion the horizon—matter, light, X rays or Disk other forms of electromagnetic radiation—is pulled inexorably in- ward by the gravity of the black hole and cannot escape. X Rays No unique X-ray signature of a black hole has yet been discovered. Cross section of an accretion disk Left behind is the rapidly spinning, In general, successful black hole of a neutron star. highly magnetized, compressed core candidates meet two requirements: of the star—a core so dense that the (i) they are luminous X-ray sources electrons have combined with the that exhibit large, rapid, and some- protons to form an object composed times quasi-periodic (a stable peri- mostly of neutrons. A sample of od would indicate a neutron star) neutron star material the size of a fluctuations on a time scale of sugar cube would weigh one billion milliseconds; and (ii) optical obser- tons. Most neutron stars appear to vations of the primary star indicate have a mass about equal to that of that it has an invisible companion the sun compressed into a ball about with a mass greater than three times twenty kilometers in diameter. The the mass of the sun, the theoretical strong magnetic field on the surface upper limit for the mass of a neutron of the star can funnel the infalling star.