The X-Ray Universe by WALLACE H

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 inward 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. matter, resulting in a hot spot which To date, a half dozen such systems manifests itself as a regularly pulsing have been discovered. The best X-ray source. estimates of the black holes in these systems are about 10 solar masses. Observations by future missions BLACK HOLES such as the X-ray Timing Explorer, For a few binary X-ray star sys- scheduled to be launched by NASA tems, the mass of the collapsed object in August of 1995, are expected to is deduced to be greater than three expand this list. times the mass of the sun. These objects are presumably the end prod- GALACTIC BLACK HOLES ucts of stars even more massive than those that produce neutron stars. The Black holes of much larger mass theory of dense matter and Einstein’s are thought to lie at the center of theory of general relativity require many and perhaps all galaxies. These that such an object would collapse supermassive black holes, which in on itself to form a warp in space could contain the mass of as many called a black hole. A black hole does as a billion suns, are thought to form not have a surface in the normal when a stellar-mass black hole swal- sense of the word. It is more like a lows enormous quantities of inter- whirlpool with a critical range of stellar gas that has accumulated in

48 SUMMER 1995 It seems certain that an understanding of the nature of dark matter could change our theories of the formation of stars and galaxies, the central regions of galaxies. As gas Is the dark matter composed of falls inward, it is accelerated to high the nature of subatomic dim stars, planets, or black holes? Or energies. This energized matter pro- particles, and the does it consist of subatomic particles duces copious amounts of electro- evolution of the that interact with ordinary matter magnetic radiation over a wide range only through gravity? We do not of wavelengths. Universe. know. But it seems certain that an If the black hole is extremely mas- understanding of the nature of dark sive and the rate at which it is pulling matter could change our theories of in matter is large, the energy release the formation of stars and galaxies, can be stupendous. This is appar- Ordinarily this would also preclude the nature of subatomic particles, ently what is happening in quasars. it from producing any appreciable and the evolution of the Universe. They radiate as much energy per sec- X-ray emission. However, in certain ond as a thousand or more normal regions of space thousands of galax- X-RAY BACKGROUND galaxies from a region about the size ies have clustered together. The RADIATION of our solar system. It is as if a small amount of gas associated with these flashlight produced as much light as clusters of galaxies may have a mass Other important clues to the evo- all the houses and businesses in the equivalent to a hundred trillion suns. lution of the universe are found in entire Los Angeles basin. The only direct way to study this gas the X-ray background radiation. In black hole models for quasars, is through the X-radiation it emits. One of the first discoveries of X-ray matter approaching the gravitational X-ray observations have shown that astronomy was an unexpectedly horizon radiates predominantly the mass of the gas in clusters of strong and uniform background glow energetic X-ray and gamma-ray galaxies is considerable—comparable of X-radiation. The uniformity of photons. A study of the intensity and to the mass of all the stars in all the the radiation suggests that it is not variability of the X- and gamma- galaxies. coming from nearby galaxies, but radiation from quasars can then Unless we are seeing a cluster of from a distance so great that all the provide information on the size galaxies at a very special time when individual sources merge into a of the black hole, the rate at which gas is exploding out of the cluster, smooth background, just as the lights it is accreting matter, and other the pressure of the hot gas must be of a distant city appear as a uniform factors that are crucial to understand- balanced by the gravity of the clus- glow. ing the inner workings of these ter. X-ray observations indicate that The current opinion is that the violent maelstroms. the hot gas in clusters cannot be X-ray background radiation must confined by the combined gravita- have been produced about ten billion tional force of gas and galaxies. An years ago. Sometime between a mil- INTERGALACTIC GAS additional, as yet unobserved form lion and a few billion years after the AND DARK MATTER of matter, called dark matter, must hot Big Bang phase from which our The matter around black holes be postulated. The implied amount Universe is thought to have evolved, radiates intense X-radiation because of dark matter is enormous, about the universe made a dramatic tran- it is highly compressed and exceed- three to ten times as much as that of sition from a smooth, featureless ingly hot. Extreme temperatures can the observed gas and galaxies. state to clumps of galaxies. The also be found in the near vacuum of If the conclusions drawn from X-ray background radiation must intergalactic space. In contrast to the observation of clusters of galaxies have been produced during this gas spiraling into black holes, the so far can be generalized to the transitional period. Radiation from intergalactic gas is hot because it is universe as a whole, then dark mat- quasars and the bright nuclei of spread out. Its low density makes it ter is the predominant component of galaxies appear to be capable of easy to heat and difficult to cool. our Universe. producing a significant portion of the

BEAM LINE 49 X-Ray Flux Nested Array of Hyperboloids

Nested Array of Paraboloids Focus

X-ray background, but it is still unclear if they can produce all of it.

THE FUTURE 0510 What is clear is that future X-ray missions will probe Meters ever more deeply into space with ever more sensitive (vertical scale exaggerated to show ray paths) and versatile instruments. The most important X-ray astronomy mission of the coming decade will be NASA’s Advanced X-ray Astrophysics Facility (AXAF), which Above right: Photograph of the largest AXAF mirror. is scheduled for launch in 1998. This observatory, with Above: Schematic of nested pairs of grazing-incidence mirrors its four sets of nested mirrors, will be the X-ray equiv- which permit the precise focusing of X rays. (Courtesy alent of the Hubble Space Telescope. AXAF and other fu- Harvard-Smithsonian Center for Astrophysics, Marshall Space ture missions will provide scientists with opportunities Flight Center, Hughes Optical Systems, and Eastman Kodak). for deeper insight into black holes, dark matter, the X- ray background, and the events that led to the formation of the elegant galaxies and colossal clusters and super- clusters of galaxies that constitute our Universe.

50 SUMMER 1995