HIGH-ENERGY RADIATIONS FROM SPACE

HIGH-ENERGY RADIATIONS FROM SPACE

Ernst Stuhlinger and Carroll Dailey

National Aeronautics and Space Administration George C . Marshall Space Flight Center

Marshall Space Flight Center, Ala. 35812

The stars, so we believed until recently, go through most of the phases of their evolution extremely slowly, with the rare exceptions of nova and supernova events. The space between the stars was assumed to contain nothing except gravitational fields, meteroids, cosmic rays, and those electromagnetic waves that can be observed at or near the surface of the earth. The first decade of space flight brought a significant change in these beliefs. Although astronomical observations from spacecraft during this first decade were still few and modest, they provided a wealth of new results that are most exciting, and that suggest that the next 20 or 30 years of space exploration may write a new chapter in the history of astrophysics and astronomy as important as the chapters initiated by Galileo or by the five-meter telescope on Mount Palomar.

The atmospheric sheath surrounding our earth is equivalent in mass to a layer of mercury two and one-half feet thick. The sheath cannot be penetrated by most of the infrared radiation, by all the ultraviolet below a wavelength of 3,000 A, by most of the gamma rays, and by many of the cosmic rays that are emitted by celestial sources. Very strangely, the atmosphere is practically impenetrable for the high-energy quanta of ultraviolet, x rays, and gamma rays, while it is transparent for the visible and some of the infrared and radio waves. In addition to the atmospheric absorption, a magnetic shielding of the earth exists; the earth-magnetic field deflects all the charged particles below certain energy levels that arrive from space and prevents them from even reaching the upper layers of the atmosphere. Rockets and satellites that travel beyond the earth’s atmosphere can detect and record all those electromagnetic radiations and particles that are not subject to absorption by interstellar matter or to deflection by the earth’s field. The farther a spacecraft moves away from earth, the lower the limiting energy at which charged particles can still be recorded. A detector for low-energy particles placed on the moon can record the “solar wind,” a flow of relatively slow (500-800 km sec-*) atomic nuclei emitted by the sun.

Observations of electromagnetic and particle radiations were made during past years with high altitude rockets, with satellites and spacecraft such as Explorer, Pioneer. Orbiting Geophysical Observatory, Orbiting Solar Observa- tory (FIGURE 1 ) , Interplanetary Monitoring Platform, Mariner, and Apollo, and particularly with OAO I1 (FIGURE 2) , the Orbiting Astronomical Observa- tory launched in December of 1968. TABLE 1 lists these projects, and also some of those projects that will be devoted to astronomical observations later in this decade.

Ultraviolet light in the form of continuous spectra and of line spectra is emitted by hot stars. The higher the temperature at the surface of a star, the farther the maximum of its energy emission shifted toward the short wavelength region, and the higher the state of ionization of its atoms. We have learned

234

Stuhlinger & Dailey: High Energy Radiation 235

FIGURE 1 . Orbiting solar observatory.

TABLE 1

HISTORY OF HIGH-ENERGY ASTRONOMY RESEARCH IN SPACE

Time Project UV X-Ray Gamma Cosmic

1946+ Rockets 0 0 0 n 1958- Explorers n n 0 n 1958+ Pioneer 0

1 9 6 b OGO 0 0 0 0

1962- oso n 0 0

1963- IMP 0

1968+ OAO 0

1969-72 Apollo 0 n 1 9 7 h SAS 0 0

1973 Skylab n n

1975- HEAO 0 0 0

(1978) LST n

236 Annals New York Academy of Sciences

FIGURE 2. Orbiting astronomical observatory (OAO 11).

from the space observation of ultraviolet spectra that the “hot” stars are much hotter than we had thought, and that some of them are emitting energy at an unbelievably high rate; at that rate of energy loss, they cannot live longer than a few thousands or even hundreds of years. Our former concepts of energy production in stars need a revision, possibly even by postulating processes of energy production still unknown today. The most successful astronomical satellite to date, OAO 11, has provided information on the blue and ultraviolet spectra of many thousands of stars; a “blue star atlas” is being assembled now on the basis of this information. The star maps in this atlas will look quite different from our well-known maps of visible stars. The principal investigators for the OAO I1 Project were A. Code of the University of Wisconsin, Madison, Wis., and F. Whipple of the Smithsonian Astrophysical Observatory, Cam- bridge, Mass.

X-ray emissions from stars had been anticipated for some time. The first opportunity to search for x rays from a vantage point above the atmosphere was offered by the launching of old German V-2 rockets at White Sands Proving Ground in 1946. These high-altitude flights revealed the presence of x rays, probably emitted by the sun. A few years later, this assumption was confirmed; Byram, Chubb, Friedman and other members of the Naval Research Laboratory detected x rays that definitely came from our sun. This discovery led to an urgent desire for a telescope that could provide an x-ray image of the sun, and possibly of other celestial x-ray sources, similar to solar and stellar images


produced by visible light through ordinary telescopes. Such imaging x-ray telescopes can indeed be built. The theory of x-ray mirrors for imaging telescopes was developed around 1940 by Wolter in Germany. The surface of an x-ray mirror follows in part a paraboloid, and in part a hyperboloid, as illustrated in FIGURE 3. The x rays impinge upon the surface under a very small angle, smaller than one degree; their angle of reflection is equally small. Several

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FIGURE 3. Typical x-ray mirror configuration.


mirrors of this kind have been carried by rockets to altitudes above the atmosphere. FIGURE 4 shows an image of the sun, photographed through an x-ray mirror in the wavelength region of about 1-20 A. The telescope was built by the American Science and Engineering Corporation and carried aloft by an Aerobee rocket. The picture shows a number of very interesting structures with a resolution of several arc seconds. It is clear that x rays are being emitted not only from hot areas of the photosphere, but also from the chromosphere and from a number of prominences and other archlike plasma regions reaching far into the corona. One of the next large satellites, the manned Skylab A, to be launched in 1973, will carry two large x-ray telescopes for the observation of solar x rays (FIGURE 5 ) . By contrast with short-lived rockets, Skylab A will

FIGURE 4. Image of the sun taken with x-rays. (Courtesy of American Science and Engineering Corp.)


FIGURE 5 . Skylab A.

remain active for about eight months. Continuous observations of solar x-ray activities will thus be possible. Another x-ray satellite, the unmanned Small Astronomical Satellite SAS-I (FIGURE 6 ) , was launched last December. It will continue circling the earth in an equatorial orbit for about one year, and it will scan the sky for galactic and possibly extragalactic x-ray sources.

A number of different processes are known that lead to the generation of celestial x rays. Very hot stars emit x rays from dense stellar matter simply as a part of the continuous spectrum of electromagnetic radiation from hot bodies that we often call Planck spectrum. Very hot gasses, consisting of ions and fast electrons, emit “bremsstrahlung” generated in the collisions between electrons and ions. Similar collision processes give rise to line spectra of x rays characteristic of the heavier elements whose ions are excited by fast electrons. A fourth process of x-ray production, first discovered during the operation of synchrotron-type accelerators, occurs when fast electrons interact with a magnetic field near a star. The electron trajectories are bent into circles by the field, and the gyration results in the emission of electromagnetic waves called synchrotron radiation. When electron energies and magnetic field strength are sufficiently high, the synchrotron radiation consists mostly of x-ray quanta. There is still another process that produces x rays in space. It is called “inverse Compton effect.” In the normal Compton effect, a high-energy quantum impacts upon an electron and transfers part of its momentum and energy to the electron;


FIGURE 6. Small astronomical satellite (SAS-I).

in the process, the electron gains and the quantum loses energy. In the inverse Compton effect, a fast electron and a low-energy quantum collide, with the result that the electron loses and the quantum gains energy. In this way, energetic electrons can produce x rays from low-energy infrared radiation.

The first evidence of a nonsolar celestial x-ray source was obtained by a sensor developed by the American Science and Engineering Corporation, and carried to high altitudes by an Aerobee rocket in 1962. The source was identified with a small blue star of twelfth magnitude in Scorpius. About 98% of the total energy emitted by this star falls into the x-ray region. If the star took the place of our sun, we on earth would receive an x-ray intensity of 2,000 kw

Stuhlinger & Dailey: High Energy Radiation 24 1

on every square meter facing this star. We do not know in detail how this tremendous x-ray production can be understood. In all likelihood, a high surface temperature of the stellar matter is the prime reason. The x-ray star in Scorpius is believed to be one component of a close binary, a remnant of a previous nova explosion.

So far, about 60 discrete x-ray sources have been found, as shown on the sky map in FIGURE 7. One source, Centaurus XR-2, was not yet detectable in 1965. In April of 1967 it was the brightest x-ray star in the sky, but by fall of that year it had disappeared. Most interesting among all the known x-ray sources are those representing the late remnants of supernovae. Several supernovae were observed during past centuries in our own galaxy, the Milky Way. After their initial explosions, they remained so bright for several months that they could be seen during the daytime. Such dramatic events were always recorded in contemporary diaries. On July 4, 1054, a supernova lighted up in the constellation of Taurus; its remnant is the well-known Crab nebula (FIGURE 8). Several independent reports of Chinese observers exist in which the “visiting star” is described. The emission of x rays from the Carb nebula was first detected in 1963; the history of its further study belongs to the most dramatic courses of events ever experienced in astronomy. In 1964, Friedman and his co-workers made an experiment to determine the magnitude of the active region within the nebula that emits x rays. They launched an Aerobee rocket with an x-ray detector during the few minutes when the moon moved across the nebula. The recording of x rays during the gradual occultation permitted the determination of the area from which the x rays are coming; this area has a diameter of about two light years, roughly one-fourth of the diameter of the visible nebula. In 1967. British astronomers discovered the

@ Identifled Optically

90

FIGURE 7. X-ray sources in the sky (1967). (Courtesy of American Science and Engineering Corp.)


FIGURE 8. Crab nebula.

first pulsar, a star that emits radio waves with pulsating intensity. Several dozen radio pulsars are known today. The frequencies of the pulsations are extremely constant; they range from about l/second to about 30/second. The Crab nebula emits radio waves that pulsate 30.2 timedsecond. In 1968, it was found that a fifth magnitude star in the center of the nebula shows a pulsation of its visible light intensity at exactly the same frequency, and in 1969 Friedman and his co-workers proved that even a part of the x-ray emission from Crab pulsates at precisely this frequency. These discoveries led to a most surprising and exciting picture of the Crab pulsar, suggested first by Thomas Gold of Cornell University, Ithaca, N.Y. When the supernova explosion occurred almost 1,000 years ago, the star underwent a “gravity collapse,” after having used up its store of nuclear energy during its red giant phase. The gravity collapse, whose last and most violent steps took only a few seconds, resulted in a superdense dwarf with a diameter of only 10-100 km, but a mass of about 10 solar masses and a density of a hundred to a thousand million tons/cubic centimeter.

Before the gravity collapse, the star turned around its axis at about the


rate the sun revolves around its axis. As the star contracted, however, the conservation of angular momentum caused its rotational speed to increase to about 50 revolutions/second by the time the collapse was completed.. The magnetic field of the star, which originally may have been of the same order of magnitude as that of our sun, increased to about 1 million million gauss. Under these very unusual circumstances, the matter within the star degenerated to pure neutron matter, while only the outermost layers retained dense plasmas of electrons and protons. These layers act now as sources for radio waves, light, x rays, and gamma rays. The pulsation frequency of those radiations we observe from earth equals the rotational frequency of the neutron star. The emission of radiative energy is not distributed uniformly over the surface; hence the pulsation of the signal received on earth. The continuous loss of energy of the neutron star in the form of x rays alone equals about 2,000 times the total energy loss/second of our sun. Because of this energy loss, the original rotational speed of 50 revolutions/second decreased during the past thousand years to the 30.2 revolutions/second we observe today. This fantastic picture of the Crab pulsar, which follows quite naturally and logically from our observations, confirms very clearly the concept of the neutron stars that was designed some years ago by astrophysicists on the basis of purely theoretical studies.

Our own galaxy, the Milky Way, is not the only place in the cosmos where we have found x-ray sources. At least two other galaxies proved to be strong x-ray emitters, although the detectable intensities are small because of their large distances. The galaxy M87 in Virgo A, shown in FIGURE 9, and also the

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FIGURE 9. X-ray emission of the galaxy M87 in Virgo A. (Courtesy of US. Naval Research Laboratory.)


galaxy Centaurus A were proven to be x-ray emitters. Both are also strong radio galaxies. The images of these galaxies imply that a beam of plasma, called plasmon. must have broken out from the galaxy a long time ago. It is possible that the emission of these plasmons is somehow related with the strong x-ray emissions from these galaxies.

A quasar, 3C273, has also been found to emit x rays. The distance of this object may be as large as 1.5 billion light years. If this estimate of distance should be correct, the total energy emitted from this quasistellar object is so enormous that all of our existing knowledge about energy production in stars and galaxies does not suffice to explain the mechanism of this energy production. Possibly, the mutual annihilation of matter and antimatter may be going on in this quasar. The x-ray energy alone emitted in each second by 3C273 is about 1,000 times greater than the total energy emitted over the entire wavelength spectrum by the Milky Way.

Several of the cosmic processes that generate x rays also produce electromagnetic radiation of higher quantum energy. This radiation is called gamma radiation. Possibly, part of the gamma radiation that we can observe from celestial sources is produced by annihilation of matter and antimatter. In addition, a prolific source of gamma rays must be seen in radioactive nuclei generated in the interior of stars.

So far, our observations of cosmic gamma rays have been very meager. Explorer 11, and later, OSO 3, recorded gamma quanta as a diffused background radiation. However, more measurements of the gamma-ray background and possibly of discrete gamma-ray sources are badly needed.

One of the most fascinating aspects of gamma-ray astronomy is the unique relationship thought to exist between the observable gamma radiation and the events occurring when a star explodes to become a supernova. These events may be the key to the synthesis of all the elements in the upper half of the periodic table that has been so difficult to explain. The reason for the intense interest in observations of gamma rays is that the various schemes proposed for nucleosynthesis phenomena differ in the prediction of specific gamma-ray energies that should be observable. In the “rapid process,” a number of radioactive isotope nuclei with short lifetimes are generated within the chains of transmutations leading to the synthesis of heavy elements; their gamma-ray emissions form characteristic gamma-ray line spectra that can be used to identify these processes. In the “slow process,” the formation of heavier elements takes a different route without the short-lived isotopes. Observation of these processes through their gamma rays may be the key to our understanding of element formation in stars. The instrumentation planned for future spacecraft should provide experimental evidence by which the theories can be judged.

To date, most gamma-ray observations have been made from balloons, and some from rockets and satellites. Although valuable measurements have been made in this way, including the identification of the first known celestial gamma- ray source in Sagittarius, the data are still too scarce to permit an assessment of the numbers and places of cosmic gamma-ray sources.

One satellite, OSO 111, has shown that the diffuse background of gamma rays at low energies appears to be concentrated in the plane of our galaxy, and that it seems to reach a maximum in the direction of the galactic center. Another satellite, Small Astronomy Satellite B, is scheduled to be launched in 1972 to further study the gamma-ray spectrum.

There is a clear need for heavier, more sophisticated satellite instrumentation


that can provide good resolution of the energy spectrum and the spatial struc- ture of gamma-ray flux.

The High Energy Astronomy Observatory (HEAO) will provide the first opportunity for detailed research of this kind. This program is presently in the planning stages. Gamma-ray experiments on the first two spacecraft, HEAO-A and HEAO-B, range in mass from about 240 kg to about 1300 kg; these spacecraft are presently scheduled to be launched in 1975 and 1976. The total payloads of these two spacecraft, which also provide x-ray and cosmic-ray research capability, are listed in Tables 2 and 3. By slowly rotating about a line that is always pointing toward the sun as it moves on its yearly journey through the ecliptic, each of these two spwecraft will scan the entire celestial

FIGURE 10. Artist’s concept of HEAO-A.

sphere for radiation sources and background. An artist’s concept of HEAO-A is shown in FIGURE 10.

There are three gamma-ray experiments associated with HEAO-A and HEAO-B. One of these, for which Dr. Lawrence E. Peterson of the University of California, San Diego, is the Principal Investigator, will concentrate on the diffuse and point sources of gamma rays in the energy range of 300 KeV- 10 MeV. His instrument uses arrays of crystals that respond to the passage of gamma rays by emitting pulses of light that can be detected by photomultiplier tubes and transformed into electronic signals for transmission to the ground.

For HEAO-B, Dr. Robert Hofstadter of Stanford University, Stanford, Calif., will use an instrument that will weigh about 1,300 kg. This instrument

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will record the behavior of high-energy radiation in a spark chamber (FIGURE l l ) , a gas-filled enclosure with layers of fine, closely spaced wires that register electrical pulses as the secondary products of the gamma rays, generated within the chamber, traverse the instrument. The physical location of each secondary particle inside the chamber can be used to reconstruct the gamma ray’s trajec- tory, and thereby identify with high precision its arrival direction. This tech- nique, together with the massive absorbing crystals, will permit determination of quantum energies, and also measurements of source location to better than

FIGURE 11. Spark chamber for HEAO-B. (Courtesy of NASA-Goddard Space Flight Center.)

one degree even at energy levels of 10 billion electron volts, where the radiation intensity is expected to be extremely low. The other gamma-ray instrument on HEAO-B uses a lithium drifted germanium detector, cooled to about 77” K to make high-resolution measurements of line spectra of gamma-ray sources in the range of 60 KeV-10 MeV. This experiment is under the responsibility of Dr. A. Jacobson of the Jet Propulsion Laboratory in Pasadena, Calif.

The third component of the highly energetic radiation reaching us from space is termed cosmic rays. The mystery attached to this radiation is illustrated by the name they were given when first discovered, “cosmic” meaning we did not know where they were coming from, and “rays” meaning we did not know exactly what they were. Although the term strictly should include all radiation

Stuhlinger & Dailey : High Energy Radiation 249

of extraterrestrial origin, cosmic rays are generally considered to be the par- ticulate component, including nuclei, fragments of nuclei, meson particles, and electrons that have been accelerated to high velocities by cosmological processes. In fact, cosmic radiation can be thought of as a tenuous, extremely hot ionized gas of high pressure, whose effects on the galactic magnetic field and the interstellar medium are considerable. Because the interstellar gas clouds probably form the raw material for star formation, the clues to the origin of the universe may be hidden in the properties of the cosmic rays that impinge on the earth's atmosphere. There is some analogy between exploration of the moon's crust to study its composition and the study of the atomic nuclei of cosmic rays to learn about the composition of the universe. Cosmic rays represent matter coming to us from inside and outside our solar system.

Observations from the ground, from balloons, and from rockets have provided much of our knowledge of cosmic rays, and small satellites such as Explorer, IMP, and OGO have contributed by unraveling many of the features of chemical abundance and energy spectra of cosmic rays. Additionally, Russian scientists have orbited a series of heavier satellites in the Proton class that have pioneered in the direct measurement of the energy and composition of cosmic rays.

One of the intriguing bits of evidence uncovered by space observations is the high abundance of the heavy elements compared to that which would be expected if the sun and other stars that have been studied spectroscopically were typical of the average composition of the universe. This could be due either to preferential acceleration of these nuclei or to sources that possess a composition different from that of the sun and many other stars. Pointing toward the latter possibility is the complexity of the violent events associated with supernovae explosions; studies of the recently discovered pulsars, which are thought to be neutron stars resulting from supernovae, tend to support this hypothesis. Another surprising feature of cosmic rays is the high relative abundance of certain specific elements such as isotopes of lithium, beryllium, and boron. Because these nuclei are likely to have been formed by fragmentation of other elements such as carbon, nitrogen, and oxygen during passage through the interstellar medium, the study of the measured ratio of abundance of the two groups should be indicative of the amount of such interstellar matter and also of the time since the formation of the nuclei.

Competing theories on the processes responsible for producing cosmic rays predict abundance peaks ,at certain locations near the high end of the periodic table. Observations that can uniquely identify such peaks will have a profound effect on our understanding of the universe. Some of these theories predict, for instance, that several stable elements beyond any elements that have been produced on earth may exist somewhere in the universe.

The study of cosmic rays at low energies (less than about 10 billion electron volts) is severely complicated by the presence of magnetic fields within the solar system. Therefore. direct measurements of galactic and extragalactic cosmic rays will be limited to the study of very energetic particles that are relatively unaffected by the solar environment. The most important opportunity for such observations will be the High Energy Astronomy Observatory. One of its instruments. which will weigh about 2,400 kg. will undertake the most detailed research yet attempted on cosmic rays to measure the relative abundances of the nuclei and to measure their energies at levels from 10 billion to 100 trillion electron volts. The massiveness results largely from tungsten plates, whose


mass is required to absorb the energetic particles, and to produce showers of nuclear fragments that can be detected and correlated with the energies of the primary cosmic rays. This instrument was proposed by Dr. Jonathan Ormes of Goddard Space Flight Center. Another instrument is designed chiefly to identify the extremely heavy nuclei, including those which may exist beyond the elements currently known. Dr. Martin Israel of Washington University in St. Louis is principal investigator for this experiment.

For HEAO-B, the primary cosmic-ray instrument is a magnetic spectrometer which was proposed by Dr. Louis Alvarez of the University of California, Berkeley, Calif. This instrument will use an intense magnetic field generated by a superconducting magnet. The measurement of energy will be based on the amount of curvature of the paths of the charged nuclei in the presence of the magnetic field. This experiment offers the exciting possibility, not represented in other types of cosmic-ray instrumentation, of distinguishing between the matter and the antimatter components of cosmic rays by the direction of the curvature. The observation of an appreciable component of antimatter would have profound cosmological significance.

During the short period of space astronomy, we have learned that the stars, and the space between them, are far more active than previously thought. In fact, hot stars are much hotter, short-lived stars live a shorter time, dwarf stars are smaller, energetic processes are even more energetic, young stars are younger, fast changes are faster, and violent events are more violent than we had known before. Obviously, mechanisms of generation and transformation of radiated energy are under way on and near stars that are still entirely unknown to us. Observation of these processes from spacecraft, however, will guide us in the design and performance of decisive experiments in our earth-bound laboratories, and they will certainly help us in finding processes which will be useful to man’s life on earth. After all, our knowledge of the hydrogen fusion reaction was obtained through observations of stellar processes, and many of the particles with which we are dealing in high-energy physics laboratories today such as positrons, muons, pions, and mesons were first discovered in cosmic-ray experiments. With its ultraviolet and x rays, its gamma and cosmic rays, and the stars where these radiations originate, nature is offering us a look into a gigantic laboratory that we will always be unable fully to duplicate on earth; however, we are using this laboratory as experimenters in our space physics and space astronomy projects. So far, this country is still holding the lead among nations in the space sciences. Skylab, the High Energy Astronomy Observatory, and other scientific spacecraft presently planned will, hopefully, help us retain this lead.

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HIGH-ENERGY RADIATIONS FROM SPACE