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2 from individuals and groups of scientists working in various areas of high energy astrophysics. Finally, our report has benefited from the advice and comment of numerous scientists who have read preliminary drafts of various sections. We gratefully acknowledge these sources of information and opinion. I I . THE NATURE OF HIGH-ENERGY ASTRONOMY AND THE SCOPE OF THE REPORT Galactic supernovae are seen as spectacular transient stars about once per century. They were noted as ominous "guest stars" in the ancient Chinese court records, recognized as stellar outbursts by Tycho Brahe in the sixteenth century, and understood for the first time in the 1930's to be cataclysmic explosions of stars. Since then, observation and theory have shown that every massive star ultimately reaches a critical condition in its evolution when its core suddenly collapses, releasing an amount of gravitational energy comparable with that which would be obtained by converting about one tenth of a solar mass into energy according to Einstein's formula E = mc2. It is generally believed that in the resulting explosion a large fraction of the energy escapes immedi- ately in a burst of neutrinos and gravitational waves. Most of the remainder is radiated as ultraviolet and visible light during the period of a few months when, as in the case of a guest star of A.D. 1054, the supernova may appear as a point source that grows brighter than Venus within a few days and then gradually fades, becoming invisible to the naked eye after several months. For thousands of years thereafter a vast expanding nebular remnant and, in some cases, a collapsed stellar core radiate predominantly in the nonoptical regions of the spectrum as powerful sources of radio, x-ray, and gamma- ray photons and energetic charged particles. The remnants of the supernova of A.D. 1054, now called the Crab nebula and the Crab pulsar, are among the most conspicuous sources of nonoptical photons in the sky and were among the first such objects to be discovered and studied in the exploratory phases of radio, x-ray, and gamma-ray astronomy during the 1950's and 1960's. The studies revealed the presence in the nebula of high-energy electrons and focused attention on supernovae and their remnants as sources of cosmic rays. They suggested that

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3 the accelerator of the electrons in the Crab nebula is the Crab pulsar, the collapsed core of the original star, now a strongly magnetized neutron star about 10 miles in diameter with a solar mass of matter compressed by its own intense gravity to a density of 10 billion tons per cubic inch and spinning at 30 revolutions per second. As this understanding grew, the Crab supernova remnant became a favorite object of "high-energy" astronomy, displaying in one event and its aftermath many of the extreme physi- cal conditions that we now know are ubiquitous aspects of the Universe--million-degree temperatures, ultra-high densities, high-energy particles, and intense gravita- tional and magnetic fields encountered in objects as diverse in size and mass as stars, quasars, and clusters of galaxies. High energy astronomy is the study of these conditions through observation of the radiations they produce. To an observer with unaided eye, or even to one equipped with an optical telescope of substantial power, the sky appears largely unchanging except for the motions of familiar nearby objects of the solar system. In contrast, to one equipped with satelliteborne detectors sensitive to high-energy photons, the sky is a place of rapid and spectacular change. Intense bursts of gamma rays, emanating from unknown sources scattered around the sky, and in some cases lasting for only a few seconds, occur at a rate of one every few weeks. Bright x-ray stars, distributed along the Milky Way, vary on time scales as short as a fraction of a second, some randomly and others with clocklike regularity. - ~very rew minutes one or another of a tew dozen peculiar faint blue stars emits a brilliant 10-see flash of x rays, the equivalent of turning the power of hundred thousand suns on and off within the time of one human breath. A distant quasar, radiating x rays with the power of a trillion suns, may wax or wane by a factor of 2 within a few hours. Rapid changes in such enormous luminosities imply highly concen- trated energy sources with temperatures and densities utterly beyond the scope of terrestrial experience. These and many other similar observations have drawn the attention of astrophysicists in recent years ever more forcibly to the phenomena of the "high-energy universe" and to the realization that the processes underlying these phenomena play critical roles in the formation and evolution of stars, galaxies, clusters of galaxies, and the Universe as a whole. ~ in, _

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4 The rapid pace of developments in high-energy astronomy during the past decade is vividly shown by a comparison of the state of knowledge as described in the report of the previous Astronomy Survey Committee (Astronomy and Astrophysics for the 1970's, National Academy of Sciences, Washington, D.C., 1972), hereafter called the Greenstein report, and the current situation. At that time, nonsolar x-ray astronomy, initiated less than a decade earlier - with the discovery of Sco X-1. was iust beoinnina to , . _ , assimilate the flood of discoveries from the first satellite x-ray observatory, Uhuru, launched in December 1970. Already available were a catalog of over 100 x-ray sources and results that revealed the existence of extremely luminous x-ray pulsators in close binary systems, evidence of a black hole, x-ray emission from active galaxies and one quasar, numerous unidentified but apparently extragalactic sources radiating predominantly in the x-ray region, and x rays from hot intergalactic gas in clusters of galaxies. _ from satellite x-ray observations were growing rapidly in number. Resorts of the findings Little was known about the soft x-ray sky except the existence of a diffuse and uneven Galactic background radiation. Nothing was known about sources of extreme- ultraviolet radiation, and indeed observations in this wavelength were believed to be infeasible. The existence of 100-MeV Galactic and extragalactic gamma rays had been established, but nothing was known of discrete gamma-ray sources, gamma-ray bursts, or lines in gamma-ray spectra that are now central issues of high-energy astronomy. It was known that cosmic rays consist of the nuclei of ele- ments throughout the periodic table, as well as electrons and positrons, with individual particle energies up to 10 million times the highest particle energies attained so far by laboratory accelerators, but the mechanisms of their acceleration, their confinement and propagation in the Galaxy, and their role in galactic evolution were poorly understood. Attempts to measure the flux of solar neutrinos had so far yielded only upper limits that were low compared with theoretical expectations. . . _ . . . . . Early reports of the detection ot astonishingly intense gravitational waves, subsequently discounted, had focused interest on the problem of improving the reliability and sensitivity of gravitational-wave detectors. In light of these assessments, the Greenstein report recommended several programs for high-energy astronomy in the 1970's that have, to a substantial extent, been

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5 accomplished. In particular, the Greenstein report listed among its highest priorities a series of four High Energy Astronomical Observatories (HEAO's). In the end, after budgetary constraints had forced a reduction in scope, three HEAD's were launched and operated success- fully. The first two were large x-ray observatories, and the second of these, the Einstein x-ray observatory, acquired the status of a national facility by virtue of the size of its user community and the breadth and sig- nificance of its results for all of astronomy. The third HEAD achieved major advances in gamma-ray line astronomy and cosmic-ray studies. However, some important scien- tific objectives in the original HEAO program, particu- larly in the areas of gamma-ray and cosmic-ray astronomy, were lost by the reduction in scope and remain to be accomplished by new programs in the 1980's. Recommenda- tions for support of efforts to detect solar neutrinos and for development of gravitational-wave detectors were acted on with the results that solar neutrinos have been detected, albeit at a surprisingly low flux value, and major progress has been made in increasing the sensitivity of gravitational-wave detectors, though not yet to the point of a positive detection. While high-energy astronomy in the United States achieved extraordinary advances throughout the 1970's, it now faces the certainty of a virtual standstill during the first half of the 1980's. The paucity of new starts on major space projects in high-energy astronomy during the past several years and the delays and reductions in funding of the few ongoing projects have caused a widening gap in observational capabilities. Research teams and their engineering support groups are disbanding at a time when the opportunities for major scientific progress are clearly defined and highly promising. As a result, leadership in high energy astronomy, which was pioneered in the United States, is now passing to other countries. Symptomatic of this trend is the problem of the Explorer satellite program, which yielded so many important results during the 1960's and 1970's. It has not been funded at levels sufficient even to keep pace with inflation, much less to accommodate the need for increased observational capabilities. As a consequence, no Explorer-class satel- lite for high energy astronomy will have been launched during a period of ten years since the last of the Small Astronomy Satellites was placed in orbit in 1975. In the area of low-cost vehicles, which were strongly recommended for vigorous support by the Greenstein committee Panel on

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6 Space Astronomy, there has been a persistent problem of inadequate funding for projects in high-energy x-ray, gamma-ray, and cosmic-ray astronomy. m e task of the High-Energy Astrophysics Panel has been to assess the current status and future prospects of the subdisciplines of astronomy that are based on obser- vations of the extreme ultraviolet region; x-ray and gamma-ray photons; energetic nuclei, neutrons, and electrons; and neutrinos and gravity waves. Together with photons in the radio to ultraviolet portion of the electromagnetic spectrum this list includes all known or expected forms of radiation that can reach the Earth from distant sources. The instruments used for measuring these forms of radiation are exceedingly diverse. For example, deep in a salt mine an enormous tank of cleaning fluid linked to a tiny detector of induced radioactivity is used to measure the flux of neutrinos generated by the fusion processes in the center of the Sun. Above the atmosphere the orbiting Einstein x-ray observatory records high- l resolution x-ray images of a distant cluster of galaxies and provides for the first time a measure of the mass and temperature of ultra-hot gas that pervades the cluster. In the HEAD-3 observatory a solid-state spectrometer observes the annihilation of positrons and electrons in the central regions of the Galaxy. Far away in the solar system, beyond the orbit of Jupiter, the transmitter on a planetary probe sends radio signals back to Earth, where frequency variations are analyzed for evidence of passage across the solar system of gravity waves from cosmic cataclysms. During the past decade all areas of high-energy astronomy benefited from rapid growth in observational capabilities based on the developing technologies of x-ray optics, radiation detection, solid state elec- tronics, space instrumentation, and data processing. In six years the sensitivity of resonant-bar gravitational- wave detectors, measured in terms of minimum detectable energy flux, has been improved tenfold, and another thousandfold improvement is imminent. Laser systems now under development offer the prospect of even greater sensitivities. Cosmic-ray astronomy has achieved remark- able improvements in the precision of mass and charge measurements that make possible the use of a variety of isotope clocks in the investigation of the acceleration and propagation of high-energy nuclei in the Galaxy. Instrumentation for the extreme-ultraviolet portion of

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7 the spectrum has evolved from laboratory test devices to sophisticated astronomical instruments for satellite observations with sensitivities comparable with those achieved by the Uhuru satellite at higher energies. In x-ray astronomy, where fewer than 100 sources were known in 1970, the Einstein x-ray observatory in 1979 had achieved more than a thousandfold increase in sensitivity, thereby bringing into the range of potential x-ray obser- vability several hundreds of thousands of discrete Galac- tic and extragalactic objects, including the most distant objects yet detected in the Universe. Such gains in observing power and the resulting progress in discovery and understanding are the benefits of investment in supporting research and technology. A vigorous program of instrumentation development is therefore essential to achieving the scientific goals of high-energy astronomy. Supporting research and technology must be considered an essential ongoing activity in each of the subdisciplines, sustained at a level of effort commensurate with the technical opportunities and relatively unaffected by the delays in funding of large projects. Looking forward to the 1990's, we see an urgent need to stimulate and encour- age the development of new technologies that will advance high-energy astronomy beyond the status we see it attain- ing in the 1980's on the bases of current instrumentation concepts. Discoveries in high-energy astronomy during the past decade have presented an overwhelming challenge to theoretical research aimed at creating the intellectual framework both for understanding what is observed with the new instruments and for providing ideas as to where and how to look for interesting new phenomena. Without adequate growth in such understanding, the efficiency of observation is reduced and its purpose unfulfilled. A significant part of the progress in the theory of high- energy cosmic phenomena has been achieved during the past decade by theoretical research supported by the data- analysis portions of project funds. But mission-oriented theoretical research suffers from inflexibility and a lack of long-term continuity. We have therefore recom- mended that a high priority be placed on providing new funding for theoretical research in high-energy astronomy, independent of specific hardware projects and missions, and at a level commensurate with the total task of interpreting the results of the observational programs. As we look to the future we see good opportunities for a continuation of the age of discovery in high-energy

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8 astronomy. Major discoveries are anticipated in gamma- ray astronomy, and it is therefore of fundamental importance that the Gamma Rat Observatory (GRO) be constructed and flown. This mission has been thoroughly studied and is now approved as the essential next step in the development of gamma-ray astronomy. m e Extreme Ultraviolet Explorer (EUVE), the essential next step in the development of E W astronomy, has been selected for early development within the Explorer program. We have predicated our consideration of future plans on the assumption that these two vitally important missions will be carried through. ~ Beyond them we see clearly defined needs for new missions on free-flying satellites of the Explorer class, launched either by the Shuttle Transporta- tion System or by independent rockets. Short-term Space- lab missions will be of great value in some investigations but will never be adequate substitutes for free-flyers in most areas of high-energy astronomy. We therefore strongly recommend that the Explorer program be substan- tially increased to accommodate the scientific needs. Among the many important new scientific opportunities that beckon high-energy astronomy in the 1980's, we believe that at present those in x-ray astronomy are the most numerous and exciting. It has become clear in the past decade that x-ray observations provide unique infor- mation not only about exotic astrophysical processes connected with collapsed stellar objects and supernova remnants but also about stars of all types, the inter- stellar medium, normal galaxies, radio galaxies, active galactic nuclei, and clusters of galaxies. These comprise most of the subjects under intense investigation in the mainstream of observational astronomy carried out in the optical and radio regions of the spectrum. To gain this information, x-ray astronomy now requires long-term observational capabilities comparable in scope with those available in the optical and radio domain. The Shuttle Transportation System will make this possible by providing the means to launch a large x-ray observatory that can be operated as a national facility with routine servicing, refurbishment, and installation of new instruments over period of many years. We have therefore come to a firm conclusion that development of the Advanced X-Ray Astro- physics Facility (AXAF) should be undertaken as the project of highest priority for astronomy in the 1980's. It should be started at the earliest possible date, and special institutional arrangements for its scientific management and operation similar to the Space Telescope Science Institute should be planned and implemented.