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12 4. Support of Rocket and Balloon Programs Experiments on sounding rockets and balloons have had central roles in the development of high-energy astronomy, both in the exploration of new domains and in the develop- ment of instruments. Their essential characteristics are comparatively low cost and short preparation times, neither of which is available in the currently planned Shuttle operations. We therefore recommend that the rocket and balloon programs be maintained at levels commensurate with the scientific needs and that adequate funds be made available to advance the capabilities of these vehicles in directions that will enhance their value to astronomy. 5. Support for Air-Shower Studies Progress in understanding the nature and origins of the most energetic cosmic radiations requires observations with large ground-based installations of the secondary particles of Cerenkov light generated in air showers. We recommend that support be given to air-shower studies that address these important problems of ultra-high-energy astronomy. IV. X-RAY ASTRONOMY A. Introduction The first extrasolar x-ray source and the isotropic x-ray background were discovered in an exploratory rocket experiment in 1962. Since then, x-ray astronomy, based on observations of photons with energies in the range from 102 to 105 eV, has developed rapidly into a major branch of astronomy. By the end of the 1960's rocket and balloon observations had revealed about three dozen discrete sources including two identified with extragalactic objects. The broad spectral character- istics of some of these sources had been determined, and large irregular variations on time scales ranging from months to minutes had been observed in a few. Several Galactic x-ray sources had been identified, six as super- nova remnants, two as faint variable stars with optical spectra resembling those of old novae that were known to be close binary systems of low total mass, and one as the

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13 pulsar in the Crab nebula. The x-ray stars, unlike old novae, were found to have x-ray luminosities that are thousands to hundreds of thousands of times the total luminosity of the Sun. Important elements of the theoretical basis for under- standing the nature of x-ray stars had been developed prior to their discovery. The end products of stellar evolution were believed to be white dwarfs, neutron stars, or possibly black holes. Also the flow of mass from one star to another in a close binary system was believed to be the cause of certain optical phenomena observed in some spectroscopic binaries. With this background the hypothesis was put forward in 1967 that x-ray stars are neutron stars accreting matter drawn from a nuclear-burning companion in a close binary system. It was recognized that the conversion into kinetic energy and subsequently into heat of the gravitational potential energy of matter falling toward the surface of a neutron star would raise the temperature of the matter to the point where it would radiate thermal x rays at a rate per unit mass of accreted material amounting to a substantial fraction of _2. m is is about 100 times more efficient than the generation of heat by nuclear fusion in the interiors of nuclear-burning stars. Very low rates of accretion, amounting to only one solar mass per 108 to 109 years, would yield sufficient power to maintain the luminosities of x-ray sources at their observed values. The proposed mechanism explained how very large x-ray luminosities could be generated with the observed spectral and variability characteristics in small regions contain- ing hot, dense plasma held together by gravity and replen- ished by turbulent accretion flows. The recognition in 1968 that neutron stars actually do exist in the form of isolated rotating radio pulsars made the accreting neutron star model of x-ray stars more plausible. Nevertheless, there remained doubts as to whether close binary systems with neutron stars actually exist and whether the model could explain the variety of the spectra and variability found among the known x-ray stars. The identification of several x-ray sources with super- nova remnants had opened a new approach to the study of the physical processes involved in the propagation of shocks in the interstellar medium. Postshock tempera- tures in typical supernova remnants are in the range for soft x-ray emission. Detailed measurements of the x-ray spectra and surface brightness can therefore provide critical tests of theoretical models of supernova. m e

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14 broad spectral characteristics of the Tycho and Cas A remnants indicated that their x-ray emissions are, in- deed, thermal radiation of hot, optically thin plasma clouds. The expectation was that their spectra would be rich in narrow emission lines characteristic of the various elements and of the thermodynamic states of the nebulae. In contrast, the x-ray emission of the Crab nebula appeared to be the high-energy portion of the synchrotron radiation that dominates the optical and radio portions of the Crab spectrum. If the latter were true, the Crab nebula x rays should be substantially polarized and have no emission lines. The only extragalactic x-ray sources that had been identified during the 1960's were a source in the Large Magellanic Cloud and the giant elliptical galaxy, M87. The latter was found to have a ratio of x-ray to optical luminosities that is very large in comparison with the ratio for our own Galaxy. There was reason to antici- pate, therefore, that more sensitive observations in the 1970's would reveal additional "x-ray galaxies" with x-ray generators far more powerful than those in x-ray stars or supernovae. The detectors used in rocket and balloon observations of x rays through the 1960's were primarily proportional gas counters and scintillation detectors with large sensitive areas and various electronic and shielding devices to suppress background counts. Mechanical collimators were used to define the fields of view. Grazing-incidence x-ray optics was under development, and preparations were under way for the use of image-forming x-ray telescopes in solar x-ray photography on the Apollo Skylab mission. However, the great power of x-ray imaging devices would not be brought to bear on the problems of extrasolar x-ray astronomy until the launch of the Einstein x-ray observatory late in the 1970's. B. Progress during the 1970's 1. Major Achievements The 1970's were heralded by the launch of Uhuru (SAS-1) the first of the small satellites devoted to x-ray astronomy that included the British satellite ARIEL-5, the third Small Astronomical Satellite (SAS-3), and the Japanese satellite, Hakucho. X-ray experiments were also carried on the Orbiting Solar Observatories-7 and -8, on

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15 the Copernicus Observatory, and on the Astronomical Netherlands Satellite (ANS). Among the major achievements of these missions were the following: (a) Comprehensive sky surveys yielding catalogs of several hundred Galactic and extragalactic sources with positions of sufficient accuracy to permit definite identifications of dozens of optical counterparts; (b) Discovery of pulsating x-ray stars in binary systems and demonstration that they are rotating, accreting magnetized neutron stars with masses near 1.4 solar masses and radii of about 10 km; (c) Discovery of x-ray bursts and demonstration that they are caused by thermonuclear flashes of material accreted onto the surfaces of neutron stars; (d) Detection of unique variations in Cyg X-1 that support the idea that it is a black hole in a close binary; (e) Detection of x rays from an isolated white dwarf and from nondegenerate stars; (f) Discovery of hot x-ray emitting intergalactic plasma in certain clusters of galaxies and demonstration that the plasma has approximately the normal cosmic abundance of iron and, in typical cases, a total mass comparable with the total visible mass of the galaxies in the cluster; (g) Detection of variable x-ray emission from active nuclei of galaxies including radio galaxies, Seyfert galaxies, and quasars. Rocket and balloon investigations were also vigorously pursued both in the United States and abroad, and among their important results were the following: (a) Discovery of the first long-period pulsating x-ray source; (b) Discovery of an electron cyclotron resonance feature in the x-ray spectrum of a pulsating x-ray star at an energy that implies a surface magnetic field of several times 1012 gauss; (c) Detection of the K line of iron in the spectrum of an x-ray binary; (d) Detection of x rays from a binary system contain- ing a white dwarf; (e) Detection of coronal emission from the nondegen- erate star Capella; (f) Detection of the K line of oxygen in the spectrum of the Puppis supernova remnant; (g) Detection of polarization of the x-ray flux from the Crab nebula;

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16 (h) Detection and mapping of soft x rays from a million-degree component of the interstellar medium. These developments demonstrated conclusively that x-ray observations are a prolific source of unique information about the high-energy universe on all size scales from stars to clusters of galaxies. Toward the end of the decade, HEAD-1 (the first of the large High Energy Astronomical Observatories) achieved a major extension of observational capabilities with instrumental techniques similar to those employed in the small satellites. With large increases in the sensitive areas of detectors and significant refinements of angular and spectral resolution over all previous missions, HEAD-1 carried out new all-sky surveys and extensive pointed observations. Among its achievements were the following: (a) Demonstration that many of the previously known but unidentified x-ray sources at high Galactic latitudes are cataclysmic variables or nondegenerate binaries of the RS CVn type, which constitute a general class of powerful coronal x-ray emitters; (b) Discovery of periodic variability in the x-ray emission of U Geminorum, a cataclysmic variable of the dwarf nova type; (c) Discovery of iron K-line emission in the spectrum of the x-ray pulsar Hercules X-1; (d) Demonstration that N-type galaxies and BL Lac objects are powerful x-ray emitters; (e) All-sky mapping of the diffuse x-ray background from 0.2 to 50 keV and demonstration that the spectrum of the component above 3 keV, which is predominantly extra- galactic in origin, matches that of an optically thin hot gas at a temperature corresponding to a value of kT equal to 40 keV; (f) Accurate measurement of the x-ray spectra of many of the nearer active galaxies and clusters of galaxies and determination of their x-ray luminosity functions, leading to the conclusion that the diffuse extragalactic x-ray background above 3 keV is not a composite of the spectra of distant unresolved objects of similar type and evolutionary development. Finally, in 1978, the first satelliteborne image- forming, grazing-incidence x-ray reflection telescope was placed in operation in the HEA0-2, the Einstein x-ray observatory. With a 60-cm-diameter objective mirror and photon-counting image detectors that provided angular resolutions as fine as several arcseconds, and with both objective and image-plane spectrographs that provided

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17 spectral resolutions up to several hundred, the Einstein x-ray observatory achieved a hundredfold increase in sensitivity over the best previous observations and broadened the scope of x-ray astronomy to encompass virtually the entire subject matter of contemporary astronomy. Several hundreds of thousands of objects were accessible to x-ray observations by the Einstein x-ray observatory. Its important results are far too numerous to describe in detail or even to list. The following are some representative highlights: (a) Detection of coronal x-ray emission from single stars of every spectral type, including pre-main-sequence stars, and M dwarfs, the most numerous stars in the Universe; (b) Detailed examination of the distribution and properties of x-ray binaries and supernova remnants in neighboring galaxies; (c) High-resolution mapping of individual supernova remnants in our Galaxy and in the Magellanic Clouds and diagnosis of their plasma conditions by spectrometry of their line emissions; (d) Comparative studies of the x-ray morphologies of clusters of galaxies and measurement of their temperature structure by line emission spectrometry; (e) Detection of x rays from the majority of known quasars including several of the most distant ones; (f) Discoveries in deep-sky surveys of many faint x-ray objects of which some are previously unknown quasars and others have no detected optical or radio counterparts; (g) Demonstration that a major fraction of the extra- galactic background in the energy range from 1 to 3 keV is the unresolved emission of distant quasars and discrete sources of an unidentified nature. From these discoveries and detailed analytical inves- tigations has come a broad understanding of the range of x-ray phenomena and the nature of the more prominent sources of cosmic x rays. 2. State of Knowledge a. Single Stars X rays have been detected from single stars of nearly every spectral type and luminosity class, including T Tauri stars, white dwarfs, and neutron stars. Thus it appears that mechanisms for significant x-ray production

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18 operate in most and possibly all stages in the evolution of single stars. These mechanisms are evidently of several different kinds, and they yield x-ray luminosities in the range up to 1033 ergs sec~1 in nondegenerate stars and up to 1036 ergs sea 1 in single neutron stars. For comparison, one solar bolometric luminosity, Lo, equals approximately 4 X 1033 ergs sec~l, and the x-ray luminosity of the quiet Sun is about 1027 ergs sec~l. The discovery of x-ray emission from single main- sequence stars of all types at x-ray lumiosity levels far above the previous theoretical expectations was a major surprise that has forced a complete reassessment of theories for stellar x-ray emission and opened an impor- tant new approach to the study of stellar evolution. m e range of x-ray luminosities observed among main-sequence stars of a given spectral type is very wide, amounting to factors of 10 to 1000, and no simple relation has yet been found between the x-ray luminosities and basic stellar parameters, though there is evidence of a corre- lation with the speed of rotation for late-type stars and with the bolometric luminosity for early-type stars. Radio pulsars are single neutron stars. Amonq the several dozen that have been examined for x-ray emission, the only one with detectable x-ray pulses is the Crab pulsar with a hard x-ray luminosity of about 1036 ergs sec~1 and a spectrum that extends into the gamma-ray region. Several others are observed to have steady soft x-ray fluxes that are probably blackbody radiation from surfaces at less than 106 K. Since their ages are much greater than the cooling times of neutron stars, they must have sources of heat. Failure to detect soft x-ray stars in sensitive searches within young nebular supernova remnants suggests that either not all supernova explosions produce neutron stars, or neutron stars that are not pulsars cool very rapidly. b. Close Binary Stars Many stars are in binary systems with separations so small that the evolutions of the component stars and their orbits are strongly affected by tidal torques and episodes of mass transfer. Some of the effects of such interactions were recognized in the optical phenomena of spectroscopic binaries prior to the discovery of x-ray stars. However, the most spectacular manifestations of interactive evolution of close binaries have been revealed by x-ray observations. A major achievement of x-ray astronomy during the 1970's was the discovery and eluci- dation of many of these effects.

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19 Within the Galaxy there are about 100 variable x-ray sources with x-ray luminosities in the range from 1036 to 1038 ergs sec~ . All appear to be close binaries with accreting neutron stars or black holes. Upper limits on their luminosities are set by the effects of radiation pressure in limiting the accretion flows. At the same time, the efficiency of heat generation by accretion of matter onto neutron stars or black holes is so high that the resulting x-ray luminosities are pushed to the radia- tion pressure limits by very small rates of mass transfer. The consequence is that the luminosities have values clustered in the range from 1036 to 1038 ergs sec 1 characteristic of a distinct group of "high-luminosity" x-ray binaries. Some x-ray binaries are produced by evolution of primordial binaries. In typical scenarios the heavier star, evolving more rapidly, expands to the point where matter in its outer layers transfers rapidly to the companion star. The remaining core continues to evolve and finally explodes as a supernova, leaving a remnant neutron star or black hole. If the binary is not dis- rupted, then the stage is set for it to become a short- lived Population I x-ray binary when the compact star begins to accrete matter drawn back from its nuclear- burning companion. Close binaries may also be formed by capture in three- star interactions or through tidal dissipation of orbital energy in two-star encounters. The most likely sites for these rare events are the cores of centrally condensed globular clusters and the central regions of galaxies where there are high densities of low-mass stars. Such regions probably have substantial numbers of dead neutron stars produced during an early epoch of star formation. Capture of a low-mass nuclear-burning companion can give such a neutron star a new lease on luminous life as a long-lived Population II x-ray binary. X-ray pulsators are binaries with strongly magnetized neutron stars that have field strengths of the order of 1012 gauss at their surfaces. When plasma, drawn from the companion and spiraling in an accretion disk toward the neutron star, enters the magnetosphere of the neutron star, it is channeled by the magnetic field through narrow accretion columns onto hot spots at the magnetic poles. Confined to these small regions, the plasma attains a higher temperature before its x-ray luminosity is limited by radiation pressure than it would if spread uniformly over the neutron star surface. Moreover, its - .

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20 radiation is emitted anisotropically so that rotation of the neutron star causes pulsations in the x-ray flux recorded by a distant observer. Orbital motion causes Doppler variations in the period of pulsations and eclipses of the x-ray flux for observers close to the plane of the binary orbit. Nonpulsating but variable high-luminosity x-ray sources found in the cores of globular clusters and the central regions of the Galaxy are generally believed to be neutron stars with comparatively weak magnetic fields and low-mass companions. Mass transfer occurs via an accretion disk that reaches close to the surface of the neutron star. As a result the accretion-generated heat is spread over a wide area that is symmetric about the rotation axis, so the resulting x-ray flux has a compara- tively soft spectrum and is unaffected by rotation. No eclipses have been seen in binaries of this type, probably because the accretion disk casts a broader x-ray shadow in the orbital plane than does the companion. X-ray bursters are also low-mass binaries, but with accretion rates below the radiation pressure limit. This lower rate, combined with the larger deposition area on a weakly magnetic neutron star results in a buildup of accreted material that has not undergone continuous thermonuclear fusion, as in the case of deposition on the polar hot spots of pulsators. After several hours, when the unfused material is several meters thick, a thermo- nuclear flash occurs that releases about 1039 ergs. This raises the surface a few meters and heats it to a temperature of approximately 30 million degrees, causing it to emit a burst of thermal x rays with a peak lumi- nosity of several times 1038 ergs sec~1 and a spectrum that is observed to change like that of a blackbody cooling during a period of about 10 sec. Cygnus X-1 is a compact object with a supergiant companion. Analyses of optical data show that the mass of the compact object almost certainly exceeds the theoretical value of the stability limit above which a neutron star will collapse into a black hole. Its spectrum extends to much higher energies than any pulsator. Moreover, the x-ray flux varies in a unique random manner on all time scales from tens of milli- seconds to hours. Its high mass and the uniqueness of its x-ray phenomena make Cyg X-1 the best candidate for a black hole of stellar mass. The phenomena of low-luminosity x-ray binaries are generally less well observed and understood. Cataclysmic

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21 variables that have degenerate dwarf components exhibit highly variable x-ray luminosities up to 1033 ergs sec 1 apparently powered by the accretion mechanism. Certain binaries without degenerate stars also have x-ray lumi- nosities up to 1033 ergs sec~l. In RS CVn binaries intense coronal emission is apparently induced in the more evolved member by a high rate of rotation resulting from tidal torques. In Algol-type binaries, the x-ray emissions may be generated in extensive gas streams between the component stars. Supernova Remnants and the Interstellar Medium A typical supernova ejects several solar masses of material with an initial bulk velocity of several thousand kilometers per second. The material plows up the ambient interstellar medium leaving behind shock-heated plasma that cools by thermal radiation, predominantly in the soft x-ray region of the spectrum. Well-resolved x-ray images of supernova remnants show a variety of features caused by inhomogeneities in the interstellar medium and by internal dynamics of the remnants. As expected for cosmic plasmas at temperatures of millions of degrees, the x-ray spectra of supernova remnants are rich in the K and L lines of highly ionized atoms of the most common elements. High-resolution spectrometry of these lines has yielded information on heavy element enrichment of the interstellar medium by supernova ejecta and on the thermodynamic conditions in the shock-heated material of supernova remnants. Evidence has been found of "super bubbles" formed by the merger of many young remnants of supernovae in regions of rapid star formation that are identified as OB associations. Remnants of ancient supernova are sufficiently numerous and so slow to cool that they merge to form an extensive network of hot plasma. All-sky surveys of the diffuse soft x-ray emission from this hot component of the inter- stellar medium, derived from rocket and satellite observa- tions, have begun to reveal its structure in the vicinity of the Earth and to cast new light on the effects of supernovae on the chemical and dynamical evolution of the Galaxy. d. Normal Galaxies Studies of the intrinsic properties and spatial dis- tributions of high-luminosity x-ray stars and supernova remnants in other galaxies have enlarged the sample of these comparatively rare objects with sources at known

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22 distances in locations whose relations to galactic structures are apparent. These studies have revealed significant differences compared to the sources in our Galaxy and have shed new light on x-ray production mechanisms and on the relation of x-ray sources to galactic morphologies. In the case of the Large and Small Clouds of Magellan the luminosity distribution of high-luminosity x-ray stars is shifted toward higher values compared with that for our Galaxy. This effect has been attributed to a lower average nuclear charge of the accreted material that raises the limits on rates of accretion flow set by radiation pressure. In the Andromeda nebula, MB1, where nearly 100 high-luminosity x-ray stars and supernova remnants have been observed, the x-ray stars in the bulge are much more centrally concentrated than in the bulge of our Galaxy. e. Active Galactic Nuclei Variable x-ray fluxes have been observed from the nuclei of a wide variety of galaxies, including normal galaxies such as Andromeda, Seyfert galaxies, BL Lac objects, and quasars. Among the latter are the most distant objects known. The implied x-ray luminosities range from the order of 1039 ergs sea 1 in the case of Andromeda to 1048 ergs sec~1 for some quasars. Varia- bility on time scales as short as hours has been observed in a few quasars, which implies that their x-ray source regions are no larger than the solar system. It is widely believed that black holes with masses in the range from 106 to 108 solar masses are at the cores of these extra- ordinary activities. The observed x-rays are apparently produced closer to these cores than any other detectable radiation, so that x-ray observations may well provide the most direct evidence as to the nature of the nuclear activity. Extensive surveys have detected x rays from more than half of previously known quasars. A typical x-ray image obtained from a long exposure of a given field at high galactic latitude by the Einstein x-ray observatory shows about half a dozen faint extragalactic objects, which turn out either to be new quasars or to have no detected optical counterparts. Such deep-sky x-ray images have provided the richest samples of candidate quasars of any search method. f. Clusters of Galaxies The discovery of spatially extended x-ray emission from clusters of galaxies provided the first direct

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27 Globular Clusters Detect and study the x-ray emission from low-luminosity sources such as white dwarfs, cataclysmic variables, blue stragglers, and flare stars in globular clusters. Determine the luminosity function of high-luminosity globular cluster sources in galaxies of the local group. 4. Supernova Remnants Determine the thermal and nonthermal structure of remnants by spectrally resolved imaging of extended supernova rem- nants (SNR's). Determine density, composition, temperature distribu- tion, and equilibrium state by spatially resolved high- resolution spectroscopy. Extend present measurements to higher energies (8 keV) to study the hot components in young SNR's and to study the Fe XXV and Fe XXVI line complex. Study temperatures and bulk velocities of turbulent elements of hot gas through measurements of line profiles. Detect SNR's in galaxies of the Local Group and cor- relate them with properties of the galaxies. Search for x-ray emission from hot neutron stars in their SNR's to obtain information on the cooling mech- anisms of neutron stars. Interstellar Medium Measure the temperature, structure, and spatial distribu- tion of the ultra-hot component of the interstellar medium. Determine the composition and distribution of inter- stellar matter from measurements of absorption edges in the x-ray spectra of Galactic sources. Determine the density and size distribution of inter- stellar grains by analysis of the scattering halos around the images of distant point sources. 6. Normal Galaxies Detect high-luminosity x-ray binaries and supernova rem- nants in galaxies out to 20 Mpc. Extend the source survey in MB1 to all galaxies in the Virgo cluster.

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28 Correlate the x-ray star content of galaxies with their morphology and state of evolution. Determine the x-ray luminosity for all known types of galaxies. Determine the distribution of low-mass stars in gal- axies from measurements of the surface brightness of x rays from M dwarfs. Study the dynamics of galactic halos by observations of their x-ray morphology and spectra. 7. Active Galactic Nuclei Determine the x-ray luminosity functions and evolution of emission-line galaxies, BL Lacs, N-type galaxies, radio galaxies, and QSO's. Search for x-ray emission by active galaxies beyond z = 3.5, and study evolutionary effects on luminosities and spectra of the most distant sources. Study the energy source and emission processes in active nuclei through measurements of variability, spectra, and polarization. 8. Clusters of Galaxies Study the formation and evolution of clusters of galaxies out to z = 2 using x-ray spectroscopy of the iron K line for direct determination of red shifts. Study the distribution, origin, and heating mechanisms of the intracluster medium by spatially resolved high- resolution x-ray spectroscopy and by spectrally resolved high-resolution imaging. Determine fundamental cosmological constants through comparison of microwave and x-ray observations of clusters 9. The X-Ray Background Measure the high-energy spectra of distant active galaxies and QSO's to determine their contribution to the back- ground above 3 keV. Determine whether a truly diffuse component of the x-ray background exists at any energy, as opposed to one due to distant unresolved discrete sources. Search for anisotropies related to superclusters and intercluster voids, which are the largest-scale structures so far perceived in the Universe.

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29 D. Inventory of Present or Approved Resources When the useful life of the Einstein x-ray observatory ended early in 1981 after more than two years of highly productive work, there remained no operational U.S. satellite for x-ray astronomy. Preliminary plans have been developed for several future U.S. satellite x-ray facilities. However, the only projects approved initially were a few short-duration Spacelab flights. These have now been cut back, leaving only an engineering feasibility study for the LAMAR facility and several x-ray spectroscopy projects, all severely limited in their scientific product by the short duration of Spacelab missions. Thus the current level of research activity in x-ray astronomy in the United States is a small fraction of the average level during the 1970's. It is apparent that x-ray astronomy, having recently reached the stage of development where it can provide critically important information bearing on nearly every topic of astronomical research, is confronted by a gap of several years during which no x-ray data will be available from U.S. satellites. The development of new satellite facilities for x-ray astronomy is therefore an urgent necessity for vigorous and balanced progress in astronomy as a whole. Meanwhile existing facilities for balloon and rocket experimentation will continue to offer valuable oppor- tunities for exploratory investigations in special areas of x-ray astronomy and for testing concepts for future satellite instrumentation. These facilities will be of vital importance in maintaining the viability of the discipline during the period when no U.S. x-ray satel- lites are operating. Improvements in the capabilities of balloon vehicles, particularly in regard to flights of longer duration, would open important new scientific opportunities. The archives of data from several recent satellite x-ray astronomy missions such as SAS-3, HEAD-1, and the Einstein x-ray observatory are a significant resource for future x-ray research. Research programs based on the use of data in these archives should be maintained at a level commensurate with their potential for obtaining significant new results. Several other countries have strong programs in x-ray astronomy, and their satellites will probably be the principal sources of new data during most of the coming decade. The Japanese x-ray observatory, Hakucho, launched

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30 in 1979 and still working, is a small rotating satellite used primarily for the study of x-ray bursters. The Japanese program also includes a larger x-ray satellite to be launched in the mid-1980's. The European Space Agency expects to launch the large and versatile x-ray satellite, EXOSAT, in 1982. m e latter is a three-axis stabilized observatory with a number of different instru- ments including two 28-cm-diameter grazing-incidence telescopes for the energy range up to 4 keV and oronor- tional counters for the range up to 60 keV. In the German space program, preliminary studies have been completed for ROSAT, a major X-ray observatory to be launched in the latter half of the 1980's. ROSAT will have a grazing- incidence telescope of 80-cm diameter, and its primary mission will be to carry out an all-sky survey of soft x-ray sources in four energy bands from 0.1 to 2 keV with a maximum positional accuracy of 20 arcseconds for bright sources and sensitivity sufficient to detect several hundreds of thousands of sources. Pointed-mode observa- tions will also be carried out with an angular resolution of 20 arcseconds and a sensitivity about three times that of the Einstein x-ray observatory. The possibility of enhancing the capabilities of ROSAT by adding a high- resolution image detector furnished by the United States is under active consideration, together with a proposal for major participation of U.S. scientists in the use of ROSAT in exchange for a Shuttle launch. E. Opportunities and Requirements for Future Programs Exceptional scientific opportunities now exist for new projects in x-ray astronomy. The exploratory studies of the 1960's and 1970's, culminating in the work carried out with the Einstein x-ray observatory, established the existence of a rich phenomenology of high-energy astro- physics, which is uniquely accessible to x-ray observa- tions and of vital importance to our understanding of the structure and evolution of the major constituents of the Universe. The technology of astronomical x-ray observa- tions has been brought to a high level of development and is now ready to support major advances in the sensitivity and precision of x-ray measurements. The Shuttle Trans- portation System will provide the means to place heavy observatories in orbit and to maintain them as long-term facilities. Finally, there exists a wide community of astronomers who need the results of x-ray observations in

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31 their research and who are skilled in the use of x-ray observatories. Most of the scientific objectives of x-ray astronomy require extended observations with instruments in free- flying satellite observatories. Conceptual designs and performance objectives for several possible satellite missions have been developed within the scientific community, and several detailed design studies have been carried out to assess the engineering requirements and costs of proposed projects. From these efforts there has emerged a program of proposed investigations that can achieve a broad and vigorous renewal of progress in x-ray astronomy in the late 1980's. The key component of this program is the Advanced X-Ray Astrophysics Facility (AXAF). We recommend that it be undertaken as the mission of highest priority in astronomy in the 1980's. It is conceived as a national x-ray observatory that will open new scientific frontiers for exploratory studies of faint x-ray sources and at the same time will provide the long-lived facilities for sensitive high-resolution x-ray imaging, spectroscopy, and polarimetry, which are now urgently needed in nearly every area of astronomical research. Among the new Explorer missions, we assign the highest priority to the X-Ray Timing Explorer (XTE), which will achieve greatly increased sensitivity in studies of variability in compact x-ray sources. The AXAF and the XTE, together with the other prin- cipal components of the future x-ray program, are described below. 1. Large X-Ray Observatories m e large satellite x-ray observatories of the future should be developed and operated with the broadest possible participation of the scientific community. The operating and management tasks for the future x-ray observatories will be similar in scope and magnitude to those for the Space Telescope (ST). We recommend that an institutional arrangement resembling the Space Telescope Science Institute be established to provide scientific guidance during development of the major x-ray facilities of the future and to act as an efficient interface between the facilities and the community of x-ray observers and instrumentalists.

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32 a. Advanced X-Ray Astrophysics Facility (AXAF) The AXAF, technical and scientific successor to the Einstein x-ray observatory, is designed to achieve the highest performance attainable within the payload capa- bilities of the Shuttle Transportation System and the current state of the art in x-ray optics and detector technology. It will operate in the energy range 0.1 to 8 keV with a grazing-incidence reflecting telescope having a diameter of 1.2 m, a focal length of 10 m, and equipped with interchangeable focal-plane instrumentation for image recording, spectroscopy, and polarimetry. Its effective area will be 1500 cm2 at 0.6 keV and 250 cm2 at 6 keV. It will provide 0.5 arcsec resolution over the central portion of a 60 arcmin field in the x-ray band from 0.1 to 8 keV. With its larger mirror area, higher angular resolution, and improved detectors, together with the longer observing times available on a long-lived facility, AXAF will be able to detect and study objects that are more than 2 orders of magnitude fainter than the faintest ones accessible to the Einstein x-ray observatory. The combination of a high-performance x-ray telescope with efficient photon-sensitive image detectors having moderate nondispersive spectral resolution will give AXAF the capability to detect and analyze active galactic nuclei out to distances corresponding to red shifts of _ = 10, and clusters to z = 3. High sensitivity spec- trometry with moderate resolution will measure the iron K lines from clusters at z = 2. These distances, comparable with or greater than those of objects accessible to ST, correspond to a range of epochs in which evolutionary effects in the development of the Universe should be clearly discernable. Indeed, it is not currently known whether galaxies and clusters were even formed at such early epochs. Individual high-luminosity x-ray binaries will be detectable out to 30 Mpc. Given the longevity of AXAF it will be feasible to study the spectra and variability of literally thousands of x-ray binaries in the galaxies of the Virgo cluster. Spectroscopy with resolutions greater than 500, possible for only a few of the brighter sources with the Einstein x-ray observatory, will be carried out routinely by AXAF on a large number of targets. Polarimetry to a precision of 1 percent will be possible for sources a thousand times weaker than Sco X-1.

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33 b. Large-Area Modular Array of Reflectors (LAMAR) The LAMAR is a multimirror modular telescope system whose principal characteristics are very large effective area and modest angular resolution. The latter, ranging from 10 to 60 arcseconds according to position in the field of view, is sufficient to avoid source confusion and to provide positions with the accuracies required for unambiguous identifications in most cases. The total collecting area at 2 keV is 3 X 104 cm2, which is 30 times larger than that of AXAF. The LAMAR will therefore be an extremely effective instrument for the study of variations in faint sources. It will be the prime instru- ment for (1) extensive sky surveys to discover, identify, and classify very faint sources; (2) detailed imaging of diffuse sources and low surface brightness features of hot intergalactic and interstellar matter; (3) studies of variations in quasars to probe as close as possible to the energy sources; (4) moderate-resolution spectroscopic studies of distant sources, including direct determina- tion of their red shifts by measurement of the iron K lines with its solid-state image detectors. The scien- tific motivations for developing LAMAR as a large-area facility following AXAF are similar to those for develop- ing new large-area ground-based optical telescopes as complementary facilities to ST, which has exceedingly high angular resolution but only a modest collecting area. X-Ray Observatory (XRO) The XRO will accomplish objectives of x-ray astronomy that are not suited to or are outside of the capabilities of the AXAF and LAMAR missions. The positions, spectra, and variability of sources in the high-energy range up to hundreds of keV may be measured with low-background detectors of large effective area and with transform image detectors employing shadow masks. The highly variable x-ray sky may be continuously monitored by survey instru- ments with broad-energy coverage to discover new phenomena and to establish the long-term behavior characteristics of known sources such as the compact Galactic sources and the nuclei of active galaxies. 2. Explorer Missions m e re-establishment of a substantial X-Ray Explorer program either in the mode of Shuttle-launched free- flyers or of independently launched satellites is of

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34 great importance to the future vitality of x-ray astron- omy. Explorers are particularly well suited to special- ized studies that complement or supplement the observing programs of the large missions. They provide much longer exposures and far more flexible observing plans than the Spacelab experiments. The Explorer missions described below require spacecraft with capabilities that are only modestly greater than that of SAS-3 (i.e., three-axis stabilization and pointing to an accuracy of a few arc- minutes). Among them the XTE has the highest priority and should be started as soon as possible. The other Explorer missions have not been ordered according to priority. X-Ray Timing Explorer (XTE) The XTE will carry out extremely sensitive and accu- rate measurements of the variabilities of x-ray sources on time scales ranging from milliseconds to years. The scientific objectives of the mission include the deter- mination of the masses, magnetic fields, and internal structures of neutron stars and white dwarfs in close binary systems; elucidation of the physics of accretion flows and stellar magnetospheres; investigation of the evolution of close binary systems; elucidation of the mechanisms of x-ray bursters, transients, and irregular variables; and the exploration of the nature of the energy sources in active galactic nuclei. The XTE mission will have a high degree of flexibility in pointing as well as detectors with large effective areas to achieve sensitive measurements that are suited to the study of a wide variety of temporal behavior. This mission has been widely discussed and is scientifi- cally well defined. It should be commenced at the earli- est possible date to alleviate the loss of observational capability in this critically important branch of x-ray astronomy caused by the terminations of SAS-3 and HEAD-1 in 1979. A large fraction of the observing time of the XTE will be made available to observers at large who will have access to high-priority sources and new targets of oppor- tunity. The XTE will be well suited to participation by many observers, since it will furnish data on one source at a time and many hundreds of interesting sources are available for study. b. Soft X-Ray Explorer A soft x-ray observatory with imaging optics that provides a sensitivity at 1 keV comparable with or better

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35 than that of the Einstein x-ray observatory, but with less angular and spectral resolution, is required to exploit the scientific opportunities for studies of objects now known to radiate strongly in the energy range from 0.1 to 1.5 keV. These include a wide variety of close binary systems containing a degenerate dwarf, binaries of the RS CVn type containing nondegenerate stars, main sequence stars with high coronal activity, isolated hot white dwarfs, central stars of young planetary nebulae, single hot neutron star remnants of recent supernovae, and x-ray pulsars. The studies would include measurements of pul- sations, aperiodic variabilities, orbital light curves, spectral continua, and the intensities of emission lines. In many cases correlated measurements at other wavelengths would be desirable. The results would enlarge our under- standing of the evolution of binary systems, the surface composition and cooling mechanisms of hot degenerate stars, and the activity and variability of stellar coronas. X-Ray Spectroscopy X-ray line emission in the energy range from 0.15 to 8 keV has been observed from main-sequence stars, x-ray binaries, supernova remnants, and clusters of galaxies. Line emission in the energy range below 1 keV is expected from the hot component of the interstellar medium. Cyclo- tron resonance features in the range from 10 to 100 keV have been observed in x-ray pulsators. Detailed spectro- scopic studies are now required to obtain the information about the physical conditions in these objects that is contained in their x-ray spectra. The demand for observ- ing time for such a program will far exceed the time available in the AXAF mission. Moreover, the study of nebular x-ray spectra and cyclotron resonance features will require special instrumentation not carried by the AXAF. An Explorer mission should therefore be dedicated to the spectrometry of both nebular and compact x-ray sources. d. Analysis of Coronas The ubiquity of stellar x-ray emission presents an important new scientific opportunity to extend the study of stellar abundances and the relations between coronal activity and basic stellar parameters into the soft x-ray range. This opportunity could be exploited effectively with an Explorer mission dedicated to the task. Such a mission would employ a grazing-incidence reflection

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36 telescope with an aperture of approximately 80 cm and an objective grating spectrometer optimized for the range from 10 to 100 ~ with a spectral resolution on the order of 100. It would probe stellar surface activities with a sensitivity that may well exceed that of optical observations the H and K lines of Ca II. For example, it could be used effectively in the study of the surface activities of M dwarfs as faint as V = 20 mag. The mission would provide observing opportunities for the wide community of stellar astronomers whose interests have been drawn to the scientific potentiality of stellar x-ray observations by the Einstein results. 3. Long-Duration Balloon Flights Successful flights of the recently developed "Sky Anchor" system by the National Scientific Balloon Facility have demonstrated the feasibility of long-duration balloon flights. It is expected that a 450-kg payload can be kept aloft for weeks near an altitude of approximately 40 km. This will open up the possibility for sensitive studies at x-ray energies above 25 keV such as (a) measurements of the spectra of active galactic nuclei (e.g., QSO's, Seyferts, N-type galaxies) to determine their contribution to the unresolved x-ray background in this energy domain; (b) the study of cyclotron resonance features in the spectra of x-ray stars; (c) accurate (arcminute) position determinations of the gamma-ray sources from measurements of their positions in the energy range from 30 to 150 keV. made from Spacelab. Because of weight constraints the balloon payloads would be considerably smaller in sensi- tive area than Shuttle payloads, but exposure times in long-duration balloon observations could be substantially longer. _ . . buck measurements can also be 4. Spacelab Spacelab will provide opportunities to make short obser- vations of a few especially interesting celestial objects and to develop and test new instruments that may even- tually be used in free-flying satellites. It can carry payloads with considerably larger sensitive areas than rocketborne or balloonborne payloads. It can also provide much longer exposures than rockets but not necessarily longer than long-duration balloon flights

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37 a. Principal Investigator Experiments Comparatively inexpensive experiments on rockets or balloons carried out by small research groups have led in the past to important discoveries as well as developments in instrumentation and research techniques. They have also achieved relatively quick responses to new ideas and unexpected astronomical occurrences. Opportunities for such experiments should be provided in the Shuttle era by means of Spacelab. b. Multiuser Facilities Instruments with which scientific objectives of interest to many investigators can be addressed should be considered as possible multiuser facilities. Prototypes of the modules that may be developed for the LAMAR and tested on Spacelab would be good candidates for deploy- ment as multiuser facilities. 5. Sounding Rockets The sounding-rocket program will continue for the forsee- able future to provide the only low-cost means for developing instruments and testing new observational strategies for x-ray astronomy in the energy range below 20 keV. It should be maintained well into the 1980's at a level sufficient to meet these needs and until it is demonstrated that the Shuttle program provides a cheaper alternative. 6. Supporting Research and Technology, Including Balloons A vigorous program for development of advanced x-ray instrumentation is critically important for maintaining the scientific productivity of x-ray astronomy and, in particular, to assure the most effective possible use of the new major facilities such as the AXAF. The balloon program provides cost-effective capabil- ities for important x-ray observations above 20 keV. It also provides essential opportunities to test experiments that may eventually be flown on the Shuttle or on free- flyers (see also Long-Duration Balloon Flights). There- fore the balloon flight program should be adequately supported, along with the developments in balloon tech- nology that offer promise of substantial improvements in capabilities for x-ray astronomy.