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43 that there is a vast number of E W -emitting stars in the Galaxy and that many of these lie within the region accessible to E W observations. High-resolution E W spectroscopy of these sources holds great promise of advances in the understanding of stellar physics and the interstellar medium. High priority should therefore be given to work in this area. The development of instrumentation for future E W observations must also be adequately supported. Among the instruments needed are large-area grazing-incidence spectrometers for high-resolution measurements in the wavelength range from 100 to 500 A, normal-incidence spectrometers for wavelengths longer than 500 A, objective gratings with grazing-incidence telescopes for moderate-resolution spectral surveys over a wide wave- length range, and high-sensitivity cameras for deep field surveys of selected areas. In addition, consideration should be given to polarization measurements, which are easier to perform in the E W region than at x-ray wave- lengths, and which may prove to be of great scientific value. E. Summary and Recommendations Recent discoveries demonstrate that E W observations provide new information of fundamental importance about stars and the interstellar medium. The preparation and launch of the E WE satellite is essential to development of the field. Meanwhile, development of instrumentation for detailed studies of E W stars, particularly spectro- meters, should be vigorously pursued, and new satellite missions should be developed. m e possibilities of ex- tending the capabilities of planned facilities to permit E W observations of stars should be carefully examined and implemented where feasible. VI. GAMMA-RAY ASTRONOMY A. Introduction Efforts to open the gamma-ray region of the electro- magnetic spectrum to astronomical observation began with balloon experiments in the late 1940's. It was recog- nized that interactions of cosmic rays with interstellar matter and starlight must produce photons in the gamma-

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44 ray region of the electromagnetic spectrum above 100 keV and that detection of this radiation would open a new approach to the study of the nature and distribution of high-energy processes in the Universe. Early attempts were frustrated by the difficulties of distinguishing between gamma rays of cosmic origin and those produced by cosmic rays in the atmosphere above the balloon or in the apparatus itself. mese difficulties were finally over- come during the 1960's in experiments carried out above the atmosphere in satellites. The first definite observa- tions of gamma rays were made with a scintillation detec- tor sensitive to photons with energies above about 50 MeV carried on the third Orbiting Solar Observatory (OS0-3). Clear evidence was obtained of a component of Galactic origin concentrated in a band of directions around the Galactic equator with a maximum toward the Galactic center and another component of extragalactic origin, which is isotropic. Gamma rays with energies in the range around 1 MeV were detected for the first time with a detector carried far away from the interfering Earth on the Ranger II Moon probe. B. Progress during the 1970's The 1970's was a period of major discoveries in gamma-ray astronomy. The second Small Astronomical Satellite (SAS-2) and the European satellite COS-B mapped the intensity of high-energy gamma rays and discovered numerous discrete sources or source regions, most of which are concentrated in the Galactic plane. Measure- ments of the diffuse component of Galactic gamma rays provided information on the distribution of cosmic rays in the Galaxy and demonstrated the feasibility of obtaining a high-contrast picture of this important aspect of Galactic structure from future observations that will be made with improved sensitivity and angular resolution. The spectrum of the diffuse component of extragalactic gamma rays was measured over the energy range from one to several hundred MeV by instruments on Apollo 15, Apollo 17, and SAS-2. COS-B detected the first identified extragalactic source of high-energy gamma rays, the quasar 3C273. Balloonborne detectors obtained evidence of low-energy (less than 10 MeV) gamma rays from the radio galaxy Centaurus A and possibly the Seyfert galaxy NGC4151, in both cases, presumably emanat- ing from an active galactic nucleus.

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45 Among the approximately three dozen Galactic sources of high-energy gamma rays now known are the peculiar x-ray binary, Cygnus X-3, several radio pulsars, and numerous objects that have not yet been identified at other wave- lengths. The gamma-ray luminosities of radio pulsars exceed their radio luminosities by many orders of magni- tude. Indeed, it appears that their radio phenomena are minor side effects of high-energy processes whose prin- cipal products are high-energy photons. Emus the most direct route to understanding the underlying mechanism of pulsars may be the detailed examination of their gamma-ray light curves and the phase dependence of their gamma-ray spectra. In the case of the Crab pulsar, the gamma-ray light curve has two peaks coincident with the radio and optical peaks. In contrast, the Vela pulsar exhibits two peaks per cycle in the gamma-ray light curve as opposed to one in the radio light curve, and neither gamma-ray peak is in phase with the radio peak. One concentrated source region detected by COS-B has been identified with the Orion cloud complex, and the contours of gamma-ray intensity measured by COS-B have been shown to coincide with features of the CO radio map. The data indicate that the cosmic-ray flux in Orion is close to the local value and pervades most of the cloud mass. Cosmic gamma-ray line emission was observed in the spectra of solar flares with scintillation spectrometers on OS0-7 and HEAD-1 and with the solid-state spectrometer on HEA0-3. Subsequent observations by the Solar Maximum Mission (SMM) of gamma-ray line emission from many solar flares provided valuable information on the dynamics of the flare processes. The positron annihilation line at 0.511 MeV was detected in the spectrum of gamma rays from the Galactic center region with a balloonborne solid-state spectrometer. Observations by HEAD-3 have shown that this line emission varies and therefore must originate in a comparatively small region. Line features in the energy range from 20 to 100 keV, which are believed to be due to cyclotron resonance of electrons in magnetic fields of more than 1012 gauss, were detected in the spectra of two x-ray pulsars with scintillation spectrometers flown on balloons and on HEAD-1. Observations of transient sources of low-energy gamma rays in the energy range from tens of keV to several MeV were initiated with the discovery of gamma-ray bursts by the Vela satellites in 1969. Since then, detectors on various satellites have recorded such bursts at a rate of

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46 about a dozen per year. An international effort is now being made to determine accurate source positions from measurements of the differences in arrival times of sharp features in the light curves of individual bursts over an interplanetary network of detectors on U.S., European, and Soviet spacecraft. Positional accuracies of about 1 arc- min have been achieved for about half a dozen gamma-ray bursts, and in all but one case the positional error boxes contain no objects observed at other wavelengths that are plausible burst sources. The one case in which a possible source was located in the error box was that of an extraordinary burst recorded on March 5, 1979, by instruments on nine widely separated spacecraft. m e position was found to coincide within an uncertainty of less than 1 arcmin with that of the super- nova remnant N49 in the Large Magellanic Cloud. However, this burst was exceptional in nearly every regard and may be fundamentally different from the usual gamma-ray burst. Its initial intensity spike was briefer, its energy flux at the detectors much greater, and its spectrum much softer. The initial spike was followed by a transient flux that exhibited periodic oscillations with an 8-see period, which is the likely signature of a rotating neu- tron star. A line feature in the spectrum at 430 keV was also reported by Soviet investigators. If the source of the burst actually lies in the Large Magellanic Cloud, then its peak gamma-ray luminosity exceeded 1044 ergs sec~l, equivalent to the luminosity of an entire galaxy emanating from an object only 10 miles in diameter. Alternatively, it may be a nearby object of much lower peak luminosity whose direction coincides by chance with N49. Evidence of red-shifted annihilation lines and cyclo- tron resonance features has been found on the gamma-ray spectra of several other bursts by scintillation detectors on Soviet spacecraft. An unusually long, 20-min transient gamma-ray event was observed in 1974 with a balloonborne solid-state spectrometer, which detected strong emission lines but no continuum in the spectrum. Thus, it appears that line emission is present in the spectra of a variety of transient gamma-ray events. Observations by ground-based detectors have discovered gamma rays with energies in the range from 1031 to 1014 eV from the Crab pulsar, Centaurus A, and Cygnus X-3. mese detectors respond to the Cerenkov light from the air showers generated high in the atmosphere by the incident gamma rays. m e observations provide direct evidence for

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47 the acceleration of particles to very high energies in a wide variety of astronomical objects. Substantial progress has been made in instrumentation for gamma-ray astronomy during the 1970's. During the early part of the decade, observations in the energy range from 0.1 to 10 MeV were made primarily with actively shielded sodium iodide scintillation detectors. Solid- state spectrometers with cryogenically cooled germanium crystals, which afforded spectral resolutions 20 to 30 times better than scintillation counters, were developed for various balloon and space instruments, and in 1979 several large detectors were launched aboard HEAD-3. The double-Compton telescope for medium-energy gamma rays was brought to an advanced state of development through bal- loon experiments. Spark chambers for high-energy gamma- ray observations were developed in balloon experiments and used on the SAS-2 and COS-B satellites. Coded-mask detectors have been developed for the purpose of measur- ing the positions of transient or persistent sources of low-energy gamma-ray photons with accuracies on the order of 1 arcmin. These and other important developments in instrumentation and vehicles provide the technological basis for the balloon and satellite missions that can accomplish the scientific objectives of gamma-ray astron- omy in the 1980's. C. Scientific Goals for the 1980's Gamma-ray astronomy addresses some of the most important questions of astronomy: How are the elements formed? How do supernovae explode? What are the properties of neutron stars? Are there massive black holes at the centers of galaxies? Is there antimatter on large scales in the Universe? It provides unique information on a wide variety of important topics such as the mechanism of pulsar radiation, the structure of the Galaxy, the pro- cesses in active galactic nuclei, and the origins of the background radiation. The observation of emission lines and cyclotron resonance features in gamma-ray spectra has opened new approaches to the study of solar flares, the central region of our Galaxy, nucleosynthesis in super- novae, and the physics of neutron stars. The gamma-ray bursts, whose origins are unknown, indicate the existence of new kinds of explosive phenomena, most likely asso- ciated with neutron stars or black holes. Observations of gamma rays at energies above 1011 eV bear on the

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48 physics of pulsars and active galactic nuclei. Emus over the entire spectral range from 104 to 1014 eV there are interesting known phenomena that should be more fully explored and systematically investigated in the 1980's, and there are undoubtedly many important phenomena yet to be discovered. In light of the discoveries and developments that have been described, and considering the current state of instrumentation, one can specify a number of feasible observational goals for the 1980's the achievement of which would greatly advance our understanding of high-energy astronomy. 1. Compact Objects Measure the gamma-ray light curves of radio pulsars and the phase dependencies of their gamma-ray spectra to elucidate the acceleration and interaction of high-energy particles in the magnetospheres of rotating neutron stars. Measure the cyclotron resonance lines in the spectra of x-ray pulsars and the nuclear lines and positron annihila- tion lines in the spectra of gamma-ray bursts and tran- sients to obtain information about magnetic-field inten- sities, gravitational red shifts, surface compositions, and the processes of particle acceleration in the vicinity of neutron stars and other compact objects. Search for sources of gamma rays with the unique char- acteristics expected from black holes, such as very short bursts signaling the final evaporation events of small black holes. Measure the spectra and variations of gamma rays from active galactic nuclei over the energy range from 104 to 1012 eV to obtain information about the processes that occur near the sources of their energy. 2. Gamma-Ray Lines from the Products of Nucleosynthesis Search for nuclear lines in the spectra of gamma rays emitted by the radioactive debris in supernova remnants. Detection of such lines would provide the most direct test of the theory of explosive nucleosynthesis in super- novae, which are believed to be the dominant sources of elements with Z greater than 2. The spatial distributions of the intensities of specific lines in a supernova rem- -

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49 n ant are expected to differ according to the radioactive half-lives of corresponding isotopes. Search for gamma-ray line emission from radioactive debris of ordinary novae to clarify the mechanisms of nova explosions, which are believed to be thermonuclear runaways in material accreted onto the surfaces of white dwarfs. 3. Gamma-Ray Bursts and Other Transient Phenomena Monitor the sky for transient gamma-ray events and measure their temporal structure, spectra, and positions with the highest attainable previsions. Extend the observations of these comparatively rare events into the x-ray and possibly other regions of the spectrum. New and powerful approaches to the study of the nature of gamma-ray bursts have been opened by the detection of spectral features identified with the positron annihilation line red shifted to about 400 keV and with cyclotron resonance of electrons in magnetic fields greater than 1012 gauss. The system- atic investigation of these phenomena is among the most important scientific goals of the 1980's. 4. Galactic Gamma-Ray Emission Determine the origins of the diffuse high-energy galactic gamma rays and assess the contributions made by inter- actions of cosmic-ray nuclei and electrons with inter- stellar matter. The results from such studies will help to elucidate the dynamical coupling between cosmic rays, the magnetic field, and the motions of interstellar matter. Search for nuclear gamma-ray line emission from inter- actions of low-energy cosmic rays with matter in large molecular clouds. Measurements of line emission would provide information on the intensity of the low-energy cosmic rays that do not penetrate into the solar cavity and on the composition of the clouds themselves. Determine the nature of the unidentified localized sources discovered by SAS-2 and COS-B and assess the contribution such sources make to the diffuse Galactic high-energy gamma rays. Determine the nature of the source of the 0.Sll-MeV annihilation radiation from the Galactic center region. Of critical importance in this study will be long-term

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50 monitoring of its variable intensity. Survey the spatial distribution of the 0.511-MeV line throughout the Galaxy, and search for line emission from discrete sources such as pulsars, supernova remnants, and active galactic nuclei. 5. Extragalactic Gamma Rays Measure the gamma-ray emissions of a wide variety of galaxies, BL Lac objects, and quasars to elucidate the nature of the energy sources in active galactic nuclei Determine the relations of their gamma-ray spectra and variability to phenomena at other wavelengths. Assess the contribution these objects make to the unresolved extra- galactic gamma-ray background radiation. Determine what portion, if any, of the unresolved gamma-ray background radiation is truly diffuse, and search for clues to its origin. . D. Inventory of Present or Approved Resources The existing U.S. resources for gamma-ray astronomy consist of a number of balloonborne instruments and several small gamma-ray burst detectors carried on the Vela satellites SB, 6A, and 6B, on the ISEE-3 satellite, and on the Pioneer Venus Orbiter. The European COS-B satellite, carrying a spark chamber telescope for high- energy gamma rays, continues to return valuable data. A French-Soviet collaborative project, Gamma-l, scheduled for launch in the early 1980's, will carry a high-energy gamma-ray telescope. Balloon experiments will continue to be essential in gamma-ray astronomy, both in the observation of gamma-ray emission from the brighter discrete sources and in the development of new instrumentation for future space- flights. Among the currently operational balloon instru- ments are scintillation and solid-state spectrometers suitable for measuring low-energy gamma-ray lines and cyclotron resonance features, and Compton telescopes and spark chambers for medium- and high-energy gamma-ray observations. The scientific productivity of these instruments could be significantly enhanced by improve- ments in the duration and reliability of balloon flights. Ground-based detectors of air showers produced by very-

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51 high-energy gamma rays have been operated in the United States by the Harvard/Smithsonian Center for Astrophysics, Bowie State College, and Iowa State University. However, these detectors are no longer operating. Very-high-energy gamma-ray astronomy is carried out at present only abroad, at the Crimean Astrophysical Observatory in the Soviet Union and the Tata Institute for Fundamental Research in India. Free-flying spacecraft are clearly the most effec- tive vehicles for all gamma-ray observations except in the energy range above about 1011 eV, where the large sensitive areas afforded by ground-based Cerenkov air- shower detectors are absolutely essential. Free-flyers provide long exposures with no interference from secondary gamma rays produced by interactions of cosmic rays with air atoms in the field of view. Thus the future develop- ment of gamma-ray astronomy will depend in large measure on observations from satellite observatories. The centerpiece of observational gamma-ray astronomy during the 1980's will be the Gamma Ray Observatory (GRO). This major new initiative in high-energy astronomy, based on the strong technical and scientific foundation estab- lished by the highly successful SAS-2, HEAD-1, and HEAD-3 missions, has been thoroughly studied and is now an approved mission. The payload of the GRO, as currently planned, consists of gamma-ray telescopes for observa- tions in the energy range from several times 104 eV to about 2 X 101 eV, including observations of gamma-ray bursts and spectroscopy of gamma-ray emission lines. The sensitivities of the GRO instruments will surpass those of previous detectors by at least one order of magnitude over the entire spectral range, and their angular resolu- tions will be substantially better. m us, the GRO will be equipped to carry out detailed analytical studies of most known gamma-ray phenomena and also to explore new domains of phenomena where major discoveries are likely to be found. Another important approved program is the inclusion of gamma-ray burst detectors in the two payloads of the International Solar Polar Mission (ISPM). While the future of this project is not certain, there is no doubt that the burst observations it would obtain would make a major contribution to the determination of the nature of burst sources. Together with the burst detectors on the GRO they would form a new time-delay network with which positions of burst sources could be measured with accuracies of about 10 arcsec.

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52 E. Comparison of Goals with Present or Approved Resources Only a few of the scientific goals of gamma-ray astronomy can be achieved with the existing resources, which consist only of balloonborne instruments and the space network of small burst detectors. The future progress of gamma-ray astronomy will depend on new space missions that include two that have been recently approved, namely the GRO and the burst detectors on the Solar Polar probes. Sensitive and detailed observations of radio pulsars in the energy range up to about 1 GeV will be made with several of the instruments on the GRO. The 0.511-MeV line and cyclotron resonance lines from x-ray pulsars, which can be detected by several of the existing balloon- borne detectors, will be studied in much finer detail by the GRO instruments. The spectrometers on the GRO will be used in a sensi- tive search for nucleosynthetic gamma-ray lines from young Galactic and nearby extragalactic supernova remnants, as well as from long-lived radioactive debris in interstellar space. m eoretical estimates indicate that gamma-ray lines from nucleosynthesis in nova explosions could be observed by balloonborne spectrometers. The observations would require flight durations of about 1 day and would have to take place within about a year after a nearby nova explosion to detect the 1.275-MeV nuclear line from 22Na decay (half-life 2.6 years). The spectrometers on the GRO are well suited for such observations, but, unfor tunately, the chance of a nova occurring close enough during the life of the mission is small. The existing network of gamma-ray burst detectors will continue to provide a means for determining the positions of burst sources with accuracies on the order of 1 arcmin by the method of timing. Accuracies on the order of 10 arcsec could be achieved with burst detectors operating on two ISPM probes and on the GRO in Earth orbit. Posi- tions accurate to a few degrees will be obtained with the gamma-ray burst monitor on the GRO. His latter detector is much more sensitive than any other existing or approved satelliteborne burst detector and should be capable of observing much weaker bursts. This will allow the exten- sion of the log N-log S curve to low values of S. which may cast new light on the spatial distribution of gamma- ray burst sources. Existing spectrometers can be used to pursue the study of gamma-ray transients in day-long balloon flights, but larger detectors on long-duration -

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53 flights are required for substantial progress in the investigation of these peculiar events. The study of high-energy gamma rays will be pursued with the spark chamber and Compton telescopes on the GRO. They will obtain detailed information on the spatial uni- formity and energy spectrum of extragalactic diffuse radiation, which is needed to determine the origin of this radiation. m ese telescopes are also expected to measure detailed properties of galactic diffuse radiation in suf- ficient detail to determine the distribution of cosmic rays in the Galaxy, to ascertain the role of molecular clouds in holding cosmic rays in the Galaxy, and to see elements of Galactic structure clearly. Hey should also provide the first large sample of data on gamma-ray emis- sion from normal and active galaxies. m ere are no approved resources, however, for studying cyclotron lines in transient and persistent sources, for observing with high spectral resolution the very narrow 0.511-MeV line from the Galactic center and diffuse very narrow lines from the Galactic plane, for observing anni- hilation and nuclear de-excitation lines from gamma-ray transients, and for carrying out ground-based observa- tions of very-high-energy gamma rays. Some of these objectives can be achieved by a balloon program, while others will require additional space missions. F. Opportunities and Requirements for New Programs Although many important objectives of gamma-ray astronomy will be accomplished by the GRO, others will remain that require the following additional facilities and programs for their realization. 1. Gamma-Ray Transient Explorer Gamma-ray transients are among the most remarkable and mysterious phenomena of high-energy astronomy. The processes in which they originate probably involve the most extreme conditions of high temperature and density that occur anywhere in the Universe. The elucidation of the nature of their sources is an exciting objective of future studies that will require the use of a special Explorer-class satellite observatory. The recent dis- covery of cyclotron resonance features and the positron annihilation line in the spectra of several bursts demon-

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54 strafes the need for a comprehensive investigation of burst spectra with high spectral resolution. Accurate position determinations for a large number and variety of transients are essential to an understanding of the origins of these phenomena. The required mission would include a hard x-ray all-sky monitor and an x-ray detector with a coded-mask collimator for measuring the positions of the sources of gamma-ray bursts with sufficient accu- racy (less than 1 arcmin) to ensure good chances for optical identifications. Line emission would be studied by means of high-resolution spectrometers with wide fields of view. Observations of solar gamma-ray transients with the same instruments would provide both important data for solar physics and in-flight tests and calibrations. 2. Advanced Gamma-RaY Experiments Following the GRO mission, there will be a need to carry out high-energy gamma-ray observations with sufficient sensitivity and angular resolution to define detailed spatial features of emission regions such as molecular clouds, Galactic arms, and nearby galaxies and to measure complex variations of compact sources. This mission should carry a large high-energy gamma-ray telescope with a comparatively narrow field of view, a collecting area of about 3 m2, and an angular resolution of about 2 arcmin or better. It should be planned to permit the spectra of some sources to be determined up to 1011 eV. A high-resolution solid-state spectrometer for nuclear line studies should also be included in the mission. Since the mission is conceived as a follow-on mission to the GRO, detailed specification of its performance objectives should begin as the results from the GRO become available. 3. Ground-Based Instruments for Very-High-Energy Gamma-Ray Observations Observations of very-high-energy (greater than 1011 eV) gamma rays by ground-based Cerenkov detectors are the only source of direct information about the presence of very-high-energy particles in objects such as supernova remnants and active galactic nuclei. Work in this area of gamma-ray astronomy should be revived in the United States, and the development of more sensitive instruments should be encouraged.