Astronomy and Astrophysics for the 1980's, Volume 2 Reports of the Panels (1983) / Chapter Skim
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IV. X-Ray Astronomy
IV. X-Ray Astronomy
Pages 12-37
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From page 12... ...
The broad spectral characteristics 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 supernova 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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From page 13... ...
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.
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From page 14... ...
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.
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From page 15... ...
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)
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From page 16... ...
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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From page 17... ...
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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From page 18... ...
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.
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From page 19... ...
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 radiation pressure limits by very small rates of mass transfer.
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From page 20... ...
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.
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From page 21... ...
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.
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From page 22... ...
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.
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From page 23... ...
Such clusters appear to be in an early phase of dynamic evolution. In contrast, CD clusters exhibit smooth, centrally condensed profiles of comparatively hard x rays, suggestive of their being more highly evolved systems.
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From page 24... ...
Its best position-determination accuracies are approximately 1 arcsec, two orders of magnitude better than the best of those in the 1960's, and sufficient to eliminate all uncertainty in the identification of optical or radio counterparts. Images with resolutions of several arcseconds have been made of the complex structure in dozens of diffuse sources such as supernova remnants and clusters of galaxies, where the best previous results were obtained with devices having angular resolutions of an arcminute.
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From page 25... ...
Transform image detectors have been developed for the study of sources of hard x rays that cannot be focused by reflection optics. Various configurations for Bragg spectrometry of hard x-ray sources and for extended diffuse sources of soft x rays have been successfully tested in laboratory experiments or on rocket flights.
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From page 26... ...
Study the accretion disks and magnetospheres surrounding compact galactic objects by high-resolution spectral and polarization measurements of binary x-ray sources. Study magnetic fields and accretion column structures of highly magnetized neutron stars by making accurate measurements of the cyclotron resonance features.
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From page 27... ...
Search for x-ray emission from hot neutron stars in their SNR's to obtain information on the cooling mechanisms of neutron stars. Interstellar Medium Measure the temperature, structure, and spatial distribution of the ultra-hot component of the interstellar medium.
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From page 28... ...
The X-Ray Background Measure the high-energy spectra of distant active galaxies and QSO's to determine their contribution to the background 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.
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From page 29... ...
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 opportunities for exploratory investigations in special areas of x-ray astronomy and for testing concepts for future satellite instrumentation.
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From page 30... ...
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 instruments including two 28-cm-diameter grazing-incidence telescopes for the energy range up to 4 keV and oronortional counters for the range up to 60 keV.
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From page 31... ...
. 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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From page 32... ...
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.
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From page 33... ...
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 instruments 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.
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From page 34... ...
The XTE will carry out extremely sensitive and accurate measurements of the variabilities of x-ray sources on time scales ranging from milliseconds to years. The scientific objectives of the mission include the determination 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.
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From page 35... ...
Line emission in the energy range below 1 keV is expected from the hot component of the interstellar medium. Cyclotron resonance features in the range from 10 to 100 keV have been observed in x-ray pulsators.
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From page 36... ...
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)
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From page 37... ...
5. Sounding Rockets The sounding-rocket program will continue for the forseeable 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.
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