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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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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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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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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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(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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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.
Representative terms from entire chapter:
supernova remnants
