Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 2
2
from individuals and groups of scientists working in
various areas of high energy astrophysics. Finally, our
report has benefited from the advice and comment of
numerous scientists who have read preliminary drafts of
various sections. We gratefully acknowledge these
sources of information and opinion.
I I . THE NATURE OF HIGH-ENERGY ASTRONOMY
AND THE SCOPE OF THE REPORT
Galactic supernovae are seen as spectacular transient
stars about once per century. They were noted as ominous
"guest stars" in the ancient Chinese court records,
recognized as stellar outbursts by Tycho Brahe in the
sixteenth century, and understood for the first time in
the 1930's to be cataclysmic explosions of stars. Since
then, observation and theory have shown that every
massive star ultimately reaches a critical condition in
its evolution when its core suddenly collapses, releasing
an amount of gravitational energy comparable with that
which would be obtained by converting about one tenth of
a solar mass into energy according to Einstein's formula
E = mc2. It is generally believed that in the resulting
explosion a large fraction of the energy escapes immedi-
ately in a burst of neutrinos and gravitational waves.
Most of the remainder is radiated as ultraviolet and
visible light during the period of a few months when, as
in the case of a guest star of A.D. 1054, the supernova
may appear as a point source that grows brighter than
Venus within a few days and then gradually fades, becoming
invisible to the naked eye after several months. For
thousands of years thereafter a vast expanding nebular
remnant and, in some cases, a collapsed stellar core
radiate predominantly in the nonoptical regions of the
spectrum as powerful sources of radio, x-ray, and gamma-
ray photons and energetic charged particles.
The remnants of the supernova of A.D. 1054, now called
the Crab nebula and the Crab pulsar, are among the most
conspicuous sources of nonoptical photons in the sky and
were among the first such objects to be discovered and
studied in the exploratory phases of radio, x-ray, and
gamma-ray astronomy during the 1950's and 1960's. The
studies revealed the presence in the nebula of high-energy
electrons and focused attention on supernovae and their
remnants as sources of cosmic rays. They suggested that
OCR for page 3
3
the accelerator of the electrons in the Crab nebula is
the Crab pulsar, the collapsed core of the original star,
now a strongly magnetized neutron star about 10 miles in
diameter with a solar mass of matter compressed by its
own intense gravity to a density of 10 billion tons per
cubic inch and spinning at 30 revolutions per second. As
this understanding grew, the Crab supernova remnant became
a favorite object of "high-energy" astronomy, displaying
in one event and its aftermath many of the extreme physi-
cal conditions that we now know are ubiquitous aspects of
the Universe--million-degree temperatures, ultra-high
densities, high-energy particles, and intense gravita-
tional and magnetic fields encountered in objects as
diverse in size and mass as stars, quasars, and clusters
of galaxies. High energy astronomy is the study of these
conditions through observation of the radiations they
produce.
To an observer with unaided eye, or even to one
equipped with an optical telescope of substantial power,
the sky appears largely unchanging except for the motions
of familiar nearby objects of the solar system. In
contrast, to one equipped with satelliteborne detectors
sensitive to high-energy photons, the sky is a place of
rapid and spectacular change. Intense bursts of gamma
rays, emanating from unknown sources scattered around the
sky, and in some cases lasting for only a few seconds,
occur at a rate of one every few weeks. Bright x-ray
stars, distributed along the Milky Way, vary on time
scales as short as a fraction of a second, some randomly
and others with clocklike regularity. -
~very rew minutes
one or another of a tew dozen peculiar faint blue stars
emits a brilliant 10-see flash of x rays, the equivalent
of turning the power of hundred thousand suns on and off
within the time of one human breath. A distant quasar,
radiating x rays with the power of a trillion suns, may
wax or wane by a factor of 2 within a few hours. Rapid
changes in such enormous luminosities imply highly concen-
trated energy sources with temperatures and densities
utterly beyond the scope of terrestrial experience.
These and many other similar observations have drawn the
attention of astrophysicists in recent years ever more
forcibly to the phenomena of the "high-energy universe"
and to the realization that the processes underlying
these phenomena play critical roles in the formation and
evolution of stars, galaxies, clusters of galaxies, and
the Universe as a whole.
~ in, _
OCR for page 4
4
The rapid pace of developments in high-energy astronomy
during the past decade is vividly shown by a comparison
of the state of knowledge as described in the report of
the previous Astronomy Survey Committee (Astronomy and
Astrophysics for the 1970's, National Academy of Sciences,
Washington, D.C., 1972), hereafter called the Greenstein
report, and the current situation. At that time, nonsolar
x-ray astronomy, initiated less than a decade earlier
-
with the discovery of Sco X-1. was iust beoinnina to
, . _ ,
assimilate the flood of discoveries from the first
satellite x-ray observatory, Uhuru, launched in December
1970. Already available were a catalog of over 100 x-ray
sources and results that revealed the existence of
extremely luminous x-ray pulsators in close binary
systems, evidence of a black hole, x-ray emission from
active galaxies and one quasar, numerous unidentified but
apparently extragalactic sources radiating predominantly
in the x-ray region, and x rays from hot intergalactic
gas in clusters of galaxies. _
from satellite x-ray observations were growing rapidly in
number.
Resorts of the findings
Little was known about the soft x-ray sky except the
existence of a diffuse and uneven Galactic background
radiation. Nothing was known about sources of extreme-
ultraviolet radiation, and indeed observations in this
wavelength were believed to be infeasible. The existence
of 100-MeV Galactic and extragalactic gamma rays had been
established, but nothing was known of discrete gamma-ray
sources, gamma-ray bursts, or lines in gamma-ray spectra
that are now central issues of high-energy astronomy. It
was known that cosmic rays consist of the nuclei of ele-
ments throughout the periodic table, as well as electrons
and positrons, with individual particle energies up to 10
million times the highest particle energies attained so
far by laboratory accelerators, but the mechanisms of
their acceleration, their confinement and propagation in
the Galaxy, and their role in galactic evolution were
poorly understood. Attempts to measure the flux of solar
neutrinos had so far yielded only upper limits that were
low compared with theoretical expectations.
. . _ . . . . .
Early reports
of the detection ot astonishingly intense gravitational
waves, subsequently discounted, had focused interest on
the problem of improving the reliability and sensitivity
of gravitational-wave detectors.
In light of these assessments, the Greenstein report
recommended several programs for high-energy astronomy in
the 1970's that have, to a substantial extent, been
OCR for page 5
5
accomplished. In particular, the Greenstein report
listed among its highest priorities a series of four High
Energy Astronomical Observatories (HEAO's). In the end,
after budgetary constraints had forced a reduction in
scope, three HEAD's were launched and operated success-
fully. The first two were large x-ray observatories, and
the second of these, the Einstein x-ray observatory,
acquired the status of a national facility by virtue of
the size of its user community and the breadth and sig-
nificance of its results for all of astronomy. The third
HEAD achieved major advances in gamma-ray line astronomy
and cosmic-ray studies. However, some important scien-
tific objectives in the original HEAO program, particu-
larly in the areas of gamma-ray and cosmic-ray astronomy,
were lost by the reduction in scope and remain to be
accomplished by new programs in the 1980's. Recommenda-
tions for support of efforts to detect solar neutrinos
and for development of gravitational-wave detectors were
acted on with the results that solar neutrinos have been
detected, albeit at a surprisingly low flux value, and
major progress has been made in increasing the sensitivity
of gravitational-wave detectors, though not yet to the
point of a positive detection.
While high-energy astronomy in the United States
achieved extraordinary advances throughout the 1970's, it
now faces the certainty of a virtual standstill during
the first half of the 1980's. The paucity of new starts
on major space projects in high-energy astronomy during
the past several years and the delays and reductions in
funding of the few ongoing projects have caused a widening
gap in observational capabilities. Research teams and
their engineering support groups are disbanding at a time
when the opportunities for major scientific progress are
clearly defined and highly promising. As a result,
leadership in high energy astronomy, which was pioneered
in the United States, is now passing to other countries.
Symptomatic of this trend is the problem of the Explorer
satellite program, which yielded so many important results
during the 1960's and 1970's. It has not been funded at
levels sufficient even to keep pace with inflation, much
less to accommodate the need for increased observational
capabilities. As a consequence, no Explorer-class satel-
lite for high energy astronomy will have been launched
during a period of ten years since the last of the Small
Astronomy Satellites was placed in orbit in 1975. In the
area of low-cost vehicles, which were strongly recommended
for vigorous support by the Greenstein committee Panel on
OCR for page 6
6
Space Astronomy, there has been a persistent problem of
inadequate funding for projects in high-energy x-ray,
gamma-ray, and cosmic-ray astronomy.
m e task of the High-Energy Astrophysics Panel has
been to assess the current status and future prospects of
the subdisciplines of astronomy that are based on obser-
vations of the extreme ultraviolet region; x-ray and
gamma-ray photons; energetic nuclei, neutrons, and
electrons; and neutrinos and gravity waves. Together
with photons in the radio to ultraviolet portion of the
electromagnetic spectrum this list includes all known or
expected forms of radiation that can reach the Earth from
distant sources.
The instruments used for measuring these forms of
radiation are exceedingly diverse. For example, deep in
a salt mine an enormous tank of cleaning fluid linked to
a tiny detector of induced radioactivity is used to
measure the flux of neutrinos generated by the fusion
processes in the center of the Sun. Above the atmosphere
the orbiting Einstein x-ray observatory records high-
l
resolution x-ray images of a distant cluster of galaxies
and provides for the first time a measure of the mass and
temperature of ultra-hot gas that pervades the cluster.
In the HEAD-3 observatory a solid-state spectrometer
observes the annihilation of positrons and electrons in
the central regions of the Galaxy. Far away in the solar
system, beyond the orbit of Jupiter, the transmitter on a
planetary probe sends radio signals back to Earth, where
frequency variations are analyzed for evidence of passage
across the solar system of gravity waves from cosmic
cataclysms.
During the past decade all areas of high-energy
astronomy benefited from rapid growth in observational
capabilities based on the developing technologies of
x-ray optics, radiation detection, solid state elec-
tronics, space instrumentation, and data processing. In
six years the sensitivity of resonant-bar gravitational-
wave detectors, measured in terms of minimum detectable
energy flux, has been improved tenfold, and another
thousandfold improvement is imminent. Laser systems now
under development offer the prospect of even greater
sensitivities. Cosmic-ray astronomy has achieved remark-
able improvements in the precision of mass and charge
measurements that make possible the use of a variety of
isotope clocks in the investigation of the acceleration
and propagation of high-energy nuclei in the Galaxy.
Instrumentation for the extreme-ultraviolet portion of
OCR for page 7
7
the spectrum has evolved from laboratory test devices to
sophisticated astronomical instruments for satellite
observations with sensitivities comparable with those
achieved by the Uhuru satellite at higher energies. In
x-ray astronomy, where fewer than 100 sources were known
in 1970, the Einstein x-ray observatory in 1979 had
achieved more than a thousandfold increase in sensitivity,
thereby bringing into the range of potential x-ray obser-
vability several hundreds of thousands of discrete Galac-
tic and extragalactic objects, including the most distant
objects yet detected in the Universe. Such gains in
observing power and the resulting progress in discovery
and understanding are the benefits of investment in
supporting research and technology. A vigorous program
of instrumentation development is therefore essential to
achieving the scientific goals of high-energy astronomy.
Supporting research and technology must be considered an
essential ongoing activity in each of the subdisciplines,
sustained at a level of effort commensurate with the
technical opportunities and relatively unaffected by the
delays in funding of large projects. Looking forward to
the 1990's, we see an urgent need to stimulate and encour-
age the development of new technologies that will advance
high-energy astronomy beyond the status we see it attain-
ing in the 1980's on the bases of current instrumentation
concepts.
Discoveries in high-energy astronomy during the past
decade have presented an overwhelming challenge to
theoretical research aimed at creating the intellectual
framework both for understanding what is observed with
the new instruments and for providing ideas as to where
and how to look for interesting new phenomena. Without
adequate growth in such understanding, the efficiency of
observation is reduced and its purpose unfulfilled. A
significant part of the progress in the theory of high-
energy cosmic phenomena has been achieved during the past
decade by theoretical research supported by the data-
analysis portions of project funds. But mission-oriented
theoretical research suffers from inflexibility and a
lack of long-term continuity. We have therefore recom-
mended that a high priority be placed on providing new
funding for theoretical research in high-energy astronomy,
independent of specific hardware projects and missions,
and at a level commensurate with the total task of
interpreting the results of the observational programs.
As we look to the future we see good opportunities for
a continuation of the age of discovery in high-energy
OCR for page 8
8
astronomy. Major discoveries are anticipated in gamma-
ray astronomy, and it is therefore of fundamental
importance that the Gamma Rat Observatory (GRO) be
constructed and flown.
This mission has been thoroughly
studied and is now approved as the essential next step in
the development of gamma-ray astronomy. m e Extreme
Ultraviolet Explorer (EUVE), the essential next step in
the development of E W astronomy, has been selected for
early development within the Explorer program. We have
predicated our consideration of future plans on the
assumption that these two vitally important missions will
be carried through. ~
Beyond them we see clearly defined
needs for new missions on free-flying satellites of the
Explorer class, launched either by the Shuttle Transporta-
tion System or by independent rockets. Short-term Space-
lab missions will be of great value in some investigations
but will never be adequate substitutes for free-flyers in
most areas of high-energy astronomy. We therefore
strongly recommend that the Explorer program be substan-
tially increased to accommodate the scientific needs.
Among the many important new scientific opportunities
that beckon high-energy astronomy in the 1980's, we
believe that at present those in x-ray astronomy are the
most numerous and exciting. It has become clear in the
past decade that x-ray observations provide unique infor-
mation not only about exotic astrophysical processes
connected with collapsed stellar objects and supernova
remnants but also about stars of all types, the inter-
stellar medium, normal galaxies, radio galaxies, active
galactic nuclei, and clusters of galaxies. These comprise
most of the subjects under intense investigation in the
mainstream of observational astronomy carried out in the
optical and radio regions of the spectrum. To gain this
information, x-ray astronomy now requires long-term
observational capabilities comparable in scope with those
available in the optical and radio domain. The Shuttle
Transportation System will make this possible by providing
the means to launch a large x-ray observatory that can be
operated as a national facility with routine servicing,
refurbishment, and installation of new instruments over
period of many years. We have therefore come to a firm
conclusion that development of the Advanced X-Ray Astro-
physics Facility (AXAF) should be undertaken as the
project of highest priority for astronomy in the 1980's.
It should be started at the earliest possible date, and
special institutional arrangements for its scientific
management and operation similar to the Space Telescope
Science Institute should be planned and implemented.
Representative terms from entire chapter:
theoretical research
