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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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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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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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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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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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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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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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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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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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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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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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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.
Representative terms from entire chapter:
cosmic rays
