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OCR for page 36
4
New Initiatives
THE WAY FORWARD
The previous chapters have outlined how astronomers have
developed a totally new view of the universe and have projected
the expected state of space astronomy in 1995. Our observa-
tional capabilities have increased steadily. New phenomena have
been revealed at each advance in sensitivity, spectrum coverage,
and angular resolution. Increasingly, the complementarily of ob-
servations in different parts of the spectrum has been revealed,
emphasizing the view that access across the electromagnetic spec-
trum is essential in advancing our knowledge of the universe. The
Great Observatory program, involving the HST, SIRTF, AXAF,
and GRO by the 1990s, meets many of the present needs. The
task group has assumed that this core program, the culmination
of many years of planning and experimentation, will have been
implemented by 1995.
The program of new initiatives for the era 1995 to 2015 fo-
cuses on improvements in capabilities in two areas: higher angular
resolution and greater collecting area.
The first of these, high-resolution imaging, requires develop-
ment of interferometric arrays to synthesize large apertures. The
goal varies with wavelength, but in general the aim is to work
36
OCR for page 37
37
toward m~croarcsecond imaging at radio and optical wavelengths
(including the ultraviolet) and to obtain milliarcsecond resolution
or better at infrared wavelengths. This would mark the logical
continuation of the program started by Galileo, when he launched
the modern era of astronomy. GaTileo's telescope, which showed
details that were a factor of 10 finer than the human eye could
see, started a process that still goes on today. As knowledge at
one level of detail is consolidated, the new questions this knowI-
edge raises justify further explorations. The program for high-
resolution imaging, described in the following section, represents
the latest step in this evolution.
The second general need, for greater collecting area, is more
accurately described as the need for high-throughput instruments.
The techniques to attain this vary greatly from one part of the
electromagnetic spectrum to another, but philosophically these,
too, are a continuation of GaTileo's program in a different aspect.
The greater collecting area of GaTileo's telescope again allowed
the observation of fainter objects than the eye could see, and
that too has continued as a major thrust in astronomy through
the building of larger telescopes and better detectors. The needs
for high-throughput instruments can be identified at subrn~lime-
ter wavelengths, in the optical, ultraviolet, and x-ray domains,
extending down to gamma rays and including the particle detec-
tors of the cosmic-ray astronomers. These needs are detailed in
a separate section. The instruments described there involve new
technologies in some cases, but are not beyond the projected ca-
pabilities for 1995 and thereafter. In some instances deployment
of the instruments could be facilitated by partial fabrication and
assembly in space. The instruments are described in order of
decreasing wavelength, but no priority ordering is implied.
The initiatives described here will demand new technology
and new capabilities in space. The only way to make sharper
diffraction-limited images will be to lengthen the baseline over
which the wave front is sampled. This can be achieved directly by
increasing the diameter of the telescope reflector, or indirectly by
coupling together radiation or signals from widely spaced reflec-
tors. To overcome quantum noise we need to provide telescopes
with larger collecting areas. These considerations set the direction
for future evolution toward still larger telescopes, and the period
beyond 1995 will bring new opportunities to pursue this evolution.
Adaptations planned for the Space Transportation System
OCR for page 38
38
(STS) will allow the launching of telescopes and components con-
siderably larger than the limit set by the Shuttle cargo bay. The
Space Station, or ultimately a lunar base, will over an arena in
which very large telescopes or arrays can be assembled, tested,
and fine-tuned in their operating environment. Transfer vehicles
will allow these telescopes to be placed and serviced in optimum
orbits.
HIGH-RESOLUTION INT1DRFEROMETRY
Introduction
When Michelson invented the stellar interferometer in 1920,
the promise of the technique was clear, and one of its principal
technical obstacles was equally clear. The sizes of stars would be
estimated from their temperature and magnitude, and the inter-
ferometer could, in principle, provide the necessary milliarcsecond
(or better) angular reduction; unfortunately, astronomy had to
be carried out at the bottom of the Earth's atmosphere, whose
turbulent behavior so perturbed the incoming wave fronts that
phase coherence was lost over apertures larger than a few centime-
ters and for times longer than a few milliseconds. The method
was little used until the radio astronomers were able to adapt
and refine the technique. Very Tong baseline radio interferometry
(V[Bl) is now used routinely, for example, to achieve astrometric
accuracy of 10 to 100 ,uarcsec (depending on source separation
and structure). The Very Large Array (VLA) in Socorro, New
Mexico, represents the present culmination of radio inferometry in
the form known as aperture synthesis, in which complete Fourier
information is obtained over an aperture 35 km in diameter, and
images are obtained with correspondingly high resolution (0.1 arc-
sec at 2-cm wavelength). The V[BA, now under construction,
will extend imaging capabilities to better than a milliarcsecond by
the same aperture-synthesis methods, but with an aperture more
than one hundredfold larger. The QUASAT mission will extend
the aperture size still further.
The advent of space science now raises the promise of us-
ing the clarity of space to achieve similar capabilities in the in-
frared, visual, and ultraviolet regions of the spectrum. Except
for technical details, none of them fundamental, the concept of
aperture synthesis carries over to the shorter wavelengths of the
OCR for page 39
39
optical domain. The ultimate goal is to achieve images with 1-
,uarcsec resolution, but this millionfold increase from our current
resolution on Earth is probably not achievable in a single step.
A thousandfold increase, however comparable to the step from
the VLA to the VI,BA does seem to be a reasonable first step.
Progress toward still higher angular resolution might then come
with the establishment of this technology. Much of the necessary
technology for imaging and astrometric interferometry is held in
common. Astrometric developments with the same or a similar
instrument would probably allow microarcsecond accuracy. An
astrometric instrument with microarcsecond accuracy would have
numerous applications including a light-deflection test of general
relativity sensitive to the effect of the square of the solar potential.
Such a test would be the first "second-order" solar system test of
general relativity. Other possible scientific uses include: a search
for extra-solar planetary systems; a direct determination of the
Cepheid distance scale; the determination of the masses of stars
in binary systems and those close enough to apply the method
of perspective acceleration; parallax measurements yielding both
absolute stellar magnitudes and, in conjunction with mass esti-
mates and other data, a sharpened mass-color-luminosity relation;
a study of mass distribution in the galaxy (and thus an improved
understanding of its dark-matter content); a strictly geometric
(i.e., coordinate and parallax) determination of the membership
of star clusters (particularly useful in the case of peculiar stars
such as blue stragglers and Wolf-Rayet stars); and a bound on or
measurement of quasar relative motions.
A workshop held in Cambridge, Massachusetts, in October
1985 reviewed the prospects for interferometry and concluded that
"imaging interferometry in space will ultimately play a central
role in astrophysics, a role comparable in significance to that
played by space observations at x-ray and infrared wavelengths."
The current level of effort in space interferometry is extremely
small, and the workshop concluded that an orderly program had
to be constructed. This would have to include the following:
technological development of structures, spacecraft control, and
optical technology; the study of a variety of instrumental concepts;
the flight of small interferometers; and the formulation of a long-
range program leading to a major observatory-cIass instrument.
The earliest observation in the milliarcsecond range would be
OCR for page 40
40
intensely interesting, and as one progresses to the microarcsec-
ond level further dramatic results can be anticipated. Figure 4.1
shows some of the phenomena that can be studied at various scale
sizes as a function of distance. Practically every class of object of
astrophysical interest appears in the diagram. Constant angular
resolution in this representation follows the diagonal lines shown,
and one can see that the HST reaches only some of the region of
interest. A milliarcsecond instrument reaches the resolution range
for several major classes of object, including stars, novae, star-
form~ng regions, and the broad-line region of quasars and active
galactic nuclei (AGN). When one progresses beyond, toward mi-
croarcsecond resolution, the fields of interest become progressively
richer.
The quasar/active galactic nucleus problem can be used as
an illustrative example. These are probably related phenomena,
differing only in scale: our own galaxy has a moderately active,
compact nucleus, possibly containing a black hole of perhaps a mil-
lion solar masses. Seyfert galaxies have more active nuclei, while
quasars with their spectacularly high energy output are the most
active of all. The power output from these objects is derived from
gravitational energy as matter in the surrounding galaxy falls into
the central black hole. The matter cannot be pulled in directly
since it has angular momentum, so it settles into an accretion
disk, where the angular momentum can be transported outward
as the material spirals inward. The phenomena are coupled and
lie at the forefront of our physical knowledge. One of the un-
expected consequences is the generation of the highly collimated
jets observed by the radio astronomers. These clearly involve the
acceleration of bulk matter to relativistic velocities. The optical
and x-ray fluctuations of quasar brightness hint at the existence of
other dynamical phenomena that can best be studied by viewing
the phenomena directly.
The angular resolution required to study the structures in an
active galactic nucleus is illustrated in Figure 4.2. For M87 (Virgo
4), the closest highly active galactic nucleus, a milliarcsecond
instrument would reach close to the accretion disk; if a resolution
of even a few tens of microarcseconds could be achieved, the actual
details of the accretion disk phenomena could be studied. The
dimensions characteristic of the black hole itself are still beyond
the observable horizon until we can achieve resolution somewhat
better than a microarcsecond. Whether the central singularity is
OCR for page 41
41
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FIGURE 4.1 Phenomena studied at various scales as a function of distance.
actually a black hole or is a still more exotic form of matter cannot
be said, but it is clear that in studying this class of phenomena
we would be drawn into a new domain of physics: the regime of
strong-field gravitation.
OCR for page 42
42
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FIGURE 4.2 Angular resolution required to study active galactic nuclei.
Optical and infrared Interferometric Arrays
The infrared and optical domains of the electromagnetic spec-
trum (including ultraviolet wavelengths as part of the optical spec-
trum) are ripe for investigation using the high-angular-resolution
capabilities of aperture-synthesis interferometry. The manifold
possibilities have been illustrated in Figure 4.1, but a program
must be formulated that can make these a reality. Both ground-
based and space-based interferometry need more intensive devel-
opment. Clearly, instruments should be located on the ground
when that is feasible, as it probably is in parts of the infrared
spectrum. Ultraviolet interferometry, however, can never be done
from the ground, nor can interferometry in those parts of the in-
frared spectrum that are blocked by the Earth's atmosphere. Like-
wise, throughout the visual part of the spectrum the disturbing
influence of the Earth's atmosphere is so great that milliarcsecond
and microarcsecond resolution seems most unlikely. Thus, visual
interferometry will also depend upon space-based instruments.
OCR for page 43
43
The first major instrument in space might well be one com-
posed of a number of medium-sized telescopes, mounted on a com-
pensating structure that would maintain their phase coherence.
The exact configuration of the array, the number and size of its
elements, and the array location (Iow earth orbit, geosynchronous,
L5, or the Moon) remain to be defined, but a reasonable represen-
tation is illustrated schematically in Figure 4.3. This shows nine
elements on a tetrahedral truss, with the image-processing equip-
ment at the fourth vertex. The mass distribution would be chosen
to yield a zero quadrupole moment, thus reducing gravity-gradient
torques. The dimensions might well be in the 5~ to 100-m range,
and the elements might be 1.5 m in diameter. Such an instru-
ment, if 100 m in dimension, would give angular resolution of 1
milliarcsec at 5000 A; at 2000 A, the resolution would be 0.4 rn~li-
arcsec. The collecting area would equal that of a 4.5-m-diameter
telescope, so its sensitivity would be comparable with that of any
ground-based telescope operating today.
An entirely different concept might consist of free-flying tele-
scopes, whose connection is only by laser beam. This might be
called a Long-Baseline Optical Space Interferometer (LBOSI) and
would achieve exceedingly high angular resolution on both bright
and faint astronorn~cal objects over a wide wavelength region (per-
haps as much as 0.2 to 500 ,um; ultraviolet to submillimetric). The
LBOSI would consist of two or more large (8-m-cIass) diffraction-
limited telescopes separated by variable baselines. For a maximum
separation of 100 km, an angular resolution of 1 ,uarcsec would
be achieved. There are major technical hurdles to be overcome,
however. The most serious one is precision station keeping and
attitude control. Studies of the limits of these technologies should
be undertaken. This concept of free-flying telescopes is probably
more difficult and more expensive than the monolithic tetrahedral
concept shown in Figure 4.3, but if it is technically feasible it
would be an exciting instrument to use. Expandability to even
Ton ger baselines is a definite advantage of this type of array.
Development of even these first-generation optical interfer-
ometers would require extensive technical progress. This would
include the development of means to measure rapidly and ac-
curately the positions of the elements, and to compensate for
path length changes in a dynamically stable fashion. Methods
for detecting fringes simultaneously at many wavelengths in the
presence of photon and background noise, with an adequate field
OCR for page 44
44
,1
1~
~ _
I:
Y it/
It
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IN \~\ a\
\\N '\\\\
at\\
V
FIGURE 4.3 A (large) space telescope array: Nine 1- to 2-m-class telescopes
on a 50- to 100-m tetrahedral truss.
OCR for page 45
45
of view, must also be developed. Since true images are desired,
the precision of the path lengths would have to be kept to a
small fraction of a wavelength during an integration period. This
may be done by using unresolved stars in the wide field of view
available to these space interferometers for sensing phasing errors.
Optical and infrared interferometry may profit from the exten-
sive phase closure and self-calibration techniques developed for
radio aperture-synthesis instruments, and their adaptation must
be carefully worked out.
Given the extent of this technical challenge, the task group
recommends outlining a developmental program over the next
decade. Some of this developmental work can be carried out on
the ground, but some will require space experiments. These would
certainly include small interferometers, probably having only two
elements and capable of assembly in space.
Extensions of Orbiting Radio VIB]
In the mode! for the space science status in 1995, sumrna-
rized in Chapter 3, the radio astronomy mission QUASAT was
projected, extending the V[B! imaging capability to baselines of
about 3 earth diameters. At present, the Soviet Union is plan-
ning a series of V[BI satellites (RADIOASTRON), and they have
indicated a willingness to coordinate their plans with NASA and
ESA. Furthermore, Japan is studying the feasibility of launching
a V[BI satellite. Again, with proper coordination, we can expect
an augmentation of resolving power surpassing 10 ,uarcsec. Radio
astronomy has had a history of uncovering surprises, and this ex-
tension of QUASAT may be expected to continue this tradition.
If the results are provocative, an extended, more ambitious array
might be contemplated for the era beyond 2000.
The natural limits of radio aperture synthesis are set by the
interstellar medium (ISM), which is an inhomogeneous plasma.
At the higher galactic latitudes, above the obscuration that affects
optical and ultraviolet extinction of intragalactic objects, the see-
ing limits imposed by the ISM are not serious down to a resolution
of a microarcsecond at a wavelength of 1 or 2 cm. This means
that an array of radio telescopes can be envisaged, extending to
baselines of 100 earth diameters or so, yielding an angular resolu-
tion of 2 ,uarcsec at 1.3-cm wavelength. If the expected advance of
microwave electronics proceeds, the minimum working wavelength
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46
might well be 3 mm and the maximum angular resolution would
then be 0.5 ,uarcsec. The scientific possibilities with respect to the
study of active galactic nuclei can be seen by referring to Figures
4.2 and 4.3.
The technical realization of plans to extend radio V[BI will
depend on the development of space technology. A current study,
known as ASTROARRAY, projects a total of 6 to 12 antennas
in a variety of orbits chosen to give full aperture synthesis. The
size of the individual antennas might well be of the order of 30
m, a size consistent with the projected state of the art in the
year 2000. V[BI is currently a strongly international activity, and
ASTROARRAY, if it becomes a reality, would almost certainly be
international in character.
.
Future Developments
The direction of developments in interferometry can be unex-
pected, as the sudden advent of VLBI demonstrated. The viewing
of the world in Fourier transform space has been a strong cur-
rent in much of modern science, and over the next 30 years one
may well see new possibilities open up as technology advances.
Microarcsecond resolution at x-ray wavelengths, for example,
would be enormously exciting, and even though x-ray interfer-
ometry is a technically difficult concept, there appear to be no
fundamental physical barriers. An x-ray interferometer with mi-
croarcsecond resolution would be of the order of a few tens of
meters in diameter. Although such an instrument has barely been
conceptualized yet, a prudent program would keep an awareness
of its potential. Flexibility of reaction is essential within the space
science program in this respect, especially since relevant experi-
ments on a much smaller scale could be carried out.
HIGH-THROUGHPUT INSTRUMENTS
The Large Deployable Redector (IDR)
The Large Deployable Reflector (LDR) will be a 2~ to 30-
m-aperture telescope dedicated to far-infrared and submillimeter
observations from space. Assembled in space, it will operate be-
tween about 30 and 1000 ,um (1 mm). It will provide angular
resolution of 1 to 2 arcsec at 100 ,um and 0.3 to 0.6 arcsec at 30
OCR for page 56
56
development program, the task group anticipates that an 8- to
loom telescope will prove to be within closer reach than a simple
extrapolation from HST would suggest.
VERY HIGH THROUGHPUT FACILITY (VHTF)
Although AXAF will be the first major observatory for x-ray
astronomy with both sensitivity and resolution comparable with
the most sensitive optical and radio facilities, it will be limited in
its capabilities for spectroscopy of faint objects as well as for high-
time-resolution studies owing to its relatively modest collecting
area. The European x-ray project (XMM), will complement AXAF
with a larger effective area but lower resolution so that fainter
diffuse sources can be studied spectroscopically. Major extensions
of this spectroscopic capability are crucial for addressing a broad
range of fundamental problems in astrophysics. A Very High
Throughput Facility (VHTF) would provide the required high-
sensitivity spectroscopy as well as high-time-resolution studies of
faint sources. The key to the VHTF is very large collecting area,
possibly at the expense of angular resolution for spectroscopy and
time variability studies. The VHTF, which would be assembled
on a space platform with support and servicing from the Space
Station, would consist of a grazing-incidence telescope system with
total elective area of about 30 m2. It would be constructed as
either a single mirror of very large diameter and focal length, or
more probably as an array of smaller telescopes of more compact
design.
With this sensitivity increase, a number of qualitatively new
investigations are possible, including the following:
. Dark matter in galaxies and clusters. VHTF would, with
its enormous sensitivity for imaging and spectroscopy of disuse
objects, allow halos of galaxies to be measured for their total
content of low-mass stars, diffuse hot gas, and total gravitational
potential (by spatially resolved studies of its hot gas). Similar
studies of galaxy clusters out to moderate red shifts (Z ~ 0.5)
would allow temperature, density, composition, and mass profiles
to be derived. This would constrain the still uncertain theories for
the origin and evolution of hot gas in clusters.
. Star formation in molecular clouds. VHTF would image
and locate pre-main-sequence stars, already known from Einstein
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57
observations to be relatively luminous x-ray sources in dark clouds.
When coupled with deep infrared observations of star formation
sites, physical conditions could be derived. X-ray heating of the
cloud by its pre-main-sequence stellar population appears to play
a fundamental role in the physics of cloud collapse and star for-
mation.
. High-time-res~olution studies of compact objects. VHTF
would provide the ultimate capability to explore the physics of
compact objects, accretion disks, and extreme field conditions in
astronomical objects. With its imaging advantages, high spectral
resolution (about 103 to 105), and large area it would study com-
pact objects in our galaxy and nearby galaxies of the Local Group
in great detail as well as QSOs at the largest red shifts. For ex-
ample, through time-resolved spectra of x-ray bursts from a larger
fraction of the burst sources in M31 and other nearby galaxies (ob-
servable within a single VHTF field), the mass and radii of neutron
stars can be derived and compared with similar results for objects
in our own galaxy. Detailed timing studies of galactic bulge x-ray
sources in our galaxy could detect pulsations at a level of 10-4 of
the persistent flux. This could detect stable pulsation periods and
thus enable searches for the gravitational waves expected if the
sources have very fast millisecond spin periods. High-resolution
spectra of QSOs and distant galaxy clusters would measure red
shifts directly from their iron-line features as well as probe the
internal dynamics of accretion disks and jets where thermal (line)
components are expected.
The category of large throughput x-ray instruments could also
include a very large area array of proportional counters provided
with mechanical collimators (about 1 degree revolution) solely to
isolate relatively bright sources. A potential design goal would
have an effective aperture of 100 me, sensitivity from about 0.2 to
40 keV, and timing resolution down to a few microseconds. Such
an instrument would allow extreme phenomena in the vicinity of
neutron stars and stelIar-mass black holes to be probed in detail.
The broad energy bandwidth would be vital in studying regions of
high opacity. Such a large array could be built in space in modular
form and assembled as a relatively Tow-cost experiment at the
Space Station. This nonimaging detector would complement both
the imaging soft/medium x-ray facility (VHIF) and a possible
Hard X-ray Imaging Facility (HXIF).
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58
HARD X-RAY IMAGING FACILITY (HXIF)
At energies above 20 keV, grazing-incidence x-ray telescopes
are impractical, and this band will still remain relatively unex-
plored in 1995. The modest-sized (about 103 cm2) hard x-ray
detectors planned for the Franco-Soviet SIGMA Satellite (1988)
and U.S. XTE mission (1992) should have made significant ad-
vances in detecting the brightest several hundred sources in the
2~ to 200-keV band by that time, but detailed astrophysical mea-
surements and exploration of the full hard x-ray/soft gamma-ray
energy band of about 20 keV to 2 MeV will not yet have been pos-
sible. This energy range contains a rich assortment of information
that can be used to address each of the field's three major sci-
entific objectives: the early universe, compact objects and stellar
collapse, and star formation. It is vital that we study this gap in
the electromagnetic spectrum in detail.
A Hard X-ray Imaging Facility (HXIF) would provide a large
increase in effective area (by a factor of about 300) and there-
fore an increase in sensitivity over any hard x-ray experiment
flown previously. It would employ coded-aperture and Fourier-
transform imaging with 10 arcsec to 1 arcmin resolution in a 5°
field of view over the broad energy range up to a few million elec-
tron volts. Systems with very long effective focal lengths (10 to
100 km) between the coded mask and position-sensitive detector
could achieve milliarcsecond angular resolution. Coded-aperture
imaging techniques, using perforated occulting aperture plates (50
percent open area) to cast a shadow on a position-sensitive hard x-
ray detector whose output is correlated with the mask, should have
been fully developed and tested in flight (including the SIGMA
mission) by 1995. With the large sensitivity increase possible with
HXIF, imaging is essential in order to eliminate source confu-
sion. It is also possible that direct (true) hard x-ray imaging over
a more limited energy range can be achieved with, for example,
Bragg concentrators. New approaches to hard x-ray imaging might
be developed with a vigorous program of flight opportunities for
low-cost experiments from the STS and Space Station.
HXIF could consist of an array of relatively simple (and self-
contained) but large imaging telescopes, each with coded mask,
shielded detector (probably scintillation crystals), and position-
sensitive readout. The full array could consist of 64 modules, each
with a 0.5 m x 0.5 m detector and mask at a focal length of 3 m.
OCR for page 59
59
(A separated detector/mask system with much higher resolution
might eventually be operated at a lunar base.) This would yield
a total effective detection area (through the mask) of 16 m2 and
would occupy a total volume of perhaps 5 m x 5 m x 4 m.
The entire array would be co-aTigned and fixed to about 1-arcmin
accuracy and then pointed with about 10-arcsec stability.
The sensitivity of HXIF would be at least 200 times that of
the experiments flown thus far and at least 10 to 30 times that
of SIGMA or the X-Ray Timing Explorer Satellite (XTE). As
such, it will be possible to attack a range of fundamental problems
including the following:
. Central engines of quasars. QSOs and active galactic nuclei
radiate most of their energy in the hard x-ray band. The 50 active
galactic nuclei, for which spectra were measured out to about 20
to 50 keV with the HEAO missions, show relatively similar power
law spectra with a spectral index of 0.7. If these spectra continue
unbroken out to about 1 MeV, the total contribution of all active
galactic nuclei would greatly exceed the hard x-ray/soft gamma-
ray background. The total luminosity (determined by the break in
the high-energy spectrum) of these sources is at present unknown.
HXIF would be sensitive enough so that with a 104-s observation
it should always detect and precisely locate one or two sources in
its field of view and measure their spectra out to at least 300 keV.
At 100 keV, the sensitivity would be sufficient to measure about
10 sources in each field.
Thus, it will be possible to measure the total energy output
of QSOs for the first time. It would also be possible to measure
changes in spectra and total luminosity as a function of cosmic
epoch (red shift). With the sensitivity to observe about eighteenth-
magnitude QSOs, HXIF could measure the brightest QSO at red
shifts of Z = 2.5 and a broad range of luminosities at Z = 0.5.
Very long exposures could reach correspondingly deeper (to Z =
3.5) so that the full spectra of QSOs at the earliest epochs could
be probed.
.
Physical properties of neutron stars and black holes. The
sensitivity of HXIF would be sufficient to detect (at about 100
keV) gamma-ray burst sources in M31. Similar observations of
the MagelIanic clouds would yield accurate source locations (about
10 arcsec) and time-resolved spectra for each burst so that their
hypothetical association with very low-accretion-powered neutron
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stars could be studied in detail. Cyclotron line features would be
detectable and neutron star magnetic fields measured. Annihila-
tion line features also could be studied in detail for burst sources in
our galaxy so that gravitational red shifts for neutron stars could
be measured.
High-time-resolution (less than 1 ms) studies at 300 keV of
stelIar-mass black hole candidates such as Cyg X-1 could be carried
out for the first time, allowing the conditions nearest the hole to be
more completely specified than with soft x-ray (~10-keV) studies
alone. Compt~onization studies versus time (in flares) in both
galactic and extragalactic candidate black holes would allow the
electron densities and temperature profiles to be derived and the
physical size of the sources to be measured.
GAMMA-RAY ASTRONOMY
Following the Gamma Ray Observatory (GRO), several tele-
scopes will again be required to meet the objectives of gamma-ray
astrophysics because of the different interaction processes involved
in gamma-ray detection over the large gamma-ray energy range,
105 to 10~' eV. Even with the relatively large instruments on the
GRO, gamma-ray astronomy is constrained by the number of de-
tected photons. Larger-area telescopes with longer exposures and
markedly improved angular resolution are necessary to meet the
objectives beyond GRO. Energy measurements are important over
the entire spectrum, with the required resolution depending on the
energy interval.
Gamma-ray observations are particularly relevant for those
phenomena in which high-energy processes reflect the underlying
energetics of the system. Active galactic nuclei, compact objects,
explosive phenomena, and the acceleration and interactions of cos-
mic rays are examples. We must obtain accurate measurements
of the gamma-ray luminosity of large numbers of active galactic
nuclei so that different production mechanisms can be identified
by class and their contribution to the diffuse radiation can be
determined accurately. Temporal variability of these sources on
time scales from minutes to years will be required to distinguish
whether the high-energy emission arises from a central engine
(e.g., a massive black hole), from interactions of energetic jets
with the ambient material in the sources, or as the result of indi-
vidual explosive events (e.g., supernovae). Observations extending
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over many years will be required to interpret accurately the rela-
tionship of the central energy source to phenomena observed at
other wavelengths, such as the relativistic jets and superluminal
expansion of knots. For some sources, we can utilize the temporal
variability to associate conclusively the gamma-ray object with ob-
servations at other wavelengths. In general, however, gamma-ray
detectors with source location capabilities approaching 1 arcmin
will be necessary.
The detailed study of compact objects black holes, neutron
stars, dwarf stars in our galaxy and nearby galaxies will demand
high-quality measurements. Nuclear line emission resulting from
reactions of energetic particles with the surface material of neu-
tron stars should provide direct information about composition
of this material and of surface red shifts from which the mass-
to-radius ratios can be derived. With estimated line fluxes from
10-5 to 10-7 ~ /cm2/s even for relatively nearby objects, substan-
tial sensitivity improvements beyond GRO are required to address
these studies satisfactorily. Spectral and temporal characteristics
of compact sources will also allow model-dependent probes of the
inner portions of the accretion disks. The dynamical and spec-
tral characteristics will provide additional information on those
sources that contain black holes, and thereby provide tests for
physical processes occurring in the vicinity of black holes. The
study of explosive events, supernovae and novae, will benefit from
the substantial improvements in sensitivity and spectral resolu-
tion anticipated with the instruments in the 1995 to 2015 era.
Significant data on extragalactic supernovae will provide direct,
quantitative information on the acceleration of cosmic rays and
nucleosynthesis of heavy elements.
Currently, the following instruments appear necessary for
progress in gamma-ray astronomy:
1. State-of-the-art spectroscopy in the 0.1- to 10-MeV spectral
region for nuclear gamma-ray line observation can currently be ac-
complished with solid-state detectors, e.g., germanium detectors.
At present, efforts are under way to develop large germanium ar-
rays that could provide the desired sensitivity-significantly below
10-5 ~ /cm2/s-if background problems can be overcome. Posi-
tion sensitivity within germanium detectors is being pursued, and
a large array combined with a coded mask could provide a system
that combines high spectral and angular resolution. The possible
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use of massively shielded detectors for use in this low-energy re-
gion will also be considered. Such an instrument should be able
to observe radioactivity in supernova remnants, electron-position
annihilation, and nuclear excitations caused by cosmic rays.
2. In the medium-energy gamma~ray range (1 to 60 MeV),
an advanced Compton telescope where both upper and lower de-
tector elements have high energy and spatial resolution and low
background would result in considerable improvements in sensi-
tivity and energy resolution compared with previous instruments.
Good energy resolution in the million electron volt range will per-
mit an accurate study of the spectra of individual sources, thereby
permitting a better understanding of their origin.
3. In the gamma-ray region above 50 MeV, where the basic
pair-production processes provide an inherently low background,
much greater sensitivity (about an order of magnitude) will come
from increased area and improved efficiency and angular resolu-
tion. Angular resolution approaching 1 arcmin is also required to
achieve the desired point-source location. Detector systems un-
der development, including large, high-position-accuracy particle-
location chambers, combined with new large telescope designs,
should provide the desired improvements. Such an instrument
would search for faint objects and make detailed studies (includ-
ing time resolution) of galactic sources. It would also dramatically
extend the knowledge of high-energy phenomena of active galactic
nuclei. Extending the energy range upward (E > 10~t eV) will
be important in understanding the origin and acceleration of the
high-energy cosmic rays.
The approaches to these instruments seem feasible, but devel-
opment of the new generation of detectors must be funded at an
adequate level now to ensure that advanced instruments will be
available in the 1995 to 2015 era.
COSMIC-RAY RESEARCH
New programs in particle astrophysics will explore energy re-
gions far beyond those currently accessible en c! will probe the
particle population in our galaxy at greatly improved levels of
sensitivity and resolution. Specific goals for cosmic-ray research
are precise measurements of the isotopic abundances at energies
(~10 to 100 GeV/nucleon) that are well above the region of so-
lar modulation; measurements of the energy spectra and isotopic
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abundances of ultraheavy particles; determination of the spectra
of electrons, positrons, and antiprotons over a wide energy range;
sensitive searches for heavy antiparticles; and measurements of
the composition of extremely energetic particles (well beyond the
"bend" at 10~5 eV). Realizing these goals will enable us to test and
specify theoretical models on the acceleration of particles in our
galaxy and beyond, to analyze key evidence for the nucleosynthesis
of the elements, to study structure and composition of the inter-
stelIar medium, and to provide observational tests for cosmological
models. A large portion of the observational program is expected
to be centered around a magnet Spectrometer Facility (ASTRO-
MAG) that should be in orbit in the early 1990s. In addition, very
large detector arrays should be assembled in near-Earth orbit to
detect expectionally rare but important particle species. Polar-
orbiting platforms should serve for investigations at Tow energies.
A particular challenge are measurements on a deep-space probe
reaching interstellar space outside the heliosphere for detailed in
situ investigation of the interstellar medium.
Magnet Spectrometer for Particle Astrophysics (ASTROMAG)
ASTROMAG will be a superconducting magnet spectrome-
ter exhibiting field integrals of several teslameters over an area of
at least 1 m2, combined with large-area, trajectory-determining
devices with better than 100-,um resolution. Such an instrument
permits precise measurements of the rigidities of high-energy par-
ticles, far beyond the capabilities of conventional instrumentation.
This spectrometer is expected to be in earth orbit for a duration
of 10 to 20 years and many require occasional servicing. To per-
form specific astrophysical observations, the spectrometer must be
combined with dedicated particle-detector systems. For instance,
isotopic abundance measurements require an accurate velocity
measurement with Cerenkov counters, in addition to the rigid-
ity measurement by the magnet spectrometer. On the other hand,
the identification of singly charged particles (protons, antiprotons,
electrons, and positrons) must be accomplished by complementing
the spectrometer with transition radiation and shower detectors.
Not all observations can be performed simultaneously, but the
dedicated detector systems should be successively accommodated
and interchanged, like focal plane instruments on a telescope. The
following briefly describes some of the observational objectives:
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. Antiprotons and Antimatter Search: to search for antipar-
ticles in the cosmic rays, e.g., antiprotons, heavy antinuclei, and
positrons. Such observations relate to the fundamental question
of matter-antimatter symmetry of the universe, the m~ssing-mass
problem in cosmology, and the production of antiprotons and
positrons by interactions in interstellar space.
O Isotopic Composition of High-Energy Nuclei: to measure
the isotopic composition. This will address questions of nucleosyn-
thesis, origin of the elements, and evolution of the galaxy. It will
also address the problem of dating with radioactive isotopes, and
the study of interactions of cosmic rays with the interstellar ~as.
Energy Spectra:
~ _ ~ ~1 ~
~ -c~ O
to determine precise energy spectra of
cosmic rays over a large energy range in order to understand the
processes of particle acceleration on astrophysical scales and of the
confinement of cosmic rays to our galaxy.
Electrons and Positrons: to measure negative and positive
electrons up to energies of a few tesla electron volts. High-energy
electrons reaching the Earth cannot have traveled large galactic
distances; thus the details of their energy spectra will reveal in-
formation on the spatial distribution of acceleration sites in our
galaxy.
Interplanetary and InteretelIar Measurements
In situ measurements of low-energy cosmic rays will be per-
formed with several detectors on spacecraft or space probes at dif-
ferent locations throughout the heliosphere. These detectors will
use proven solid-state detector technology, or they may employ
new devices now in development that permit much larger detector
areas, with a corresponding increase in sensitivity. These instru-
ments will study the three particle populations in the heliosphere:
galactic cosmic rays, energetic particles of solar or planetary ori-
gin, and the anomalous cosmic-ray component. These phenomena
will be studied at detection levels that permit investigations of
very rare species, including ultraheavy particles, and with high-
mass resolution sufficient to identify rare isotopes (a related role
will be played by detectors on a polar platform; see the section
below on Experiments on Polar Platforms).
One or several space probes will reach nearby interstellar
space, outside the region of solar modulation. A dedicated in-
tersteliar probe will make it possible to pursue one of the most
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important and most challenging goals in the coming decades: in
situ measurements in interstellar space. Recent measurements on
the Pioneer and Voyager spacecraft have dramatically expanded
our understanding of the solar environment. The size of the helio-
sphere, estimated to be 5 to 10 AU before these missions, is now
estimated to be 50 to 100 AU in the ecliptic plane.
Large Detector Arrays in Space
Some of the most critical questions in particle astrophysics
will only be answered by the exposure of arrays of detectors larger
or more massive than those that can be carried on a single Shuttle
mission. These arrays will have to be assembled in space from
separate modules carried up individually. The following are exam-
ples:
· High-Energy Array: 10~5 to 10~6 eV. The deployment of an
exceptionally massive array is required in order to study cosmic-
ray particles in the energy range beyond 10~5 eV, where air-shower
data suggest a break in the energy spectrum. This break might
reflect the large-scale structure of the galaxy and the escape of
high-energy cosmic rays from the galactic magnetic fields, or it
may signify limitations in the galactic acceleration mechanism.
To answer these questions, precise measurements of the elemen-
tal abundances are necessary. This will require an array that is
large enough to detect a significant number of particles at these
high energies and is able to measure their energy. Only a large
calorimeter appears to meet these requirements. Such a calorime-
ter would have a mass of 60 tons and a diameter of 5 m, or even
more.
· Ultraheavy Nuclei: Z > 30. For the very rare ultraheavy
(UH) nuclei, individual elemental abundances will be determined
with the Heavy Nucleus Collector (HNC) (see Chapter 3), but
practically nothing will be known about their energy spectra. Only
by achieving major increases in the collecting power of the detec-
tors will it be possible to acquire the data needed. Such data
will not merely supplement what we already know but will add a
new dimension because of the different histories of nucleosynthesis
and propagation of these heavier elements. An array has been
proposed that is composed of a large number of relatively simple
detectors, arranged in a sphere with a diameter of 30 m and a
mass of 30 tons.
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Experiments on Polar Platforms
Exposure of large instruments in high-incTination orbit for ex-
tended periods of time is required to address several remaining
frontiers of cosmic-ray research. Examples include measurements
of cosmic-ray positrons and antiprotons from 0.1 to approximately
4 GeV and measurements of the isotopic composition of ultraheavy
nuclei (Z > 30~. The ultraheavy isotope measurements are con-
sidered in more detail below.
For those elements occurring beyond iron and nickel in the
periodic table, elemental composition measurements will provide
partial information on the nucleosynthesis, acceleration, and trans-
port of ultraheavy cosmic rays in the galaxy. Further details can
be deduced from the abundances of individual isotopes. Similarly,
studies of the time history of ultraheavy cosmic rays, from their
nucIeosynthesis and acceleration through propagation throughout
the galaxy, will require the determination of the abundances of
individual radioactive isotopes.
The investigation of the isotopic composition of the ultra-
heavy elements requires the achievement of good mass resolu-
tion in large-area detectors. It also requires the availability of
long-duration (several years) exposures of such instruments to the
galactic cosmic-ray flux. Typical detectors will require a very large
area (several square meters) and will most likely be restricted to
low energies (< 1 GeV/n). Because of the geomagnetic cutoff,
these measurements cannot be performed aboard a Space Sta-
tion in low-aTtitude/Iow-inclination orbit. The ideal vehicle would
be capable of getting out of the magnetosphere. However, since
the instruments to be flown will necessarily be large and heavy,
access to such an orbit may not easily be available. A satisfac-
tory alternative would be a near-Earth platform in a near-polar
orbit. Even at the lowest energies of interest for galactic cosmic-
ray studies (around 50 MeV/AMU), data will be collected over
approximately one third of each orbit.
Representative terms from entire chapter:
cosmic rays
