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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
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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
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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
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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
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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
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41 10 9_ 8 lo] ~ 6 in ~ 5 - o J 4- 3 2 O -3C273 -VIRGO (M87) CEN A 106Mo BH IN MW Nuc. _ URIC SMC '2','~.. . :; BLACK.;.. :;: o' %/..' `0~ .\\\~\A~ \~] ~) ~\~UKU"U] As' LINENS i^REG0N' SUPEF Nova ... AT; '100' .; DAYS ,.,. . ~ MW Nut. ;N\~oA\\~\N\~ ~ MAIN: ; ~SE- . L QUENCH N .,,,T.,ii' . ,., -. WHITE DWARF C A; :'.: L: ~ . \,1,j, ~- ~> ~ ~- out STARS 8 10 12 A: As, EGIONS Air ~ -` ;. .' `# ~Co`, `'SN 1987a ,` (2 DAYS) I I 1 1 14 16 18 20 LOG DIMENSION ( cm ) 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.
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42 ,131 Dis tent DIs tent Virgo Coma 3C273 Galaxies 1IS()'s ~ " "' 1 " " 1 1 1 1 1 1 "' ~ ~ 1 ~ ~ I' ~ 1 ~ I,,, . I,,,,,, . 1 .,, 1 1 2 NARRON L I NE RE G. I ON ~B ROAD L I NE RED 1 ON H.. ~ JO H_ TtIICK DISr ~4 ._ qO 1/2 / / / / / / / / _ / / / / / / / / / /ACCRETION / / / / / / / / ~ ~ / //// = /~//////// / / / / / / / /~ / / / / / / / / / / / / / / / /~;'c~// / / / // / / // // /// / /~/ //// /////////// it, , 1 , , , , 1 1 1 1 1 1 , , , , 1 , , , , 1 , , , , 1 , , , , 1 , , , , 1 , , , , 1 -4 -3.5 -3 -2.5 -2 -1.5 -1 -.5 0 .5 -2 103AU 1 AU ~ ~ / - / / / / / ' so / / / / / / / / / / //~ / / / / / / / / ~ / / / / / DISItS / //// //////// _4 // / _ / / , , Log z 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.
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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
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44 ,1 1~ ~ _ I: Y it/ It \\ l ,:~41 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.
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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
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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.
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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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60 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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61 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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62 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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63 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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64 . 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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65 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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66 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: