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Astronomy and Astrophysics in the New Millennium: Panel Reports 1 Report of the Panel on High-Energy Astrophysics from Space
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Astronomy and Astrophysics in the New Millennium: Panel Reports SUMMARY X rays and gamma rays are emitted by the hottest gases and the most energetic events in the universe. Because of their penetrating power, they enable us to see into regions that are inaccessible in other wave bands, and because of their energy, they probe matter under the most extreme conditions. They also allow us to see out to large distances, observing the universe when it was much younger than it is today. X rays and gamma rays can only be observed from space, so their use for astronomy is young compared with other wavelengths. Still, dramatic discoveries of cosmological gamma-ray bursts, magnetars, baryon-rich clusters of galaxies, iron lines from accretion disks, and microquasars have led to a better understanding of these energetic environments and have taken us closer to a number of long-range scientific quests: finding the first light of the modern universe, elucidating relativistic gravity by directly imaging black holes, and understanding the origin of the elements that are critical for forming planets and life. The technological capability is at hand to take the next steps toward these goals. Accordingly, the Panel on High-Energy Astrophysics from Space of the Astronomy and Astrophysics Survey Committee recommends a program for the coming decade that will require the building of three new telescopes: The Constellation-X Observatory (Con-X) is a major, high-spectral-resolution, broad-bandpass, x-ray spectroscopy mission. It is proposed as a launch of four telescopes on two rockets well away from Earth. Their combined sensitivity will improve upon that of existing and imminent x-ray missions by factors of 20 to 100, depending on wavelength. The top-priority, intermediate-class mission is the Gamma-ray Large Area Space Telescope (GLAST), which will use technology developed for particle physics experiments to detect high-energy gamma rays from quasars, pulsars, and gamma-ray bursts. The second-priority, intermediate-class mission is the Energetic X-ray Imaging Survey Telescope (EXIST), which will be attached to the International Space Station. It will monitor the whole sky at hard x-ray energies every 90 minutes. In addition, the panel proposes a prioritized, advanced technology program that will comprise three missions: the Microarcsecond X-ray
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Astronomy and Astrophysics in the New Millennium: Panel Reports Imaging Mission (MAXIM), Generation-X, and the MeV Spectroscopy Mission. Such a program would do three things: Lay the foundation for ultimately resolving black holes using x-ray interferometry in space. Develop the mirror and detector technology required to create a 10-m-diameter, focusing x-ray telescope that can detect emission from the first galaxies and stars in the universe. Develop instruments sensitive enough to perform extensive gamma-ray spectroscopy of the sites of element formation. The six missions are listed in Table 1.1. A healthy high-energy astrophysics program should also embody a balance between larger and smaller projects. Accordingly, the panel presents four unprioritized recommendations: Maintain the Explorer program, which offers timely opportunities for opening up fresh territory, including nuclear line spectroscopy, x-ray surveys to map the “missing baryons,” and continuous x-ray monitoring of the entire sky. Develop ultralong-duration ballooning, a cost-effective approach to hard x-ray and gamma-ray astronomy. Increase the investment in laboratory astrophysics so as to be ready to interpret the results anticipated from the observing program. TABLE 1.1 New Major and Intermediate Missions Considered in Chapter 1 Mission Specialty Recommendation Con-X X-ray spectroscopy 2008 launch EXIST Hard x-ray survey 2005 deployment on ISS Generation-X Large-aperture x-ray telescope Technology development GLAST Hard gamma-ray survey 2005 launch MAXIM X-ray interferometry Technology development MeV Spectroscopy Mission Gamma-ray spectroscopy Technology development
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Astronomy and Astrophysics in the New Millennium: Panel Reports Support focused theoretical challenges centered on the principal targets of observation. This would also enhance the scientific return from Con-X, GLAST, and EXIST. Finally, the panel advocates three unprioritized policy actions: Provide sustained support for data analysis groups. Support instrumentalists in junior faculty positions. Maintain the exemplary record of public outreach. A DECADE OF OPPORTUNITY Our universe is an astounding place, and we are privileged to be alive when its scope, contents, and history are being revealed. From the almost-perfect microwave background radiation to immense clusters of galaxies; from the first quasars to the sleeping, giant black holes that they leave behind; from dense hydrogen gas clouds, where stars and their scalding planets are discreetly born, to the life-giving elements that these stars spawn—we have discovered worlds more wondrous than our boldest prophecies and more subtle than our most careful predictions. This flow of enduring discovery has been sustained by applying ingenious technology to increasingly sensitive telescopes operating throughout the electromagnetic spectrum as well as by exploring the universe using cosmic rays, neutrinos, and—soon, it is hoped—gravitational radiation. The panel was charged with surveying x-ray and gamma-ray astronomy and recommending new initiatives for the coming decade at a particularly exciting time. As astronomers absorb the momentous discoveries of the U.S.-led Compton Gamma-Ray Observatory (CGRO) and Rossi X-ray Timing Explorer (RXTE), the Japanese-led spectroscopic satellite ASCA, the German-led low-energy survey satellite ROSAT, and the Italian-led broadband x-ray to gamma-ray mission Beppo-SAX, they are starting to make fundamental discoveries using the recently launched Chandra X-ray Observatory and the European-led X-ray Multi-Mirror Observatory (XMM-Newton). In addition, the European gamma-ray observatory INTEGRAL and the U.S. missions Hete-2 and Swift will also be launched and are expected to make major advances in gamma-ray astronomy. However, these current missions have nominal lifetimes of 5 years, and the scientific opportunity and technology are already in hand to go
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Astronomy and Astrophysics in the New Millennium: Panel Reports well beyond their capabilities. It is therefore imperative to plan now for their successors. To focus its conclusions, the panel has organized its report around three long-term scientific “quests”: To see the first light at the end of the universe’s dark age and comprehend our cosmic origin; To image black holes and elucidate relativistic gravity; and To understand the origin of the elements essential for forming Earth-like planets and life. These quests will probably take several decades and will involve observations over the whole electromagnetic spectrum. For this reason, the panel has selected 12 associated near- to mid-term challenges (items A through L in Table 1.2) that are specific to high-energy astrophysics and that could be met over the next 10 to 15 years. EMERGENCE OF STRUCTURE The central task in contemporary observational cosmology is to reconcile the ancient and the modern universe. By detecting tiny fluctuations in the microwave background radiation, astronomers expect that they will soon have comprehensive measurements of the minor irregularities in the expanding universe from a time when it was less than a million years old. These irregularities grew, under gravity, to form the structure that we see around us now. Measurements such as these will also allow us to estimate the size and shape of the ancient universe. Meanwhile, observations of nearby stars and galaxies reveal the size and shape of the modern universe as it ages from roughly 1 to 13 billion years. Although the full story is not yet in, there is confidence that, within a few years, it will be possible to link these two views, using cosmological theory, for a universe containing cold dark matter (CDM) and, perhaps, dark energy. This will give us a description of the overall expansion of the universe—the stage upon which great cosmic dramas are enacted. However, even if this endeavor is brilliantly successful, it will not tell us how, when, or where the first stars and galaxies formed. Indeed, we still do not know if the first luminous objects are stars in developing dwarf galaxies, as most theory predicts, stars in normal galaxies like our own, or accreting black holes in galactic nuclei. Although there has been impressive progress in recent years using optical observations of very distant galaxies and quasars, these observations are proving difficult to interpret,
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Astronomy and Astrophysics in the New Millennium: Panel Reports TABLE 1.2 High-Energy Astrophysics Challenges to 2015 Designation Challenge Sections in Which Discussed A Find and weigh the first clusters of galaxies Emergence of Structure, Hot Intergalactic Medium (Con-X), Generation-X B Detect local intergalactic gas and measure its density and temperature Emergence of Structure, Hot Intergalactic Medium (Con-X) C Observe the first generation of gamma-ray bursts, perhaps associated with the first massive stars Emergence of Structure, Cosmic Rays (GLAST), Gamma-ray Bursts (GLAST), Gamma-Ray Bursts (EXIST) D Find the first active galactic nuclei (AGN) Emergence of Structure, Cosmic Rays (GLAST), Obscured AGN and the X-ray Background (EXIST) E Form an indirect image of the flow of gas around a black hole Gravity Power, Black Holes and Neutron Stars (Con-X), Cosmic Rays (GLAST), Obscured AGN and the X-ray Background (EXIST), Galactic Survey (EXIST), MAXIM, All-Sky Monitors F Understand how jets are created and collimated Gravity Power, Black Holes and Neutron Stars (Con-X), Blazars (GLAST), Cosmic Rays (GLAST), Gamma-ray Bursts (EXIST), MAXIM, All-Sky Monitors G Measure accurately the variation of neutron star radii with mass Gravity Power, Black Holes and Neutron Stars (Con-X), Galactic Survey (EXIST) H Solve the mystery of gamma-ray bursts Gravity Power, Gamma-ray Bursts (GLAST), Gamma-ray Bursts (EXIST), All-Sky Monitors I Balance the cosmic energy budget of galaxies and their active nuclei Gravity Power, Black Holes and Neutron Stars (Con-X), Blazars (GLAST), Gamma-ray Bursts (GLAST), Obscured AGN and the X-ray Background (EXIST), Generation-X
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Astronomy and Astrophysics in the New Millennium: Panel Reports Designation Challenge Sections in Which Discussed J Use x-ray and gamma-ray observations to associate evolving stars with their post-supernova remnants and the elements they form Origin of the Elements, Nucleosynthesis (Con-X), Galactic Survey (EXIST), Gamma-ray Bursts (EXIST), MeV Spectroscopy Mission, Nuclear Line X-ray Spectroscopy K Determine accurately the relative abundances and distribution in the interstellar medium of the 20 most common elements Origin of the Elements, Nucleosynthesis (Con-X) L Understand the cosmic history of element production and dispersal Origin of the Elements, Hot Intergalactic Medium (Con-X), Nucleosynthesis (Con-X), MeV Spectroscopy Mission, Nuclear Line X-ray Spectroscopy, Soft X-ray Surveys for three reasons. First, we do not understand how the distribution of luminous galaxies relates to that of the dark matter. Second, the first stars create heavy elements that quickly condense into dust grains and lead to variable and uncertain obscuration of optical light. Third, the formation of galaxies involves much complex physics that is difficult to quantify. As a result, future progress is believed to depend on observations at both longer (infrared) and shorter (x-ray) wavelengths. The practical approach to the first light quest is to investigate how structure emerges on all scales. The pivotal discovery—that the largest collections of galaxies, called “clusters,” are luminous x-ray sources— gave us a probe of large-scale structure that avoids all three of the above problems. This is because the penetrating x-ray photons allow us to measure the mass of both the gas and the dark matter and because the formation of clusters is believed to be simpler than the formation of individual galaxies. X-ray observations of local clusters of galaxies provided the first indication—and, in many respects, the strongest argument yet—that the universe contains too little dark matter to arrest its expansion. A central tenet of the CDM theory is that large structures were formed from the merging of smaller structures. In other words, clusters of galaxies should be relatively young. Some support for this view comes
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Astronomy and Astrophysics in the New Millennium: Panel Reports from the observation that many local clusters appear to comprise colliding subclusters, which may be visible by virtue of the strong x-ray-emitting shock waves that they develop. However, the similarity of distant x-ray clusters to those that we see around us now and the discovery of an apparently very dense x-ray cluster, which must have formed when the universe was less than 8 billion years old, suggest that much of the large-scale structure was in place earlier than had once been thought. To understand what really happened, we need to know the size of these young clusters, when they were formed, and how they themselves congregated. These considerations naturally motivate our first challenge, namely to find and weigh the first clusters of galaxies (A). Another way to understand the development of structure is to find the gas that does not condense into stars, galaxies, and clusters. Optical astronomers have probably found much of this gas—at epochs when the universe was only a few billion years old and temperatures were around 30,000 K—through its absorption of quasar light. However, they also know that most of this gas is no longer in this form and that it is necessary to understand what has become of it. We already know, from microwave background observations, that its current temperature must be less than 30 million K and that the recently launched Far Ultraviolet Spectroscopic Explorer (FUSE) will detect any gas with a temperature around 300,000 K. However, numerical simulations suggest that the temperature ought to be closer to a few million kelvin, which is well suited to x-ray observation. Therefore, in order to describe most of the matter in the expanding universe, we need to detect local intergalactic gas and measure its density and temperature (B). A more direct approach to finding the first light of the universe comes from observing gamma-ray bursts. These are now known to be ultraluminous explosions, probably associated with massive stars and already seen from when the universe was only 2 billion years old. They should be visible from much earlier times and may turn out to be a signature of the very first stars. If so, we should be able to use observations of gamma-ray bursts to study the history of formation of these stars and to determine whether or not they are localized in normal galaxies. Consequently, we desire to observe the first generation of gamma-ray bursts, perhaps associated with the first massive stars (C). Like supernovae, gamma-ray bursts can serve as invaluable cosmological probes even if we are unsure how they work. We now know that most normal galaxies contain nuclei that, although mostly dormant now, were very active in the past. The most
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Astronomy and Astrophysics in the New Millennium: Panel Reports luminous of these are the quasars, and they can be recognized using x-ray and gamma-ray observations. Quasars have already been seen from when the universe was only a billion years old and should be observable from much earlier epochs. There is no sign yet from the x-ray observations (in contrast to the optical searches) that we are seeing the onset of quasar activity. It is even possible that quasars formed before stars. X-ray and gamma-ray observations are particularly important for finding distant quasars because they are much less susceptible to absorption than optical emission. The final challenge associated with the first quest is, then, to find the first active galactic nuclei (AGN) (D). GRAVITY POWER The study of black holes began as a theoretical consequence of Einstein’s General Theory of Relativity more than 80 years ago. During the past decade, the evidence that they exist in abundance—in the nuclei of most normal galaxies as massive (a million to a few billion solar masses) black holes and as 5- to 30-solar-mass products of stellar evolution in close binary systems—has become overwhelming, and their masses have been confidently measured in roughly 10 cases. (More recently, there have also been reports that black holes with masses several thousand times that of the Sun may have been detected, and these might have a cosmological origin.) An astrophysical black hole is described by just two parameters, a mass (which also measures its size) and a rate of rotation. Black holes have to be observed indirectly through their effects on nearby matter. As relativistic objects, they accelerate gas near their surfaces to speeds close to that of light and so can convert mass into radiant energy with an efficiency a hundred times greater than nuclear reactions. This happens in quasars, which sometimes outshine their host galaxies by a factor as large as 1000 to 10,000. It also happens in the nuclei of “normal” galaxies like our own, which contains a 2.6-million-solar-mass black hole (Figure 1.1). And it happens in binary stars, where the gas swirling around the black hole can be much brighter than the regular stellar companion. Indeed, far from being seen as an end point, the formation of a stellar black hole is now regarded as a beginning—the start of a new phase when it can convert matter into radiant energy with far greater efficiency than was possible for it as a star. We are deeply curious about how they function. This is our second quest. Black holes were predicted to swallow their gas via accretion disks,
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Astronomy and Astrophysics in the New Millennium: Panel Reports FIGURE 1.1 Chandra x-ray image of the galactic center. Infrared observations have demonstrated that there is a 2.6-million-solar-mass black hole at the dynamical center of our galaxy. This massive black hole accretes gas from its surroundings and heats it to x-ray-emitting temperatures. It is spatially coincident with the x-ray point source at the center of this image. The intensity of this source is surprisingly small. The other sources in the image are mostly associated with gas and stars near the black hole. Courtesy of NASA/Massachusetts Institute of Technology/Pennsylvania State University (PSU).
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Astronomy and Astrophysics in the New Millennium: Panel Reports which orbit the hole rather like the rings that encircle Saturn (Figure 1.2). The existence of disks has been substantiated by several observations, most notably the measurement, by ASCA, of relativistically broadened iron line profiles (combined with narrow absorption features) from nearby galactic nuclei. These measurements have also been used to argue that the black holes must be spinning very rapidly. The gas in accretion disks is believed to sink toward the central hole and be heated by the frictional effect of a magnetic field. This magnetic field can also sustain an active corona rather like that observed around the Sun and from which extremely energetic x-ray photons are produced. Roughly half of the coronal radiation is reprocessed by the accretion disk, and much of the x-ray spectrum of nearby Seyfert galaxies (which resemble low-power quasars) has been interpreted in this manner. However, the arrangement and thermal states of the absorbing and the emitting gas are unknown. A new diagnostic of the hole-disk-corona connection has been studied in great detail in accreting stellar black holes using RXTE. It has been found that these sources often exhibit quasi-periodic oscillations (QPOs), which are probably derived from the excitations of waves in the accretion disk. Interestingly, the x-ray photons are so energetic that they must have been created in the corona. Some QPOs are associated with bursts that happen on neutron star surfaces. These provide good measures of the (high) spin frequencies of the neutron stars and test our understanding of their properties. To understand how gas accretes onto a black hole, we must use observations from binary stars and AGN and combine these with laboratory atomic astrophysics investigations and three-dimensional, numerical magnetohydrodynamical simulations. In essence, what we are trying to do is to form an indirect image of the flow of gas around a black hole (E), in much the same way that a geophysicist might analyze seismic waves, gravity data, surface geology, and so on to create an image of Earth’s interior. The x-ray counterparts of these diagnostics include the variable iron line profiles and the QPOs, which ought to be characteristic of the mass and spin of the hole and the rate and manner by which gas is supplied to it. In addition to disks, many accreting black holes form a pair of jets that appear to flow with speeds close to that of light along opposite directions that are perpendicular to the disk (Figure 1.3). These jets were first found associated with galactic nuclei using radio astronomy. However, they have also been seen emanating from stellar black holes and
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Astronomy and Astrophysics in the New Millennium: Panel Reports of it), so these gamma-ray observations can monitor the history of star formation in the universe. An understanding of the absorption on the infrared background is also needed to measure the intrinsic high-energy spectrum of gamma-ray bursts. SECOND-PRIORITY PROPOSED INTERMEDIATE MISSION: EXIST MISSION DEFINITION An imaging survey of the hard x-ray sky is our second candidate for a moderate mission. There has been no hard x-ray survey of the whole sky to match the existing ROSAT soft x-ray survey since that performed by the High-Energy Astronomical Observatory (HEAO-1) satellite. The International Space Station-attached EXIST9 would carry out this survey using eight wide-field (~40 deg), coded-aperture, hard x-ray (~5 to 600 keV) telescopes with good energy resolution (E/ΔE~100) that image the whole sky every 90-min orbit. (This is a particularly important feature because the hard x-ray sky is so variable and is enabled by having eight telescopes.) The final survey limit would be roughly 100 to 1000 times fainter than the HEAO-1 limit and roughly 10 times fainter than the anticipated Swift hard x-ray survey (with a much broader energy range and superior angular resolution). EXIST would provide ~30 arcsec source localization for bright sources and angular resolution of ~5 arcmin for bright extended sources as well as ~1 µs photon timing. The International Space Station provides a nearly ideal platform for this fixed-pointing, scanning telescope (although it could also be a free flyer). A single telescope could be prototyped on an ultralong-duration balloon flight. This project is enabled by recent advances in hard x-ray Cd-Zn-Te detectors (which are also used for medical imaging). The most difficult technology challenge is to construct ~1m2 arrays of these detectors for each telescope. Other challenges concern data handling and integration with the International Space Station. EXIST was selected as a New Mission Concept in 1994, and the proposal has developed considerably since that time. An EXIST science 9 See <http://exist.gsfc.nasa.gov>.
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Astronomy and Astrophysics in the New Millennium: Panel Reports working group has been formed. Because of its strong synergy with Con-X and GLAST, a 2005 launch would be optimal. The projected phase C/D cost is $120 million, and the MO&DA for a 2-year mission is $30 million, for a combined estimated cost of $150 million. OBSCURED AGN AND THE X-RAY BACKGROUND (D, E, I) Perhaps the most compelling survey science is the first deep survey of AGN above ~20 keV. There are already strong indications from ASCA and Beppo-SAX that many AGN are heavily obscured. EXIST should discover more than 3000 such self-absorbed AGN, which can then be subjected to deep follow-up study. (These sources are probably quite variable, and EXIST is well suited to monitor them.) This will be necessary to understand the source composition of the hard x-ray background (at around 40 keV, where it is most luminous). In addition, EXIST observations will facilitate a secure calculation of the contribution of accreting black holes in AGN to the “luminosity density” of the universe and a direct comparison with the galaxy luminosity density that should be measured by the Next Generation Space Telescope (NGST). Further comparison with the distribution of black hole masses in dormant galactic nuclei will lead to a quantitative understanding of the evolution of black holes in different types of galaxy. GALACTIC SURVEY (E, G, J) With its very large field of view, EXIST is well suited to detect the “soft x-ray transients” that are usually associated with black holes. Studying these objects and understanding why they behave as they do will lead to a census of stellar mass black holes within our galaxy. In addition, its good energy resolution should allow EXIST to perform the first high-sensitivity, high-energy galactic search for supernovae that are hidden inside molecular hydrogen clouds by seeking gamma-ray emission lines from radioactive titanium. If these lines are seen, they should help us to understand the overall supernova rate and relate this to the theory of advanced stellar evolution. Sensitive, hard x-ray observations of accreting neutron stars should measure their magnetic fields. In addition, it will be possible to observe QPOs from the disk coronas around neutron stars and stellar black holes at the high energies where they are most prominent.
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Astronomy and Astrophysics in the New Millennium: Panel Reports GAMMA-RAY BURSTS (C, F, H, J) EXIST is also well matched to the study of GRBs. With a projected sensitivity 20 times better than CGRO and 4 times better than Swift, EXIST should detect GRBs every 4 hours and furnish arcminute positions for all and ~10 arcsec positions for the brightest cases. If there is a large population of low-power GRBs associated with supernovae, like the recent example 1998bw, then EXIST should find them. It also has the sensitivity to detect bursts from when the universe was less than a billion years old, and this will provide a direct probe of early star and galaxy formation. EXIST can time tag each photon with microsecond accuracy, which will be a useful diagnostic tool for exploring the kinematics of the expanding, ultrarelativistic blast waves and jets. INVESTING FOR THE FUTURE The missions that have just been described are large steps towards the long-term goals with which this report began. To go further requires investing in selected technologies that will be needed by missions that could be launched after 2010. The panel has identified three important areas where there has recently been considerable progress and where the prospects for future advances seem particularly good. In order of priority, they are MAXIM, Generation-X, and the MeV Spectroscopy Mission. MAXIM (E, F) The second quest, imaging a black hole, could succeed, in principle, using x-ray interferometry, as proposed for MAXIM10 (Figure 1.13). Success would require ~0.1 µarcsec angular resolution, a seven-order-of-magnitude improvement over Chandra. At first sight, this seems unrealistically ambitious. However, new interferometer designs, using grazing incidence mirrors, suggest a clever way of using widely separated spacecraft to form an interferometer in a manner that will combine photons with different energies and accommodate source variability. One spacecraft holds the mirrors, which are separated by a fixed 10 See <http://maxim.gsfc.nasa.gov/>.
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Astronomy and Astrophysics in the New Millennium: Panel Reports FIGURE 1.13 In the MAXIM approach to x-ray interferometry, beams of x rays separated by several meters are formed by the “optics spacecraft.” The x-ray photons in these beams are then detected by a second “detector spacecraft” roughly 500 to 1000 km away. Ultimately it is proposed to use x-ray interferometry to form images of gas flow around a black hole. Courtesy of NASA/ MAXIM/GSFC. “baseline” as large as ~100 m. These mirrors reflect the x rays onto a second spacecraft up to 1000 km away, where the interference fringes are formed. One possible intermediate goal is a pathfinder mission designed to demonstrate ~100 µarcsec resolution at ~1 keV, comparable to what is achieved at radio wavelengths using very long baseline interferometry (VLBI). In order to have an adequate flux of x-ray photons, a collecting area of 100 cm2 and a baseline of ~2 m are required. The detector would have to be ~500 km away. The biggest technology challenges for
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Astronomy and Astrophysics in the New Millennium: Panel Reports such a pathfinder mission include developing x-ray optics based on large optical flats and fabricating large, two-dimensional cryogenic detector arrays with sufficient energy resolution. The pointing (~30 µarcsec) and metrology (~0.1 nm) demands are similar in character to those of the Space Interferometry Mission (SIM) and the Laser Interferometer Space Antenna (LISA), and much of the technology should be transferable. GENERATION-X (A, I) Attaining the first goal of seeing directly the very faint first galaxies and stars requires much larger collecting areas to capture enough photons to form an image or attempt spectroscopy; x-ray detectors are already approaching 100 percent efficiency. Furthermore, to make the identifications, it will be necessary to localize sources to within an arcsecond. These requirements motivate the fabrication and deployment of x-ray mirrors with effective areas exceeding ~100 m2, a hundred times larger than Con-X and comparable to a single Keck telescope. Large-format detectors capable of sub-eV energy resolution will also be necessary. A paced program of mirror and detector technology development directed toward these long-term goals is recommended. MEV SPECTROSCOPY MISSION (J, L) As discussed above, the measurement of nuclear gamma-ray lines provides a direct probe of the formation of the elements in supernovae. Most of the lines have MeV energies, and measuring them poses an unusual instrumental challenge because it is not possible to use focusing optics. The INTEGRAL mission will be an important pathfinder for this field. It appears that in order to attain its scientific goals, it will be necessary to surpass INTEGRAL in sensitivity by a factor of roughly 30. The panel recommends support for technology leading to a future mission with sensitivity matched to our scientific goals for reasonable cost. SMALLER PROGRAMS A healthy high-energy astrophysics program must embody a balance between larger and smaller projects. Accordingly, the panel also endorses four smaller programs.
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Astronomy and Astrophysics in the New Millennium: Panel Reports POTENTIAL EXPLORER RESEARCH There is strong support in the high-energy astrophysics community for the Explorer program, as demonstrated by the large number of exciting proposals submitted by high-energy astrophysicists. As the central purpose of this program is to encourage innovation and rapid responses to emerging scientific opportunities, the panel does not endorse any specific mission proposals. However, three exciting research areas appear to be particularly well suited to Explorer missions. NUCLEAR LINE X-RAY SPECTROSCOPY (J, L) Recent advances in multilayer coatings used at grazing incidence make it possible to concentrate soft gamma-rays onto a small detector, greatly reducing the background and allowing unprecedented sensitivity with a much more modest instrument than previously imagined. Several important nuclear lines occur in the accessible energy range, below ~200 keV, including three isotopes of Ti, Co, and Ni that are stringent diagnostics of the explosion mechanism for type II supernovae. Many supernovae could be observed in this manner, providing unique diagnostics of how elements are created and disseminated. SOFT X-RAY SURVEYS (L) A systematic low-energy survey of a substantial area of sky to depths significantly beyond that investigated by the all-sky survey ROSAT could map distant clusters and the hot structures surrounding them to probe the missing baryons. Spectroscopic surveys of diffuse emission offer another approach to observing the intergalactic medium that would complement absorption line measurements by the large observatory missions, and they could also help determine the nature of the hot interstellar medium in our own galaxy. ALL-SKY MONITORS (E, F, H) Soft x-ray transients, microquasars, and AGN are all highly variable. Continuous monitoring of large numbers of these sources at low and intermediate x-ray energies will complement the high-energy studies proposed above using EXIST and help produce an indirect image of accretion disks and their coronas. Continuous monitoring of a large
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Astronomy and Astrophysics in the New Millennium: Panel Reports fraction of the sky should also discover new and important rare phenomena. “Lobster eye” optics promises an order of magnitude gain in sensitivity, enabling thousands of AGN, binary x-ray sources, variable stars, and so on to be monitored simultaneously. ULTRALONG-DURATION BALLOONING There is a good history of performing high-energy astrophysics from balloons. A particularly fine example was the rapid response to supernova 1987a, which exploded in the Large Magellanic Cloud in 1987. Hard x-ray and gamma-ray missions were successfully mounted from the Southern Hemisphere and were able to confirm directly the production of radioactive nuclei within the expanding supernova remnant. Since that time the capabilities of balloons have increased considerably. The ultralong-duration balloon (ULDB) program offers the prospect of carrying several ton payloads to ~40 km altitudes for flights of several months’ duration. The panel recognizes that this capability offers relatively cheap access to near space for certain classes of hard x-ray and gamma-ray payloads and urges that this capability be developed further. It is noted that ballooning is particularly well matched to the needs of the best younger scientists, who require experience in building and flying instruments with relatively rapid turnaround. Additional support of roughly $5 million per year is needed for the development and operation of the balloons. LABORATORY ASTROPHYSICS High-energy astrophysics missions will return spectroscopic data over the next decade with unprecedented breadth and detail. This will take us into virgin territory largely unexplored by theoretical chemists and experimental astrophysicists. In addition, the laboratory study of high-energy-density fluid dynamics, magnetohydrodynamics, and plasma physics appropriate to the interpretation of supernovae, jets, and GRBs is in its infancy. There is need for an increased, though still comparatively small, investment (the panel estimates roughly $2 million per year for high-energy astrophysical studies) in both computational modeling and experimental facilities in order to make the best scientific use of the new observatories. A cross-agency initiative involving the Department of Energy (DOE), NASA, and the National Science Foundation (NSF) is recommended.
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Astronomy and Astrophysics in the New Millennium: Panel Reports THEORETICAL CHALLENGES The contributions of theory to the development of high-energy astrophysics are legion. Black holes, neutron stars, cosmological GRBs and relativistic blast waves, supernova nucleosynthesis, gamma-ray jets, cosmic-ray acceleration in supernova remnants, and the hot intergalactic medium were all widely discussed in the theoretical literature before observations established their reality. However, what was found was not usually exactly what had been predicted, so the theory had to be modified. To continue this symbiotic relationship, the panel proposes that each of its three recommended missions sponsor a particular theory challenge (see further discussion in Chapter 6) in order to refine mission planning and to obtain the best scientific return. The expenditure on this program should be matched to the perceived benefit, which may vary from mission to mission. Candidate challenges for Con-X, GLAST, and EXIST are those designated E, F, and G, respectively, and elaborated upon in Chapter 6. POLICY ISSUES The panel identified three policy issues for which it makes focused recommendations. LONG-TERM SCIENTIFIC SUPPORT FOR OBSERVERS The NASA data support centers serve the critical function of writing and maintaining the data analysis software needed for the dissemination, interpretation, and archiving of high-energy data. The concomitant large pool of scientists creates a critical mass for broad scientific investigations. The role of academic researchers in high-energy astrophysics has historically been to propose single observations, and they receive incremental funding to carry out the specific observations at hand. However, the lack of long-term, stable funding makes it difficult to create critical masses of researchers in universities similar to those in other countries, and it is also hindering the training of graduate students, a potential loss for the field. Accordingly, the panel recommends that NASA invest in a small number of focused, high-energy astrophysics data analysis groups within universities and research institutes. In particular, computing facilities and an
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Astronomy and Astrophysics in the New Millennium: Panel Reports appropriate mix of graduate students and postdoctoral scientists needs to be supported. JUNIOR FACULTY INSTRUMENTATION PROGRAM Space missions are becoming increasingly complex and their organization concentrated in a few centers. This has the unfortunate side effect that instrument and spacecraft development is becoming remote from most universities, in particular from graduate students, who are eager to enter the field at a time when there are great opportunities and a need for new blood. As evidence for this, the panel cites the apparent shortage of U.S.-trained postdoctoral scientists with experience of high-energy instrumentation. To alleviate this problem, the panel proposes that NASA initiate a modest program specifically directed at supporting a small number of carefully selected junior faculty working in high-energy instrumentation. EDUCATION AND PUBLIC OUTREACH In recent years, high-energy astrophysicists have established an enviable record for responding to a strong public interest in their discipline. Black holes, supernovae, and gamma-ray bursts have captured the public imagination like few other topics in the physical sciences and are at least as firmly established as cosmology and the search for extraterrestrial life. High-energy astrophysicists have experimented successfully with a variety of new education and outreach initiatives, including the High-Energy Astrophysics Learning Center and the Astronomy Picture of the Day. Individual missions such as Chandra, Swift, GLAST, and Con-X maintain interactive Web sites and have distributed compact disks and brochures widely. In recent months the early-release images from Chandra have been broadly accessed and disseminated in the news media. The panel recognizes the importance of these outreach activities and recommends that they continue to be encouraged, financially supported, and rewarded in a manner that is described in more detail in the survey committee report.
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Astronomy and Astrophysics in the New Millennium: Panel Reports ACRONYMS AND ABBREVIATIONS ACIS —Advanced X-ray Astrophysics Facility (now Chandra) CCD Imaging Spectrometer AGN —active galactic nuclei ASCA —Advanced Satellite for Cosmology and Astrophysics mission (Japan) ASTRO-E —fifth in a series of Japanese x-ray astronomy satellites; with help from the United States AURA —Association of Universities for Research in Astronomy, Inc. Beppo-SAX —Satellite per Astronomia X, a European collaboration x-ray mission CCD —charge-coupled device CDM —cold dark matter CGRO —Compton Gamma-Ray Observatory Chandra —Chandra X-ray Observatory (NASA, launched in 1999) CXC —Chandra X-ray Center at the Smithsonian Astrophysical Observatory DOE —Department of Energy EGRET —Energetic Gamma Ray Experiment aboard CGRO EPIC —European Photon Imaging Camera (on XMM-Newton) ESA —European Space Agency ESO —European Southern Observatory EXIST —Energetic X-ray Imaging Survey Telescope FUSE —Far Ultraviolet Spectroscopic Explorer GLAST —Gamma-ray Large Area Space Telescope GRBs —gamma-ray bursts HDF —Hubble Deep Field HEAO-1 —High-Energy Astronomical Observatory HETE-2 —High-Energy Transient Explorer (launched in 2000) HST —Hubble Space Telescope INTEGRAL —International Gamma-Ray Astrophysics Laboratory ISS —International Space Station LISA —Laser Interferometer Space Antenna MAXIM —Microarcsecond X-ray Imaging Mission MO&DA —mission operations and data analysis (NASA) NASA —National Aeronautics and Space Administration NGST —Next Generation Space Telescope NSF —National Science Foundation OSS —Office of Space Science (NASA)
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Astronomy and Astrophysics in the New Millennium: Panel Reports QPOs —quasi-periodic oscillations, x-ray pulses from compact objects ROSAT —Roentgen Satellite (German-U.S.-U.K. collaboration) RXTE —Rossi X-ray Timing Explorer SAO —Smithsonian Astrophysical Observatory SIM —Space Interferometry Mission STScI —Space Telescope Science Institute ULDB —ultralong-duration balloon VERITAS —Very Energetic Radiation Imaging Telescope Array System VLBI —very long baseline interferometry XMM-Newton —X-ray Multi-Mirror Observatory, a European collaboration x-ray space mission
Representative terms from entire chapter: