| Stephen S. Murray | Chairperson - Smithsonian Astrophysical Observatory |
| Alexander Brown | University of Colorado |
| Webster C.Cash | University of Colorado |
| Deepto Chakrabarty | Massachusetts Institute of Technology |
| William R. Forman | Smithsonian Astrophysical Observatory |
| Gordon Garmire | Pennsylvania State University |
| Fiona Harrison | California Institute of Technology |
| Frederick K. Lamb | University of Illinois |
| Richard Mushotzky | Goddard Space Flight Center |
| Robert Petre | Goddard Space Flight Center |
| Wilton T. Sanders | University of Wisconsin |
| Melville P. Ulmer | Northwestern University |
| Frederick M. Walter | State University of New York |
| Martin C. Weisskopf | Marshall Space Flight Center |
The Working Group endorses and recommends a new start for the Constellation-X mission in 2005. To meet this goal, we recommend that the necessary advanced technology development be funded now, and continued at the level needed to assure the new start date.
The need to strengthen the underlying infrastructure of Mission Operations, Data Analysis, Supporting Research and Technology, Theory, Laboratory Astrophysics, and Facilities is identified as a area of concern, and the XAPWG suggests some constructive changes to improve their state. We particularly single out the success of NASA's ADP and LTSA programs that encourage multi-wavelength studies and provide support for many bright young researchers and recommend that these opportunities be expanded.
We view Explorer Missions as an extremely important component of a balanced program in X-ray astronomy. A review of recent missions submitted in all categories of Explorers (UNEX, SMEX, and MIDEX) demonstrates the creativity of the community in utilizing this opportunity for carrying out first rate science investigations. The Working Group recommends a significant increase to the Explorer budget over the next few years to allow more access to space on short time scales.
In the mid-term and beyond time frames, XAPWG'99 sets forth in this report a Grand Challenge that culminates in the ability to image the event horizon of a black hole, and to detect the first X-ray emitting objects of the early Universe. These goals require great advances in technology, most likely centered around X-ray interferometry for very high resolution imaging, a new breed of high quality, light-weight optics to provide large collecting areas necessary for high sensitivity observations of distant and faint sources, and a new generation of detectors to take advantage of these optics.
We present a road map for the discipline of X-ray astronomy that encompasses the plans described in this report. We stress the importance of beginning now to explore the techniques that will be needed to accomplish this Grand Challenge, and the valuable contributions to the field that can be made through a series of missions (large and small) as we march towards that goal.
This report examines the current state of X-ray Astronomy as we
prepare to enter the first decade of the 21-st century. In 1994, the
X-Ray Astronomy Program Working Group published a ``15-Year Plan for
X-Ray Astronomy'' that was to serve as a roadmap for this same time
period. We review the progress that has been made since the 1994
Report, and update that report to reflect recent changes in scientific
emphasis and technology.
In October 1998, NASA requested that the Ad Hoc X-ray Astronomy Program Working Group (XAPWG'99) reconvene in order to update its 1994 report. The Working Group was asked to formulate plans in X-ray Astronomy for three time frames: 2003-2007 (near-term of the next NASA Strategic Plan); 2008-2013 (mid-term); and 2014 and beyond. The XAPWG'99 is submitting this report to the Structure and Evolution of the Universe Subcommittee (SEUS) of the Space Science Advisory Committee (SScAC) and also to Dr. Alan Bunner, SEU Theme Director at NASA.
The original 1994 membership was revised somewhat, and is given in Table 1. XAPWG'99 met three times (Austin, TX January 1999; Greenbelt, MD March 1999; and Charleston, SC April 1999) and held one open meeting for community input at the January AAS meeting in Austin, TX. In addition, a World Wide Web site (http://www-hea.harvard.edu/XAPWG/) was created to provide community access to the draft report and facilitate comments and discussion. A number of comments and suggestions for topics to be included in the report were received via this public access conduit, which also included e-mail. The committee is very appreciative of the community involvement in this report and hopes that there will be continued dialogue resulting from this final report.
| S.Murray | A.Brown | W.Cash |
| SAO-Chair | U. of Colorado | U. of Colorado |
| D.Chakrabarty | W.Forman | G.Garmire |
| MIT | SAO | PSU |
| F.Harrison | F.Lamb | R.Mushotzky |
| CalTech | U. of Illinois | GSFC |
| R.Petre | W.Sanders | M.Ulmer |
| GSFC | U. of Wisconsin | Northwestern |
| F.Walter | M.Weisskopf | A.Bunner |
| SUNY | MSFC | NASA HQ |
Continued progress in astronomy and astrophysics requires systematic planning for new observations from space in the kilovolt X-ray regime. Such observations are indispensable for the study of leading problems in astrophysics, including large-scale structure of the Universe; galaxy formation and evolution; the interaction between stars and the interstellar medium; the structure of accretion disks surrounding collapsed objects, the physics of neutron stars and black holes; and the chemical evolution of the Universe. Below we discuss some key areas where X-ray observations play a particularly significant or unique role, keeping in mind that these are not an exhaustive nor complete set.
Fundamental questions regarding the chemical composition of the Universe include how elements are synthesized; how the elements are distributed and recycled; and how these processes have evolved over cosmic time. X-ray observations are particularly well suited to address these questions.
Frequently, X-ray emission is the primary radiation from an astrophysical system. For example, in clusters of galaxies, the bulk of the luminous baryonic mass is in the form of hot gas visible only in X-rays, in early type galaxies the ISM is predominantly in a hot X-ray emitting phase, and in young supernova remnants, the shock heated interstellar medium and reverse shock heated supernova material are most easily detected in X-rays. Since the X-ray band is rich in atomic transitions from nearly all charge states of cosmic metals, high resolution spectroscopy can be used to measure abundances in these systems. In many cases the emitting plasmas are optically thin simplifying the spectroscopic data interpretation. In addition, X-rays are often only weakly absorbed by intervening material between a source and the observer. The details of that absorption are included in the spectrum so that correcting for this absorption is possible with good accuracy. Thus, it is possible to perform detailed plasma diagnostics to measure density, ionization state, and other physical parameters at the source of emission. With spatially resolved spectroscopy abundance gradients in clusters of galaxies, individual galaxies, and SNRs can be measured, mapping the flow of enriched material as it mixes with its environment.
At higher X-ray energies nuclear transitions from recently synthesized elements are detectable, thus permitting the identification of the sources of this material. In particular, for supernovae and their remnants, high energy X-ray imaging and spectroscopic observations directly measure the synthesis and distribution of heavy elements.
Stellar coronae and winds are examples of astrophysical systems where the X-ray emission is critical to understanding the processes whereby enriched material is mixed into the interstellar medium. X-ray spectroscopy of these coronae and stellar winds directly maps the flow of material from a star into its neighborhood. Observations of the interstellar medium, both hot and cold phases, can be made in X-ray emission and absorption.
Temperature maps of the ISM along with elemental abundance distributions will shed light on the processes that mix enriched stellar material, transport it, and maintain the energy balance of stellar neighborhoods. In the case of clusters of galaxies, groups of galaxies, and early type (elliptical) individual galaxies the hot X-ray emitting gas provides a fossil record of the star formation history and, therefore, chemical evolution that can be traced over cosmic time.
Fundamental issues include how baryons are distributed in the universe and their relation to the dark matter; how does the distribution of dark matter change with cosmic time, and how does this test physical models of structure formation and evolution; and what is the relationship between the formation of stars and galaxies, galactic structure, and the dynamics of the interstellar medium?
A census of presently observed baryons at low z shows that over 80% are only visible in X-rays. Theoretical models predict that most presently undetected baryons reside in a highly ionized IGM that should be visible in the soft X-ray band. Large scale galactic structure, (the Galactic Bulge, Ridge, galactic winds, and halos), groups and clusters have virial temperatures > 106K and are uniquely observable in X-rays. For groups and clusters, X-rays are the dominant emission. X-rays readily identify distant clusters which are resolvable with moderate angular resolution at all redshifts. The X-ray emission from most bound structures reflects the gravitational potential, hence allowing a detailed theoretical prediction of the form and temperature of that emission. Studies of virialized clusters determine the dark matter distribution and measure the baryonic fraction of the Universe. X-ray spectral images of the hot baryonic component of the Universe will provide a 3-D map of virialized, highly compressed, and shock heated structures.
In addition to deriving mass distributions of already relaxed systems, X-ray imaging and spectroscopy are ideally suited to studying the formation of collapsing and merging structures. In particular, large cluster mergers, the most energetic events in the Universe since the Big Bang, involving up to 1063-64 ergs (exceeding gamma ray bursts by 10 orders of magnitude!) can best be studied in X-rays. Shock structures, entropy distributions, and electron-ion temperature diagnostics provide otherwise unavailable information about details of the virialization process.
Large scale surveys provide an intermediate ( z~1 ) map of structure for comparison to optical surveys ( z~0.2 ) and CMBR ( z~1000 ) maps, testing models for the growth of structure. X-ray observations of SNR's and superbubbles (the by-product of wind-blown bubbles) are a unique way of measuring their contribution to the galactic energy budget. X-rays identify and map fossil star formation regions in our galaxy and nearby galaxies and provide a picture of recent star formation and its relation to large scale galactic structures.
High redshift AGN are easily selected through their X-ray properties and permit efficient optical follow-up. X-ray imaging provides an easy and efficient method for identifying young low mass stars. X-ray observations of large scale galactic structures facilitate measurements of the temperature, density, and abundance variations, allowing determination of the origin and dynamics of these structures.
Fundamental questions include how do black holes form and what determines their mass and spin; how do the masses and spins of black holes evolve with time; is the turn-on of accretion luminosity from these galactic nuclei a tracer of galaxy formation; which properties of accreting black holes are universal and which properties scale with mass; what is the physics of matter and radiation under extreme conditions of gravity, density, and magnetic field; and are there fundamental differences in the physics of low luminosity and high luminosity types of black hole candidate X-ray emitting sources?
X-rays are the predominant radiation produced near black hole event horizons and neutron stars, and from neutron star surfaces. These X-rays can penetrate the gas surrounding the compact object, and hence X-ray luminosity traces black hole locations in both galactic and extragalactic environments. The X-ray surface brightness from black holes and neutron stars is often higher (many million times) than in other radiation bands, facilitating the study of many more compact objects in X-rays. The iron K-line is the only known spectral feature that originates in the innermost regions of a black hole, and thus provides unique information on the structure and dynamics of the central regions.
Detecting and measuring matter in the extreme is an exciting area of astronomical research. The detection of black holes closest to earth was first and best done with X-rays, for as strong X-ray sources, the objects stood out clearly in the X-ray sky. Neutron stars were discovered independently by both X-ray and radio astronomers, but the only direct measure of the magnetic field close to the neutron stars' surface has been done by X-ray measurements of cyclotron absorption features.
X-ray observations led to the discovery of micro-quasars which may provide crucial insights into how black holes produce jets. Especially important for the fundamental understanding of the nature of accretion in these sources has been the diagnostic information gained from the continuous monitoring of their transient behaviors over long time intervals. Observations of the spectral energy distribution, coupled with high frequency quasi-periodic oscillations seen in the micro-quasar sources have the potential to constrain the mass and geometry of space time (Kerr or Schwarzschild geometry) as inferred from the spin of the underlying black hole.
Time-resolved high-resolution iron line spectroscopy will map the innermost regions near black holes (reverberation mapping). Timing of rapid X-ray variability of black holes and neutron stars will measure the properties of strongly curved spacetime, and constrain the properties of matter under extreme conditions. X-ray spectroscopy will determine the geometries of accretion flows, and the interaction of radiation with relativistic particles and flows, near black holes and neutron stars. X-ray spectroscopy will measure the surface temperature and composition of isolated neutron stars and constrain the nuclear equation of state.
Many of the uncertainties in theoretical models of these sources have to do with the unknown geometry of the accretion flow (size and shape of a scattering corona, the possible existence of magnetic flaring regions, and winds and jets). X-ray polarization provides a unique diagnostic on geometry.
As demonstrated by many of the discoveries in the past two decades, knowledge obtained from the X-ray band about various cosmic objects is an essential component leading to understanding their nature. Virtually every kind of object emits X-rays, from solar system objects to the most distant quasars. Understanding the interrelationship among the emission mechanisms across the electromagnetic spectrum leads to a fuller understanding of these objects and new physical insights. The synergistic relationship between observations in the X-ray and other wavebands will intensify dramatically over the next few years as CXO (Chandra X-Ray Observatory) and XMM provide the deepest views of the X-ray universe, and, together with ASTRO-E, reveal via spectroscopy previously unobserved astrophysical phenomena.
In main-sequence stars, including the Sun. although only about 10-7 of the bolometric stellar flux emerges in soft X-rays, this emission provides information about the stellar dynamo that compliments observations at other wavelengths. In the case of young SNR's, the abundances of the reverse shock heated ejecta can best be measured in X-rays. This information, when subsequently combined with radio and optical data, leads to a clearer picture of the properties of the progenitor star. Gamma-ray bursts are a recent and dramatic example of where the relatively small fraction of flux emitted as X-rays has enabled localization of the events, and led to the resolution of one of the great astrophysical mysteries of our era.
In the modern astrophysical context detailed understanding of most objects requires a panchromatic approach incorporating data from the entire spectrum of radiation. There are very few astrophysical sources for which detailed X-ray observations do not add a major component to our understanding. While it is clear that there are many classes of objects which are best studied with X-ray timing, imaging or spectroscopy there are also many other classes of objects for which the X-ray band provides vital pieces of information not accessible in any other way. Thus even for objects in which the X-ray emission is only a small fraction of the total energy budget the diagnostic power of X-ray emission is often crucial for a true astrophysical picture to be obtained. There are several classes of astrophysical work in which the X-ray and other wavelength bands are especially synergistic. For example the study of highly absorbed dusty sources such as galactic nuclei and star forming regions: the source of most of the emitting energy in universe. The only radiation that reaches us from these areas is the X-ray which penetrates the dust and gas and the sub-mm/radio which is the result of the reprocessing of the original radiation by the dust and gas. The combination of these two spectral regions have proven to be a powerful diagnostic of the origin of the energy and the nature of the radiation. Another area of powerful synergism is the study of particle acceleration in the universe where the radio, X-ray and gamma-ray radiation each trace a different component of the object (magnetic fields, electrons photons and protons) and it is the combination of the data that are required to understand the objects. Finally it is the combination of UV,EUV and soft X-rays that produce the ionization that are vital to understand the origin of the main visible property of active galactic nuclei the strong broad optical and UV emission lines.
For some phenomena, combined observations incorporating X-ray and other data allow otherwise difficult or impossible measurements. X-rays surveys can produce large unbiased samples of clusters of galaxies, but these data (at least for the present) require optical and/or X-ray follow-up to obtain redshifts and galaxy data. Combinations of techniques in optical and X-ray give total mass estimates for clusters that are especially interesting from a cosmological perspective. Millimeter radio and X-ray data can be used to measure the Sunyaev-Zeldovich effect in clusters, and hence, provide an independent measurement of the Hubble constant. In another example, simultaneous X-ray, optical, and UV data for reverberation mapping of active galaxies allow us to study the structure of these objects on physical scales inaccessible using direct imaging. In neither case can the measurements be performed using data from a substitute waveband for the X-ray.
X-rays can be used for pathfinding observations, most efficiently locating some classes of objects, which are then best studied in a multiband context. These can be objects in which the X-ray emission is dominant, such as galaxy clusters mentioned above. Isolated neutron stars are another example of objects best discovered in the soft X-ray band, where their flux is highest. For these objects a combination of X-ray data with UV and optical fluxes is essential for determining M/R and constraining the equation of state of nuclear matter.
As already discussed above, even when the X-ray emission is only a small fraction of an object's bolometric luminosity, X-ray studies can play an important role. For example, X-ray observations have revealed that the true population of low mass pre-main sequence stars in star forming regions outnumbers the classical T Tauri stars by about a factor of 10. X-ray source counts imply extensive rates of low mass star formation in nearby OB associations. Many new associations, and an unexpected population of young stars (perhaps associated with Gould's belt) have been seen in X-ray observations. The detailed physics in these populations can only be addressed after the pathfinding X-ray observations are followed by careful optical spectroscopy.
Crucial to research in X-ray astronomy is a balance of programs, continuity of opportunity, and an investment of resources in areas that support and bring to fruition the actual conduct of observations. Additional key components are a strong program of theory, laboratory astrophysics, analysis, and interpretation to transform data into useful knowledge. Equally important is the maintenance of appropriate levels of support for specialized X-ray facilities and the technology development essential for the advancement of astrophysical research.
NASA currently employs a mix of small and modest missions (Suborbital, UNEX, SMEX and MIDEX) with large, less frequent observatory-class missions (the Great Observatories, GLAST, Constellation-X, NGST). These program elements can be used to strike a balance encouraging the incorporation of new technology into flight opportunities. It is important that a viable number of X-ray astronomy mission opportunities be maintained in the overall mix to ensure both progress in the field, and optimal use of limited funding resources to benefit astronomy and astrophysics in general.
In the near-term frame, we recommend that the Structure and Evolution of the Universe Subcommittee (SEUS) include in its SEU 2003-2007 Strategic Plan the following:
The above list is not intended to represent a prioritized list. It is generally ordered along chronological lines in that the XAPWG supports the need to complete on-going projects (or at least get them into operation) represented by the first three items. The Working Group is unanimous in its support for the Constellation-X mission and gives this the highest priority for a new start. In order to achieve this goal by 2005 advanced technology funding is needed now. The last three recommendations are needed to sustain the vigor of X-ray astronomy into the next millennium, and to continue providing its unique contributions to astrophysics. The experience of the last several decades shows how important observations made at X-ray wavelengths have become to a broad understanding of virtually all astronomical objects and astrophysical processes. Just as there are many optical and radio telescopes needed to carry out a variety of observations, there is a growing need for multiple observing facilities in the X-ray band. A program plan with balanced mission opportunities, large and small, will provide this needed capability. More detailed comments are given below.
The Chandra X-ray Observatory (aka AXAF) is scheduled for launch in 1999. With its sub-arcsecond angular resolution and high spectral resolution ( R » 1000 ), Chandra will open new areas of discovery and astronomical research. The majority of Chandra observing time is open to the entire community through a guest investigator program. A high level of interest in Chandra has already been demonstrated through the overwhelming response to the first announcement for observations. It is important that the resources needed to obtain the best possible science return from Chandra be maintained over the lifetime of this mission. An initial 5 year mission lifetime has been included in NASA strategic planning. The possibility for extended operation to 10 or 15 years needs to be introduced into long range planning without impacting the development of other X-ray missions and the technology development to support those missions. The very high angular resolution of CXO is a unique capability that will not be surpassed (or even approached) by any mission prior to the end of this mission time frame. It is recommended that the SEUS include this longer operational lifetime for Chandra in its report to NASA.
With RXTE, powerful diagnostics have been discovered for regions of spacetime where strong field GR is required to describe the motion of matter. The kilohertz QPOs are related to orbital motion only a few km above the surface of neutron stars, and similar phenomena may already be seen near black holes as well. Several more years of operations of RXTE will be required to reap the fruits of these exciting discoveries. We recommend that the SEUS endorse the recent NASA Senior Review plan to continue RXTE operations through at least the year 2002.
We must continue a significant US involvement in international missions. The successes of missions like ROSAT (Germany/US/UK) and ASCA (Japan/US) demonstrate the wisdom of this approach. In the near-term, XMM (ESA/US), ASTRO-E (Japan/US) and Spectrum X-Gamma (Russia/Denmark/US) continue to prove the great advantages provided by this type of initiative; continued strong US participation in future international X-ray experiments is essential.
The return on investment to NASA and the US Science Community from international participation is high. The coordination and cooperation involved in these programs helps to further the science goals of X-ray astronomy while minimizing duplication of effort. The complementarity of Chandra and XMM illustrate this point. NASA and other national space agencies are encouraged to continue to find new avenues for cooperation in the field.
We recommend that the SEUS report suggesting NASA facilitate broader bilateral opportunities. Providing a new budget line for international participation (or non-zero funding for such an existing budget item), or ensuring that proposals for international collaborative missions can compete at all levels within the NASA funding system would be a significant step towards this goal.
We recommend that the next major new mission in X-ray astronomy be the Constellation-X project. Constellation-X is a high-throughput spectroscopic mission addressing the need to study in detail the physical processes responsible for X-ray emission from celestial objects. The mission provides throughput needed to carry out high resolution spectroscopy over the band of X-ray atomic transitions. A unique aspect of Constellation-X is the inclusion of a high energy telescope for observing non-thermal processes and the continuum spectrum simultaneously with the softer line rich band. This broad energy response is needed to understand the physical processes and conditions in the sources being studied.
The Constellation-X mission is now in pre-Phase A planning, (as part of the current NASA Strategic Plan for the next 5 years). This is an ambitious mission, it will bring increases in throughput and spectral resolution relative to Chandra and XMM that hold great promise for following up the discoveries from these missions as well as making major new contributions to the field. This mission corresponds to the recommendation of the 1994 XAPWG Report for a High Throughput Mission as a candidate for a major new mission.
The advanced technology development needed for this mission is well underway, but further work is needed to transform laboratory proofs into flight readiness. The Constellation-X Project Office has formulated a technology roadmap that accomplishes this goal in a well conceived program with low risk and high yield. We fully endorse this approach to a new mission; consistent with NASA's guideline of investing in early technology development to reduce risks and costs.
The initial deployment of the Constellation-X fleet of 4 similar satellites, using two launchers, spreads development costs and reduces risk. The combined capability of the Constellation-X fleet meets all of the requirements to carry out the scientific program that has be outlined and reviewed as part of the NASA Strategic Planning process. The XAPWG is in full agreement with the project as planned.
As discussed in Sections 4 and 5 of this report, there are science objectives the the field of X-ray astronomy that go beyond even the capabilities of the Constellation-X mission. We note that this mission could be extended though additional launches in the later part of the decade. Each additional two satellites, launched together, would increase total area by 50% and allow new technologies to be employed, further increasing the utility of this mission while maintaining a continuous high-throughput capability. Opportunities for improved angular resolution,; higher energy resolution; expanded field of view for the microcalorimeter array; extension of bandpass to both higher and lower energies; as well as possibilities for new advanced instruments, are examples of the benefits that such an extended mission might have. The planned infrastructure of the initial configuration (operations and data analysis) could easily absorb additional Constellation-X elements, making this a cost effective and evolutionary program.
The mission is described in detail in the Appendix to this report. A broad range of science is addressed through an evolution of technologies currently achievable. We urge the SEUS to strongly recommend NASA carry out the Constellation-X technology plan in order to be positioned for a new start for the mission in FY 2005. The details for this technology plan are included in Science with the Constellation-X Observatory. Constellation-X is the only new start being recommended in the near-term of this report. It must be started soon and made operational quickly if we are to avoid a hiatus in our national X-ray astronomy observing capability and maintain a leadership position in the field that was pioneered in the US.
3.1.4.1 Science Examples with Constellation-X
Science with Constellation-X is discussed in great detail in the included Appendix. Highlights from this report include:
3.1.5.1 Supporting Research and Technology
Any plan spanning time scales of a decade must recognize the importance of basic technological research as an enabling prerequisite for innovative missions. Up front investment in development can open new vistas for observational capability while managing risk at a time when the costs are relatively low. By the time a mission is selected for flight new development can be kept to a minimum by building on an existing base. Examples of the importance of early technology development funded via SR&T are the cryogenic detector and high energy telescope activities that were the immediate precursors to the Constellation-X initiative. Without the leading edge technical feasibility efforts supported by SR&T, new mission initiatives will be significantly hampered.
Strengthening the research and technology base remains a challenge. SR&T is fundamental to new instrumentation and training for young experimentalists - no other NASA program gives students actual, hands-on experience with state-of-the-art hardware. With the notable exception of computing, all the equipment needed for SR&T projects has become more expensive through technical inflation. In addition, the easy experiments have all been performed - to compete successfully at the forefront of the field new technology must be more complex and financially demanding than in the past. Funding for X-ray detector development, X-ray optics, and unique X-ray test facilities must be augmented substantially if NASA is to foster maintenance of its pool of hardware-trained young scientists and continued progress in its technology base.
3.1.5.2 Theory
There is a serious shortage of support for theoretical studies. After some improvements in funding as a result of the recommendations of the Bahcall Panel, the situation has again become critical. Only 10-15% of proposals are being accepted, and funding of accepted proposals is at a level (in real dollars) much reduced from the 1980s. The lack of support for theoretical studies is having a negative impact not only on understanding the results of missions, but also on planning of future missions. In addition to augmentation of the Astrophysics Theory Program, NASA should consider funding individual investigators and groups, and funding theory fellowships as part of each mission, beginning with the development phase of the mission. This action would encourage theoretical understanding before missions are flown assuring that they return relevant, valuable data.
3.1.5.3 Laboratory Astrophysics
The upcoming launches of Chandra, XMM and ASTRO-E will bring a revolution in astrophysics due to vastly improved, highly sensitive, spectral capabilities. However, the interpretation of this avalanche of data will be compromised because of uncertainties and/or gaps in our understanding of the details of the relevant atomic physics. The problem is not one of a limitation of the basic physics of the underlying processes (although one can never be completely sure!), but rather one of completeness and precision. By and large, state-of-the-art atomic codes are capable of generating relevant parameters for particular processes with reasonable accuracy. However, there are a large number of possible charge states, a large number of transitions per charge state, and a large number of microphysical processes per transition which can contribute to an astrophysical spectrum. Reliable calculations exist for very few of these and reliable measurements, absolutely critical for benchmarking the codes, identifying computational errors and focusing attention on the relevant processes are still only available in a limited number of cases.
The modest program for X-ray relevant laboratory astrophysics has historically been funded as part of the High Energy Astrophysics SR&T program. For the future, NASA has consolidated all laboratory astrophysics into the new Space Astrophysics Research and Analysis program under conditions that might make it difficult to maintain even the existing levels of effort. We urge NASA to take into account the need for more, rather than less, research in X-ray laboratory astrophysics when preparing budgets and guidelines under the new policy.
3.1.5.4 The MSFC X-Ray Calibration Facility
The MSFC X-Ray Calibration Facility (XRCF) provides unique capabilities for the characterization and calibration of space optics and instrumentation. Built originally for the HEAO-2 (Einstein) program and later substantially modified for the Chandra (AXAF) program, the XRCF represents a substantial investment of scientific and engineering effort, as well as capital resources. Several programs e.g., Constellation-X, Solar X-ray Imager, and the Next Generation Space Telescope, currently utilize this facility either for evaluation of prototypes or calibration of flight instrumentation.
In view of its demonstrated utility and the cost of re-establishing such capability for future missions, we recommend that NASA maintain the XRCF as a national facility for testing space optics and instrumentation. Furthermore, we urge NASA to fund the facility so that it will be available and affordable to any program with appropriate scientific justification.
Another perennial challenge has been that of ensuring support for Mission Operations and Data Analysis (MO&DA). The X-ray community consists of both users and builders and the users rely on MO&DA funding, in part, to do their research. MO&DA funding is used to pay salaries of postdocs and other soft money workers (graduate students, summer salaries of faculty) in addition to supporting other research expenses (computers, travel, supplies, etc). Continuing and enhancing the guest observer programs is vital to continued activity in this area. The recent reductions in ROSAT, ASCA and RXTE funding combined with the Chandra delays have had a very deleterious effect on the community. Only with stable MO&DA funding at a level consistent with the large national investments in the hardware efforts, can high energy astrophysics continue to attract researchers so vital for the intellectual success of the field.
3.1.6.1 Mission Operations (MO)
NASA must sustain the operation and data analysis for existing missions that continue to produce excellent science. Relative to building new missions, the operation costs of existing missions that provide sustaining science are low. We recommend that NASA encourage developments for low cost mission operation so that it will be economically possible to prolong those missions that are still scientifically productive. Participation in international collaborations is an important means of leveraging U.S. investments in missions, and these arrangements should continue to be encouraged.
We must assure vigorous Guest Observer programs for current and future missions; maintain support for mission science centers; and ensure support for archival data analysis through such programs as the Astrophysics Data Program, the Long-Term Space Astrophysics Program, the Astrophysics Theory Program, and the Astrophysics Data System. The science return of a mission depends on rapid analysis of data and access to the mission by as many scientists as possible. It is time to review policies regarding proprietary data rights and exclusivity with a mind towards encouraging participation while still protecting the contributions of individuals bringing missions to fruition. There are various types of missions or observations where the rapid release of data and results is easily accomplished and brings great benefit to the community. The Hubble Deep Field is an excellent example of such a situation. However, this example also highlights the need to be certain that instrument performance and calibration are well understood before such a broad release can be made.
The XAPWG believes there is a need to protect the intellectual property rights of facility guest observers though the use of a proprietary data rights policy giving these observers exclusive rights for a period of 12 months early in a mission, perhaps decreasing to a little as 6 months when the mission data system is more mature (e.g., after the first two years of mission operations). There are various situations where the need to fully understand the calibration and performance of a mission requires additional time before accurate science analysis can be done. It is important to make sure that quality is high before broad dissemination is made of the detailed data set. In the case of instrument developers for large (facility class) missions, the Working Group recommends the policy of guaranteed observing time be continued, but limited. We consider the GTO policy for the Chandra X-ray Observatory a good example for future missions. The GTOs may consist of Instrument Teams and Interdisciplinary Scientists selected for the mission through a peer reviewed process. In the first two years of mission operation 30% of the observing time is reserved for the GTOs. For the first year the GTO observations are based on the research topics included in the instrument or interdisciplinary science proposal. For the second of these two years, the observations are subject to peer review to assure high quality science observations are carried out. After the second year a much smaller time (about 5%) is allocated to the Instrument Teams for carrying out long term science goals and monitoring the state of health of their detector systems.
3.1.6.2 Data Analysis (DA)
As scientific missions become more complex, and the data quality improves, it becomes more difficult to fully analyze data immediately. Planning for DA funding must recognize the fact that analysis often proceeds for years after the formal end of a mission. A proper, steady state needs to be defined and maintained within the NASA sponsored community of X-ray astronomy research.
Scientists find themselves writing large numbers of observing proposals annually in order to sustain an on-going research effort. As a result, too much time is spent writing proposals, budgets, and reports, when it would be to the best interests of all (including NASA) to be doing science; data analysis is protracted and results appear later than they would had they been adequately funded.
Stable, predictable funding levels that result in better science are needed. In the current situation most data analysis funding is tied to actual missions, on an annual basis, while a much smaller portion is available for longer periods, up to 5 years. NASA must be willing to expand the experiment with multi-year funding, started in the ADP and LTSA programs, to encompass more of the mission-oriented data analyses. New mechanisms should be explored for providing sustaining support spanning broad research areas, subject to periodic peer review.
3.1.7.1 Explorer
The Explorer Program supports missions with specific scientific goals for which a large facility is not appropriate. Such missions often complement larger X-ray missions and observations at other wavelengths, providing context for better understanding of various phenomena. Advances in spatial, spectral, polarimetric, and temporal resolution should be supported through the Explorer Program to help build the framework for future progress.
Many mission concepts that address exciting issues in astrophysics fit within the boundaries of the Explorer Program (UNEX, SMEX, and MIDEX, as well as the Mission of Opportunity). Small and medium missions act as pathfinders and surveyors for future missions and can also carry out cutting-edge science. For example, a deep, wide-field 0.2-2 keV survey, an all-sky spectroscopic survey of the soft X-ray diffuse background, a hard X-ray survey, and a high resolution nebular spectrometer are all mission concepts included in the 1994 Report that continue to have high scientific merit and fit within the envelope of the Explorer program. Pathfinding missions for polarimetry and X-ray interferometry are also well within this scope and should be included in any list of potential missions.
The quality and quantity of proposals in response to recent announcements for the Explorer Program demonstrate that there are many more outstanding proposed projects than the current funding level can support. The need for more opportunities is justified by the quality of the science being proposed and the desire to maintain the research infrastructure that will sustain a high level of competence and experience. We urge that the SEUS recommend to NASA the Explorer budget be substantially increased over the next three to four years so that there are more opportunities for flight, and that larger and more complex missions can be accommodated. Contingent upon the size of any increase in the overall Explorer budget, we recommend an increase in the cost ceiling for MIDEX to accommodate missions that fall well below the $500M cap for those larger missions needing new start approval but currently exceed the MIDEX limit. This change would allow the ``Observer'' mission mentioned in the 1994 report to be supported without necessarily reducing the number of mission opportunities already dangerously low. We stress that the need to increase the number of mission opportunities within the Explorer Program has a higher priority than increasing the cost cap for missions. We also suggest reconsideration of the pending decision to lower the current UNEX cost ceiling, as that severely limits the potential for X-ray missions in that category.
NASA's current approach to selecting Explorer missions includes a detailed scientific and technical review of all proposals, this makes proposal preparation expensive and inefficient. A scientifically and technically excellent proposal requires a substantial effort by a science and engineering team, and a large investment by a spacecraft provider. We recommend a two-stage selection approach, the first is made primarily on the basis of scientific merit and technical feasibility, and the second on scientific merit, technical approach and management plan. Only after initial selection is made would strong involvement by aerospace contractors be expected. This approach would facilitate better leveraging of the aerospace contractors, who could concentrate their effort on a smaller number of proposals, and with the probability of selection being higher, would be more likely to invest a more substantial effort.
Additionally, we suggest during each cycle a small number of Explorer missions, offering strong scientific promise but requiring further definition, be selected for extended Phase A study or technology development funding so that they can be reconsidered for later flight opportunities.
3.1.7.2 Suborbital
An active suborbital program must be sustained. This area of research can provide a low cost testing ground for new techniques. It is also a training ground for the next generation of scientists in the field. In these settings, students and young investigators can learn how to run a mission, how to manage a project, and how to interact with various agencies before being confronted with these challenges for the first time on a major satellite mission. Sounding rockets and long-duration balloons offer important science opportunities such as rapid response to new or transient phenomena. The Ultra Long-Duration Balloon (ULDB) program holds great potential for high energy X-ray observations. We suggest these opportunities be expanded, perhaps by including ULDB's in the UNEX and/or SMEX mission category.
3.1.7.3 Explorer Mission Capabilities
We describe below a range of science capabilities that can be carried out within the scope of the Explorer Program. Some of these examples were taken from mission concepts that have been submitted over the past several years. They were ranked ``very highly'' scientifically, but for various reasons not selected for flight. The XAPWG thanks the various PIs of these missions for allowing us to include their concepts in this report. These examples are not meant to be exhaustive, nor are they presented in any particular priority order. They are provided in order to make the point that the quality of mission concepts for X-ray astronomy Explorers, and their relevance to astronomy and astrophysics in general is very high.
The study of large scale structure in the Universe is particularly
well-suited to large area X-ray surveys where distant objects are
readily detected as compared to more local foreground objects
(galactic stars and low redshift galaxies). Groups and clusters of
galaxies, as well as active galactic nuclei and QSOs can be used as
powerful tracers of the structure of the Universe on scales of 10's to
100's of Mpc. The large area surveys in X-ray astronomy that have thus
far been carried out had limited sensitivity due to their
instrumentation or optical design. Missions that are currently
underway with angular resolution below 15 arcseconds have relatively
small fields of view. As general facilities, they are unable to devote
the large amounts of time needed to achieve the necessary combination
of sensitivity and sky coverage to address the science objectives of
1) understanding how structure in our Universe emerged from the Big
Bang and 2) how dark and luminous matter determine the geometry and
fate of the Universe .
X-ray observations provide the unique ability to select groups, clusters, and AGN to high redshifts. With a comprehensive optical follow up program, redshifts for many sources would be determined so that the true three dimensional structure could be mapped in a large volume. These data provide a unique intermediate anchor point between COBE, MAP, and PLANCK, which probe the grand cosmic design and observe newborn cosmic structures at z » 1000 , and the Sloan Digital Sky Survey which describes these structures at the current epoch.
Astronomical observations at all wavelengths have revealed the
existence of numerous types of objects where extremely energetic and
exotic phenomena occur. These include X-ray binaries and active
galactic nuclei (AGN) among others, where accretion onto a compact
object (collapsed star or massive black hole) is thought to be the
basic mechanism for the release of large amounts of energy. In
accretion processes, energetic radiation is emitted that can be
detected in the soft X-ray region. In general, these objects combine
the presence of strong magnetic fields, extended electron atmospheres
and/or large accretion disks. The emitted radiation will be polarized
by means of electron acceleration in the presence of rotating magnetic
fields, electron scattering, and/or nebular reflection. Polarimetric
observations in the X-ray band will help to determine the nature of
the physical processes powering these and other exciting objects.
A polarimeter design well matched to the Explorer Program could yield definitive scientific results based on observations of selected classes of soft X-ray sources. Polarization investigations should not be limited to the brightest objects in our galaxy. Rather, such a mission should have the sensitivity to detect polarization at the level of a few percent at the 5s level for extragalactic sources, particularly AGN and BL Lac objects. This capability will open up a new observational window and may herald new astrophysical discoveries.
There still exists no imaging hard X-ray sky survey. Hard X-ray
spectroscopy also has been rather limited - yet these are very
interesting regimes, both for studying highly obscured AGN and for
studying fundamental physics from reflection and scattering. The
apparent discrepancy of soft survey (ROSAT) and harder survey (GINGA)
source counts remains an unresolved problem.
With new technologies for building grazing incidence hard X-ray telescopes (e.g., using multilayers on foils), and with efficient high energy imaging detectors, a hard X-ray survey with good sensitivity would explore a new region of the sky. With modest spectral resolution, such a survey could provide important information on the nature of the X-ray background, the hard continuum of Blazars, and the frequency of cyclotron features in neutron star X-ray binaries.
Isolated, non-pulsing neutron stars are important for understanding
the physics of neutron stars. The surface of an isolated neutron star
is simply a high pressure stellar atmosphere, perhaps of exotic
composition. High resolution X-ray spectroscopy (possible with AXAF,
XMM, and Constellation-X) will reveal the surface composition and
surface gravity. Modeling of the stellar atmosphere will give the
angular diameter. These objects are close enough that parallax's can
be measured directly, so we can determine the masses and radii. This
gives the interior equation of state of a neutron star. The proper
motions, if traceable back to known supernova remnants or star
formation regions, can reveal the ages of the neutron stars.
There is only one such object known, with 3 or 4 additional candidates. We would like to expand the sample size. With a larger sample, we could estimate the typical neutron star birth mass (it is only known now for neutron stars in binary systems), and the cooling curve for neutron stars. This can only be done by obtaining a deep soft X-ray survey. Such a survey, going a factor of 10 deeper than the RASS, with an instrument sensitive (and calibrated) below 0.1 keV, would turn up both somewhat more distant hot young objects, and nearby older cooler objects.
The X-ray band is unique in its ability to contribute to understanding clusters of galaxies, the interstellar medium, and supernova remnants. The large angular size of many of these sources makes their observation with observatory class missions difficult. An explorer class mission, specifically designed for spatially resolved X-ray spectroscopy with an appropriately large field of view, is a requirement for optimally carrying out these studies. The power of high resolution spectroscopy arises from its ability to measure basic astrophysical properties by providing quantitative diagnostics and from the fact that these usually optically thin sources allow particularly unambiguous interpretation of the results. Scientific goals of a large FOV, spatially resolved spectroscopy mission include 1) detailed physics of cluster atmospheres, e.g., gas bulk flows, turbulence and ion-electron equilibria 2) interaction of SNR with the ISM and measurement of the shock structure 3) mapping the galactic center region in the 6.7 keV line to determine the activity history of the black hole at the center of the Galaxy and, 4) mapping the large scale filamentary structure of the Universe by detecting the warm baryons in these structures
An advanced X-ray all-sky monitor to explore timescales longer than those accessible by RXTE is essential to complement the capabilities of existing and planned narrow-field imaging and spectroscopy missions. It is important that such monitors push down substantially deeper than BATSE and ASM have done, reaching a sensitivity of a mCrab or better in a day or less. The behavior of galactic transients at low luminosities (e.g., ADAF) is largely unexplored. AGN variability is also largely unexplored. A sensitive monitor could revolutionize these fields in the same way that BATSE and ASM did for galactic transients.
The great success of the Rossi X-Ray Timing Explorer (RXTE) has demonstrated that fast timing measurements are a unique and powerful way to probe strong gravitational fields and the physical properties of black holes and neutron stars. A ``next-generation'' X-ray timing satellite with a 60,000 cm2 detector array-ten times larger than RXTE-should be possible as a MIDEX mission. Such a satellite would measure accurately the waveforms of the millisecond periodic oscillations during X-ray bursts, constraining further the compactness of neutron stars and the equation of state of superdense matter; make precise observations of kilohertz quasi-periodic oscillations from black hole and neutron star low-mass X-ray binaries, which could establish securely the presence of strong-field gravitational effects; and, study rapid variability in the spectra of relativistic jet sources, which should advance substantially our understanding of jet emission mechanisms.
Even as we map out our plans for the coming decade, we must begin looking ahead to the decade beyond. Bold new initiatives are needed to bring X-ray observations into the era of the ``many square-meter class'' missions that will be contemporaneous with NGST and will be capable of simultaneous very high resolution imaging and spectroscopy. Such ``facility class'' missions will have broad user constituencies and can be expected to herald fundamental breakthroughs in astrophysics.
X-ray interferometry and X-ray polarimetry (beyond SXG) are examples of new observational approaches that may require long term development before they can be considered for flight programs. Mid-term funding for advanced technology development will be absolutely crucial in meeting the challenges that lie ahead. In this area it is important to allow support for as many approaches as possible with periodic reassessments so that high risk (but high return) concepts can be attempted. If the past has shown us anything, it is that innovation and a solid research base are the best foundation for being prepared for the future.
The mid-term needs for X-ray astronomy also include significant small to medium mission opportunities through the Explorer program. The active combination of science and technology carried out at Universities, Federally Funded Research Centers, and NASA Centers can only be sustained if there are flight opportunities that occur on the short time scales offered though this approach. In addition to an enhanced Explorer Program, and strong international collaborations, we anticipate the need for flight opportunities through the New Millennium Program for technological demonstrations and tests. The variety of missions and capabilities that can be accomplished within the scope of these programs has already been illustrated in Section 3 of this report. The Working Group urges NASA to recognize the value of theses types of missions, their intrinsic scientific merit, and their contributions to the technological base upon which new large programs can be built; reiterating the recommendation for increased funding and more frequent flights.
In almost all cases of X-ray observing there is a dearth of photons that can only be addressed by increasing the collecting area of the system being used. X-ray detectors are approaching 100% efficiency and further gains in that direction will, at best, be modest. Similarly, the efficiencies of dispersive elements (e.g., gratings) are also nearing their theoretical limits. Only by substantially increasing the size of telescope collecting areas can we hope to achieve the major gains in throughput needed to probe the X-ray Universe in more detail. When coupled with advances in spectroscopic and angular resolution the development of very large area and large field of view optics will enable a new class of observing capabilities leading to new scientific insights.
With large throughput of > 10 m2 , and modest performance (angular resolution ~5-10 arcsecond, field of view ~1 deg2, and energy resolution ~100 ), it will be possible to carry out surveys over large solid angles for flux limited samples, including clusters and groups. With modest resolution spectroscopy and sufficient throughput, the redshifts of groups and clusters can be determined directly from the X-ray observations. Similarly, the redshifts of the AGN that dominate the point-like source population can be determined. Even with 10 arcsecond resolution positions of point sources can be determined to a few arcseconds allowing the brighter AGN to be uniquely identified with optical counterparts. It should also be possible to map the filamentary structure of the cosmic web predicted by cosmological numerical simulations.
High throughput can be used to monitor the time variability of faint sources on rapid time scales to study QPOs. RXTE is limited to detecting very rapid orbital motions in a statistical sense, as a superposition of several frequencies. An instrument of much larger area could go from measuring the frequencies of these orbits to actually mapping them in space and time and thereby verify GR in the strong field regime. The ability to measure the properties of kHz QPOs on time scales shorter than their coherence time would, for example, allow us to see the change in the orbital frequency of a clump of matter as it spirals down to the compact object.
With very large area >> 10 m2 , good angular resolution ( ~2-5 arcsecond), high spectral resolution ( ~1eV ), and modest field of view (~100 arcmin2) the spectra of faint objects, either low luminosity relatively nearby sources or bright but very distant sources can be measured. Both the low luminosity AGN and low luminosity clusters appear to be fundamentally different from their higher luminosity counterparts. Spectroscopy will provide the information needed to understand these differences and determine if these are new classes of objects or not.
Mapping the 3-D filaments and baryonic matter distribution out to redshifts of order 1 will provide observational checks on the complex numerical models being developed for the evolution of the Universe. By varying cosmological parameters the predictions of these simulations can be tested against observations to constrain these parameters.
The X-ray sky is time-variable on all timescales. It is important to design at least some level of time-resolved readout in future missions, even if it is at timescales of seconds (as opposed to milliseconds). This applies to spectroscopy as well; for example, iron line reverberation mapping of AGN will require studying the time evolution of the line profile on timescales of less than a minute. Large-area/high-sensitivity missions have the advantage of an instantaneous signal-to-noise sufficient to study how systems (e.g. disks) respond to impulsive changes, in terms of reflection, reprocessing, etc. - it is important to exploit this advantage on the time-resolved side of things.
Sub arcsecond angular resolution and large area >> 10 m2 are needed to study very faint X-ray sources that might be associated with the most distant objects found at other wavelengths, or only discovered in X-rays at the detection limit of a mission. In order to accurately measure flux and spectra for the faintest low luminosity black hole sources or brighter more distant sources, high angular resolution is needed to avoid source confusion. Based on estimates of their early luminosity, it should be possible to detect the earliest supermassive black holes as they ``turn on'' at high redshift.
Many cosmic sources are expected to have significant structures on sub-arcsecond scales as shown spectacularly by HST and VLA images. Most of these should have X-ray counterparts and many other types of structure are expected to show up on these scales only in the X-ray band. It is only by having a large collecting area follow-up to Chandra that the breakthroughs that we expect from this mission can be exploited.
Improvements in angular resolution have always lead to great advances in astronomy. In the case of X-rays we have already seen the gains in sensitivity for source detection that come about through improved angular resolution. In the next decades the need for higher resolution is driven by the science goals listed below. Sub-arcsecond imaging will permit detailed studies of physical processes that will extend our understanding of various classes of objects. New (SR&T funded) technological advances suggest that extremely high angular resolution can be achieved using X-ray interferometry techniques. Many of the technological issues associated with interferometry are already being addressed in the optical SIM project. A recent workshop on X-ray interferometry concluded that SIM performance requirements are applicable at X-ray wavelengths.
At this resolution it becomes possible to examine stellar plasma interactions in close binary systems, and stellar wind interactions in early type stars and proto-stellar systems. This scale is beyond the best capabilities of Chandra's grazing incidence large telescope, and will require the development of X-ray optics technologies that are significantly different from those currently in use. For example, normal incidence multilayered optics may allow this angular resolution to be reached but only over narrow wavelength bands.
There is an indication that low luminosity black hole systems have their disks truncated at some ``transition radius'', such that inside this radius there is no cold gas, only hot gas. The evidence is not very strong. Measuring fluorescent iron line profiles or the reflection component would be very useful to determine the location of the transition radius. ADAF models for low luminosity systems predict thermal emission from as far as 103-105 Schwarzschild radii from the center, whereas the standard disk model predicts that nearly all of the emission comes from close to the back hole. 100-1000 micro-arcsecond imaging (perhaps using X-ray interferometry techniques) could distinguish these models by directly resolving the emission regions in some nearby sources. Sub-milliarcsecond imaging would directly measure the regions responsible for producing the X-ray radiation from black holes without recourse to indirect arguments.
At this resolution studies of stellar coronal morphology of nearby (<100pc) stars are possible. A high throughput, large effective area, high spatial resolution X-ray instrument is needed to resolve stellar coronae. It would take a 24m diameter telescope operating at 10Å (1 keV) to have a diffraction limited resolution of 10 micro-arcsecond by the Rayleigh criterion. Other methods for achieving such high image quality could involve X-ray interferometric techniques with multiple baselines and high throughput.
A 10 micro-arcsecond telescope would resolve the Sun at 50 pc into 100 independent points, while the active star in an RS CVn binary would be about 3 times larger. With 10 m2 of collecting area the Sun ( Lx~1027ergs sec-1 ) would be detected at 0.2 cts s-1 and an RS CVn system would yield 2×103 cts s-1 at the same distance. dMe stars are 100 times brighter than the Sun, giving tens of counts/sec at 50 pc.
At this resolution we could obtain images of stellar coronae of (nearly) the quality of the Skylab ATM X-ray pictures of the Sun. This would immediately allow us to determine the magnetic morphology of all types of stars. With a non-dispersive low energy spectrograph one could easily obtain spatially-resolved spectra of the various magnetic structures of stellar atmospheres, and put to rest all the questions that vex us today about the relation of solar magnetic structures to those of the most active stars. At the limit of this resolution, one could examine the coronal morphology of pre-main sequence stars (~150 pc) to distinguish among possible emission mechanisms including whether the X-rays arise in small solar-like structures, in high latitude accretion columns, or in a large global dipolar-like magnetic canopy.
X-ray emission is naturally associated with compact structures. High concentrations of energy in small regions lead to high temperatures, and short timescale events from small objects lead to high energy non-thermal emission. The surface brightness of these emissions can exceed by factors of millions to billions all but the brightest non-thermal sources in the visible, IR, and radio. We can take advantage of this high surface brightness in X-rays to image distant structure in the Universe on kilometer scales instead of parsec's or astronomical units.
The black holes in the center of M87, and possibly the Milky Way, have angular scales of a few micro-arcseconds and there are many more AGN's with extents in excess of a tenth of a micro-arcsecond. To capture actual images of event horizons would fulfill a central goal of science: to peer into a truly different part of space-time. It is hard to imagine an experiment more exciting for science or exploration.
At scales near, or just below, a micro-arcsecond we can image directly hot gas transfer in close binary systems, even resolving the disks of tiny white dwarf stars! We can watch a supernova blast wave evolve in the early days after explosion, even though it sits in a distant galaxy. The list of basic new observations is lengthy. As our resolution continues to improve below a micro-arcsecond the presence of gravity waves will make itself apparent in our images. As the waves cross the image they will create ripples, making our observations akin to standing above a swimming pool, reading a newspaper at the bottom, below the waves. This will give us a sensitive new probe of gravitational physics, but will complicate super high resolution imaging, particularly at cosmological distances.
The performance requirements discussed in the previous sections can only be achieved through the development of new technologies. Advances in materials and fabrication are needed to make high performance and light weight optical systems. New concepts that lead to practical, flight-capable methods for implementing laboratory techniques, such as X-ray interferometry, need to be developed and tested. The enabling technology driven by the scientific goals of the Long Range X-ray Astronomy Program Plan is listed below:
To focus the technological efforts needed to meet the scientific objectives discussed above, we have set as long-range goals the challenges of a) truly imaging the event horizon on a black hole, and b) detecting X-rays from the Early Universe ( z~10 ), either from supermassive black holes as they first form, or from the radiative cooling that occurs as the first galaxies form. To achieve these goals we need to call on all of the technological developments listed above. We also require a logical program involving a mix of small, modest and large missions that prove out these innovations. In this process, new science capabilities will become available addressing some of the key science topics discussed in this report so that progress occurs on all time scales and on several fronts. We expect in addition to the major missions depicted on the X-ray Astronomy Road Map, there will be Explorer and Millennium class missions to act as testbeds, pathfinders, and independent scientific experiments, progressing towards this grand challenge.
The first steps of a long range plan for X-ray astronomy have been discussed in Section 3 of this report. Some of those items, such as the successful launch and operation of the Chandra X-ray Observatory, represent milestones. Others, such as the sustaining SR&T, MO&DA, and Theory programs, represent continuing needs for the field over any time frame being considered. In our long range program we assume that these ``first'' steps have been accomplished and the sustaining efforts are being supported. Thus, the new components of the long range plan are:
The list of technological areas was given in Section 4.3 of this report. It is essential a technology development effort be started during the current NASA Strategic Plan (the near-term) in order to explore the variety of approaches that may lead to the capabilities needed in the long term. The XAPWG recommends that the SEUS request NASA to include this technology support in its near-term plan.
The need to have a series of missions is one that the Working Group feels is key to the ultimate success of the plan. A combination of Explorer, Millennium, and new starts will allow innovative concepts to be proven out at acceptable costs and risks. At the same time, scientific results from a series of missions will be valuable in many ways. New information will help focus the science objectives of successive missions. The science return of each mission will add to the growing body of knowledge and provide insights into the fundamental questions driving the field as discussed in Section 2.
The grand challenges produce a road map extending beyond the time frame of the near-term NASA Strategic Plan (2003-2007).