Final Report March 2002 An Ultra High Throughput X-ray Astronomy Observatory With a New Mission Architecture Grant-16717066 Prepared for NASA Institute for Advanced Concepts USRA Smithsonian Institution Astrophysical Observatory Cambridge, Massachusetts 02138 The Smithsonian Astrophysical Observatory The Smithsonian Astrophysical Observatory is a ismember a member of the of the Harvard-Smithsonian Center for Astrophysics Abstract An Ultra-High Throughput X-Ray Telescope (UXT) Observatory With a New Mission Architecture Paul Gorenstein Harvard-Smithsonian Center for Astrophysics Over a two year period with the support of a grant from the NASA Institute for Advanced Concepts (NIAC/USRA Grant No. 7600-035) we addressed the question of how a space based X- ray astronomy observatory with an effective area of over 2 million square centimeters and an angular resolution of one arcsecond or better could be constructed and deployed in space. The Chandra X-Ray Observatory, XMM-Newton, and RXTE have reinforced the motivation for undertaking this enterprise by demonstrating its relevance to cosmology and fundamental physics. We considered three fundamentally different designs for the optics with several variations within each approach. All of them require “formation flying” or synchronized positioning of a large telescope aboard one spacecraft and detectors aboard others. One approach is an adaptation of the conventional filled aperture Wolter Type 1 X-ray telescope with a focal length of about 200 m. Another is a family of “sparse aperture” Kirkpatrick-Baez geometry telescopes with focal lengths up to 30 km. A third is a system based upon Fresnel zone plates and refractive lenses with focal lengths up to 3000km. All may require adaptive optics on some level to satisfy the angular resolution goal. The sparse aperture telescopes may be able to satisfy the angular resolution goal more easily but the very long focal length imposes limitations. The advantage of the Fresnel optics is two orders of magnitude lower mass but its extremely long focal length and severe chromatic aberration are important issues that limit their performance. The best site for all three designs is the Sun-Earth L2 region. We also considered a lunar-based observatory. The Moon is competitive with free space only if there already exists a lunar base with an infrastructure that can provide at no or very low cost transport services, materials, and construction services. 1. Introduction 1.1 Goal 1 1.2 Importance of X-ray Measurements in Astronomy 2 1.3 The Early Universe and Sources of X-ray Emission 4 2 The Ultimate High Throughput X-ray Telescope Observatory 2.1 X-ray Observatories 5 2.2 Astrophysics and Fundamental Physics 5 2.3 Effective Area of UXT 6 2.4 Relation of UXT to XEUS 9 3 The Observatory Architecture 3.1 Options 11 3.2 Telescope Architecture 12 3.4 Formation Flying-Pointing Accuracy and Stability 13 3.5 Detector Spacecraft:Traffic Management 14 3.6 Formation Flying Accuracy 15 3.6 Changing Pointing Direction 17 4 The X-ray Optics 4.1 Requirements of UXT 18 4.2 Filled Aperture Telescopes: Wolter Type 1 18 and Kirkpatrick-Baez Designs 4.3 Fabrication of KB and Wolter Optics 22 4.4 Sparse Aperture Telescopes 26 4.5 Fresnel Optics 29 4.6 Correcting Chromatic Aberration of Fresnel Optics 33 5. Adaptive Optics 5.1 Introduction 38 5.2 Effect of Telescope Geometry 39 6 Observatory Sites 6.1 Introduction 40 I 6.2 Low Earth Orbit 41 6.3 High Eccentricity Orbit 42 6.4 Heliocentric Orbit 43 6.5 Sun-Earth L1, L2 LaGrange Points 43 6.6 Halo Orbit about L2 44 7 A Lunar Based UXT 7.1 Introduction 46 7.2 The Architecture of a Lunar Based UXT 46 7.3 Components of a Lunar Based UXT 48 7.5 Construction of the Telescope 49 7.6 Construction of the Telescope on the Moon: Method 2 50 7.7 “Freezing” the Liquid 53 7.8 Uncertainty in the Assumptions 55 8 Launch and Propulsion 8.1 Requirements 57 8.2 The Journey from LEO to L2 57 9 Future Activities 9.1 Enabling technologies Required 60 9.2 Pathfinder Missions 60 9.3 Propulsion, End Note 61 10. References 62 Appendix A: Detecting Distant Quasars Appendix B: (Ion Engine Fuel Consumption and Power Levels in Station Keeping and Target Changes ) Appendix C: Lunar Glass Manufacturing and Mining References II 1. Introduction 1.1 Goal Reports of the NAS sponsored survey committees in astrophysics and physics have identified the most important questions of their disciplines. They are: what is the history of the early universe? In particular, when did the era of reionization begin and did it unfold rapidly or gradually? Were the first luminous objects, stars or AGN black holes? What is the distribution of mass in the universe and how did its structure evolve? How does matter behave in the strong gravitation field of a black hole? Do quantum gravity models succeed in unifying the gravitational force with the standard model for the strong, weak, and electromagnetic forces? Results from the Chandra X-Ray Observatory plus XMM-Newton and RXTE indicate that these questions can be addressed with considerable confidence of success in the X-ray band with telescopes that are two or three orders of magnitudes higher in throughput than the current missions or about 2 million square centimeters effective area and excellent angular resolution, about 1 second or arc. The objective of this two-year study is to develop conceptual models for what such an X- ray observatory would look like, plus how it can be developed and launched into space. We consider several options for the X-ray optics that differ radically from each other. Only one of these options is an extension or a scaled-up version of the current X-ray telescope geometry, the Wolter Type-1 optic. We have not limited ourselves to only those options allowed by current technology. Rather we identify what new technologies are needed in order for them to be feasible. This two-year study of an Ultra-High Throughput X-ray Observatory has occurred during a very eventful period in X-ray astronomy. During this time two major X-ray observatories have been operating in space, the Chandra X-Ray Observatory developed by NASA and the X-Ray Multi Mirror-Newton Observatory (XMM) of the European Space Agency. The two are complementary in that the Chandra telescope has very high angular resolution and the XMM telescope system has very high throughput. Each is the product of some fifteen years of effort and is considered among the greatest success in astrophysics for NASA and ESA. X-rays have been detected from virtually every type of astronomical object ranging from comets and planets in our solar system, clusters of galaxies, the largest known structures short of the universe itself, and active nuclei of galaxies out to very large distance, which are believed to be black holes. The Chandra deep surveys (Giacconi et al, 2001, Brandt et al, 2001) suggest that the first generation of luminous objects will carry X-ray signature that are not attenuated above 2 keV by the intergalactic medium or local dust shrouds as their visible light counterparts may be. They could be star formation regions, accreting black holes in nascent AGNs, or inverse Compton radiation from high energy particles scattering off the microwave background during a higher frequency epoch (Barkana and Loeb, 2001, Schwartz, 2001). X-rays are very specific indicators of gravitational lens effects and become increasingly effective at larger distance as probes of mass distribution (Munoz, Kochanek, and Falco, 1999). The X-ray band is optimum for testing models of quantum gravity. They predict distance dependent effects such as line broadening that are more pronounced at higher photon energies (Di Stefano et al, 2001). X-ray sources throughout a large range in z contain spectral lines as indicators. 1 Figure 1. Hubble Deep Field (left) and Chandra Deep Field (right), the HDF objects are mostly galaxies of finite size while the CDF objects are point sources, most likely black holes at the centers of distant galaxies. The level of detail that appears in the gallery of high resolution images obtained by Chandra http://chandra.harvard.edu/photo/chronological.html) is particularly impressive not only in appearance but also for providing a physical insight that is often not seen in other wavelength bands. They clearly demonstrate that high angular resolution has to be an integral feature of a very high throughput X-ray Astronomy to merit receiving the considering funding it requires. This program is a study of what should be the scope and architecture of the ultimate X-ray telescope and how to develop it while satisfying the need for both high throughput and high angular resolution. 1.2 Importance of X-ray Measurements in Astronomy X-rays provide unique information about the structure and evolution of the universe. The most distant X-rays originate from the epoch when the first objects appear. The youngest objects that can be detected in any wavelength band are likely to be X-ray emitting infant black holes at the centers of young galaxies shrouded by light obscuring dust, or high redshift gamma ray bursts and their X-ray afterglows. X-rays from later epochs reveal the growth of structure, and the evolution of the abundance of chemical elements unambiguously. The absorption properties of elements in the foreground along the line of sight to distant sources are relatively insensitive to whether they are in the gaseous or sold and to their temperature. The growth of structure in the universe can be traced up to the present by imaging the extended X-ray emissions of clusters of galaxies and the high temperature halos of massive elliptical galaxies.