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Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2021)

Chapter: Appendix J: Report of the Panel on Electromagnetic Observations from Space 2

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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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Suggested Citation:"Appendix J: Report of the Panel on Electromagnetic Observations from Space 2." National Academies of Sciences, Engineering, and Medicine. 2021. Pathways to Discovery in Astronomy and Astrophysics for the 2020s. Washington, DC: The National Academies Press. doi: 10.17226/26141.
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J Report of the Panel on Electromagnetic Observations from Space 2 EXECUTIVE SUMMARY The Panel on Electromagnetic Observations from Space 2 (EOS2) was charged to review space mission opportunities involving electromagnetic observations across the full spectrum, excluding the narrow band from the near infrared (IR) to the near ultraviolet (UV). In addition, the panel was asked to consider potential new space programs in gravitational radiation and cosmic particles. The panel discussed a wide array of white papers submitted by the community that are germane to this charge.1 In particular, the panel examined materials in support of two proposed Flagship mission concepts, Lynx and the Origins Space Telescope, that provide new capabilities for X-ray and far-IR observations, respectively, as well as a suite of Probe mission concepts covering a variety of fields. The panel requested and received Technical, Risk, and Cost Evaluations (TRACE) (see Appendix O) of both Lynx and Origins. The panel resonated with three key considerations that emerged from the community’s white papers:  A panchromatic approach to the future of space astronomy is the only way to address many of the high-priority science questions of our times.2  The current paradigm for selecting, funding, and managing Flagship missions will not support the simultaneous development and operation of the multiple observatories required to provide such panchromatic coverage. A radically different approach is needed.3,4,5,6  As the community moves forward to address more detailed and specific questions, it would be beneficial for NASA to enable and plan for a new approach of coordinated programs, involving multiple missions on multiple platforms.7,8 1 See Appendix A for the overall Astro2020 statement of task, for the set of panel descriptions that define the panels’ tasks, and for additional instructions given to the panels by the steering committee. 2 S.T. Megeath, L. Armus, M. Bentz, B. Binder, F. Civano, L. Corrales, D. Dragomir, et al., 2019, The legacy of the great observatories: Panchromatic coverage as a strategic goal for NASA astrophysics, white paper submitted to the Astro2020 Decadal Survey. 3 J. Tumlinson, J. Arenberg, M. Mountain, L. Feinberg, J. Grunsfeld, K. Sembach, N. Levenson, J. O’Meara, and M. Postman, 2019, The next great observatories: How can we get there? white paper submitted to the Astro2020 Decadal Survey. 4 J.A. Crooke, M. Bolcar, and J. Hylan, 2019, Funding strategy impacts and alternative funding approaches for NASA’s future flagship mission developments, white paper submitted to the Astro2020 Decadal Survey. 5 J. Hylan, M. Bolcar, and J. Crooke, Managing flagship missions to reduce cost and schedule, white paper submitted to the Astro2020 Decadal Survey. 6 M. Smith, “Zurbuchen: JWST Will Not Launch in March 2021,” Space Policy Online, last update June 10, 2020, Zurbuchen: JWST Will Not Launch in March 2021—SpacePolicyOnline.com. 7 N.A. Levenson, L.J. Storrie-Lombardie, and B.J. Wilkes, 2019, Scientific advancement through flagship space missions, white paper submitted to the Astro2020 Decadal Survey. 8 M. Elvis, J. Arenberg, D. Ballantyne, M. Bautz, C. Beichman, J. Booth, J. Buckley, et al., 2019, The case for probe-class NASA astrophysics missions, white paper submitted to the Astro2020 Decadal Survey. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-1

The vision this panel offers in response to these issues involves three major components that are summarized below. Table J.3, which appears at the end of this report, indicates the connections between each of these components and the high-priority science questions that emerged from the Astro2020 Science Panels. The EOS2 vision addresses 23 of the 30 questions, at least in part, while 19 are addressed extensively and in detail, or are design drivers for the program. 1. Joint X-Ray/Far IR Flagship Program: While both Lynx, an advanced X-ray observatory, and Origins, an advanced far-IR observatory, are exciting concepts, neither an X-ray nor a far-IR mission alone is sufficient to address the most pressing science of Astro2020. A combined program involving both X-ray and far-IR components would be much more powerful. The panel envisions a coordinated Flagship program, built around the especially compelling theme of studying the “cosmic dance” between black holes and galaxies—the intricate relationship between the growth of black holes in the universe and the evolution of galaxies that form and evolve around them. Because many galaxies are obscured by dust, it takes the synergy of two distinct kinds of observations to peer into their central regions: A high-sensitivity and high-angular resolution X-ray imaging mission that can detect accretion onto the black holes themselves, and a far-IR spectroscopic mission that detects and pinpoints both the effects of the intense black hole radiation and the effects of star formation and evolution on galactic energetics. For the purposes of this report, these notional missions are called “Fire” and “Smoke,” respectively. Fire and Smoke are based on the proposed flagship missions Lynx and Origins, respectively, but are scaled to fit into a single flagship program organized around the investigation of the cosmic dance science. While optimized for that central focus, however, they will still enable a broad array of other science highlighted by the science panels. This program is scientifically compelling and daring. While achievable, it poses significant technological challenges that will empower NASA to stretch U.S. capabilities well beyond the state of the art. 2. Time Domain Astrophysics: A coordinated program to ensure a continuous U.S. presence in space for the study of transient and time-variable phenomena in the universe. This could involve different platforms, ranging from a single probe to a suite of much smaller or medium size missions, potentially including foreign missions. The required capabilities include: All- sky monitoring at hard X-ray/gamma-ray energies, transient localization capability that can position events at the few arcsecond level, and fast slew and follow-up imaging and spectroscopy at ultraviolet, X-ray, and near-IR wavelengths. Much of this suite of capabilities exists now with the Neil Gehrels Swift Observatory, but it is essential that it be preserved and enhanced through new launch opportunities for the future, and that it be optimized to support the new science that will come from the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Cherenkov Telescope Array (CTA), and IceCube. 3. Early Universe Cosmology: A mission designed to provide high-precision measurements of the polarization of the cosmic microwave background at a range of frequencies. This would complement major ground-based facilities in exploring several of the greatest mysteries of fundamental physics—inflation, dark matter, dark energy, and neutrino mass, as well as the growth of structure with cosmic time. Last, the panel reemphasizes the importance of NASA maintaining a healthy balanced program of space astrophysics mission opportunities on all scales, from suborbital rocket and balloon experiments, through Astrophysics Pioneers and Explorers, and the new class of Probes. In addition, the panel endorses the currently envisioned NASA participation in the European Space Agency (ESA) upcoming major missions, the Advanced Telescope for High Energy Astrophysics (Athena) and the Laser Interferometer Space Antenna (LISA). PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-2

J.1 INTRODUCTION Panchromatic (multi-wavelength and multi-messenger) observations are the only path to disentangle and decipher the nature and history of our complex universe. The realization of that goal has been the basis of twenty-first-century space astrophysics. No single telescope alone can answer all of the most pressing questions in the field, from the nature of the Big Bang to the emergence of life on planets. The EOS2 panel is inherently the most panchromatic of the various program panels convened for the Astro2020 Decadal Survey. The charge to the panel was to review and evaluate a large suite of proposed space mission concepts designed to address astronomy and astrophysics questions primarily by means of radio, far-infrared, and high-energy electromagnetic observations from space. A wide variety of phenomena are uniquely observable in the bands under this purview, ranging from the dust and spectral line emission in galaxies prominent at millimeter and far-IR wavelengths (~15–500 μm), to the decays of radioactive nuclei visible at MeV gamma-ray energies. Although not reflected in its name, the panel was also charged with considering nonelectromagnetic investigations in space, such as those designed to detect relativistic particles and gravitational waves. The panel reviewed a total of 55 white papers from the community covering a range of diverse topics. Proposed space-based missions included experiments devoted to GeV and MeV gamma rays, hard X rays, high-resolution X-ray imaging, spectroscopy, timing, and polarimetry, far IR, millimeter and MHz interferometry, the cosmic microwave background, cosmic rays, neutrinos and gravitational waves. Other white papers made the case for technology development, including cryocooler technology, advanced X-ray optics, heterodyne receivers, and fully active telescopes in space. Several white papers addressed program balance between large and small missions and wavelength coverage. The case was made for the continued importance of Flagship missions, but with a need to control cost growth through early investment in development, and continuing assessment of technical and schedule realism. The wisdom of creating a new Probe class of missions was argued to close the gap between Flagships and Explorers. One white paper recounted the strong legacy of the Great Observatories, and it argued for panchromatic coverage as a strategic goal for NASA astrophysics. The charge to the panel also included a review of the current status of the field. Over the past three decades, many space experiments in the EOS2 relevant wavebands have been developed and launched. A summary of missions currently operating, or approved for development, is provided in Table J.1. While the broad array of missions listed suggests that there is already a wealth of observational capability across the spectrum, those missions do not possess the appropriate mix of sensitivity, or the spatial and spectral resolution, necessary to address all of the observational needs for the next decade. As the white papers emphasized, the future of the field is ripe for further investment in new facilities with significantly enhanced capabilities. This report is organized as follows: In Sections J.2 and J.3, the Flagship mission proposals that the panel reviewed, Lynx and Origins, are discussed with their TRACE analyses. In Section J.4, a rationale is provided for reformulating these mission concepts into a single Flagship program consisting of two notional missions, Fire and Smoke, which are jointly optimized for the study of the “cosmic dance,” the complex interaction of galaxies and the giant black holes at their cores. A new approach is suggested for the co-development of these missions, within realistic NASA budget profiles, that would allow them to be operating contemporaneously. In Section J.5, the Probe class of missions is discussed, and arguments are presented for the development of two targeted Probe programs, one in Time Domain Astrophysics, and the other in Early Universe Cosmology. In Section J.6, the panel discusses the balance of mission sizes and considerations for on-orbit servicing. Section J.7 is focused on two ESA missions in development, with U.S. participation: Athena and LISA. Last, a summary and final thoughts are given in Section J.8. TABLE J.1 EOS2-Related Missions Operating or in Development Mission Agency or Capabilities Spectral Coverage Expected Country Launch PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-3

Large Chandra NASA X-ray imaging and spectroscopy 0.2–10 keV Transmission grating spectroscopy 0.08–10 keV Fermi NASA γ-ray imaging and spectroscopy 30 MeV–300 GeV Spectroscopy 8 keV–30 MeV SOFIA NASA/DLR 2.7 m telescope 0.3–1600 µm IR imaging and spectroscopy XMM- ESA X-ray imaging and spectroscopy 0.15–12 keV Newton Reflection grating spectroscopy 0.33–2.5 keV UV/visible monitor 170–650 nm INTEGRAL ESA X- and γ-ray imaging and 3–35 keV; 15 keV–10 MeV spectroscopy 500–850 nm Visible monitor SRG DLR/Russia X-ray imaging and spectroscopy 0.2–10 keV; 5–30 keV HXMT China X-ray imaging and spectroscopy 20–250 keV; 5–30 keV; 1–15 keV γ-ray monitoring 0.2–23 MeV DAMPE China γ- and cosmic ray imaging and 5 GeV–10 TeV; 100 GeV–100 TeV spectroscopy Medium SWIFT NASA X- and γ- ray imaging and 0.2–10 keV; 15–150 keV spectroscopy 170–650 nm UV/visible imaging ASTROSAT India X-ray imaging and spectroscopy 0.3–100 keV UV/visible 200–300 nm; 130–180 nm Visible 320–550 nm ISS-MAXI JAXA X-ray imaging and spectroscopy 2–30 keV; 0.5–12 keV GECAM China X- and γ- ray all sky monitor 6 keV–5 MeV Small NuSTAR NASA X-ray imaging and spectroscopy 3–79 keV Mission of Opportunity ISS-NICER NASA X-ray timing and spectroscopy 0.2–12 keV Approved JWST NASA IR imaging and spectroscopy 0.6–28.3 µm 2021 GUSTO NASA IR high-resolution spectroscopy 63, 158, and 205 µm 2021 IXPE NASA X-ray polarimetry 2–8 keV 2021 SVOM China/France X- and γ- ray imaging and 0.3–10 keV; 4–150 keV 2022 spectroscopy 15 keV–5 MeV Visible imaging 400–950 nm EP China/DLR X-ray imaging and spectroscopy 0.5–5 keV; 0.3–10 keV 2022 XRISM Japan X-ray imaging and spectroscopy 0.4–13 keV; 0.3–12 keV 2022 SPHEREx NASA IR spectroscopy 0.75–5 µm 2024 HERD China/ESA γ- rays and electrons Tens of GeV–10 TeV 2025 (?) member states Cosmic rays Up to PeV eXTP China/ESA X-ray imaging 2–50 keV 2027 member states Polarimetry 2–10 keV Spectroscopy 0.5–10 keV; 6–10 keV ARIEL/CASE ESA/NASA IR spectroscopy 1.25–7.8 µm 2029 Visible/IR photometry 0.5–0.55 µm; 0.8–1.0 µm; 1.0–1.2 µm Athena ESA X-ray imaging and spectroscopy 0.3–10 keV; 0.1–12 keV Early 2030s LISA ESA Gravitational waves 2 × 10-5–3 × 10-2 Hz 2034 PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-4

J.2 PROPOSED X-RAY FLAGSHIP MISSION CONCEPT: LYNX Lynx is one of the four Flagship concepts studied by NASA in preparation for the Astro2020 Decadal Survey.9 The mission concept is based on the X-Ray Surveyor notional mission envisioned in the NASA Astrophysics Roadmap Enduring Quests, Daring Visions.10 The Lynx observatory would operate in the 0.2–10 keV energy band with 100 times higher sensitivity than the Chandra X-Ray Observatory over a much larger field of view. The mission technical requirements are defined by the three scientific pillars described below, that map directly onto many of the Key Science Questions and Discovery Areas from the Astro2020 Science Panels, especially the panels on Compact Objects and Energetic Phenomena, Cosmology, Galaxies, Interstellar Medium and Star and Planet Formation, and Stars, the Sun, and Stellar Populations. Pillar 1—The Dawn of Black Holes. Discover massive black holes (BHs) formed in the very first galaxies (z ~ 10) and determine the mechanism(s) by which they were able to so quickly assemble into the super massive black holes (SMBHs) seen at lower redshift (z ~ 6). While the host galaxies of the seed BHs will be found and characterized in deep optical and IR surveys obtained either with JWST or the subsequent Roman Observatory, only Lynx can reach the X-ray flux limits required to detect BHs (Figure J.1). Pillar 2—The Invisible Drivers of Galaxy Formation and Evolution. Characterize the diffuse baryon population of galactic halos, observe the effects of AGN feedback on galaxies, determine the state of the gas feeding the central BH, and measure the energetics and mechanics of the resulting outflows. Pillar 3—The Energetic Side of Stellar Evolution and Stellar Ecosystems. Study X-ray emission associated with stellar birth, evolution, and death over the entire initial mass range. Characterize stellar coronae to address crucial questions on planet habitability, owing to potential lethal effects of coronal activity on life on planets orbiting close to their stars. Study compact stars through surveys of X-ray binaries and supernova remnants in the Milky Way and other nearby galaxies. Lynx would also play a major role in time domain/multi-messenger astrophysics by following up LIGO merger events, studying X-ray chirps in merging SMBH systems, and monitoring the evolution of tidal disruption events. J.2.1 Instrumentation Required for Lynx The Lynx Mirror Assembly (LMA) technical requirements flow from Pillar 1. The detection of BH seeds at z ~10 translates into a detection flux limit of 1 × 10-19 erg cm-2 s-1 for a 104 M◉ BH accreting at the Eddington limit, which is ~100 times fainter than the Chandra deep fields. To meet this requirement, the LMA must achieve sub-arcsecond (~0.5" or better) angular resolution in the energy range 0.2–10 keV, with 50 times the Chandra effective area, over a factor of 10 larger field of view (FOV) (22' diameter). 9 NASA Marshall Space Flight Center, Lynx X-Ray Observatory Concept Study Report, 2019, NASA Science and Technology Definition Team, Huntsville, AL, https://wwwastro.msfc.nasa.gov/lynx/docs/LynxConceptStudy.pdf. 10 NASA, 2013, Enduring Quests, Daring Visions: NASA Astrophysics in the Next Three Decades, NASA Astrophysics Subcommittee, Washington, DC, https://science.nasa.gov/science-red/s3fs-public/atoms/files/secure- Astrophysics_Roadmap_2013_0.pdf. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-5

Permission Pending FIGURE J.1 Representation of SMBH formation models, showing the sensitivity required to detect BHs early in their evolution (z > 10). Athena (see description in Section J.7.1) begins to detect such sources only when the BHs reach sizes of 107 M. Lynx can see them much earlier in their history. SOURCE: Lynx concept study. FIGURE J.2 Simulated deep surveys of 3' × 3' (2 percent of the total area) fields. Left: Athena (5" resolution). Middle: Lynx (0.5"). Right: JWST (0.1"). Lynx would not be affected by source confusion. Every X-ray source in the Lynx FOV can be uniquely identified with its host galaxy. SOURCE: Lynx concept study. Courtesy of the Lynx Team. The High-Definition X-Ray Imager (HDXI), the Lynx wide-field imager, would be a silicon detector array with ~100 eV spectral resolution and 0.3" pixels (0.2–10 keV); it would provide good sensitivity and spatial resolution across the FOV and would exploit the full imaging potential of the optics (Figure J.2). The Lynx X-Ray Microcalorimeter (LXM) would provide high-resolution imaging spectroscopy of point and extended sources using large arrays of microcalorimeters. It would comprise three different arrays: (1) Main Array: FOV 5', pixel size 1", energy resolution ~3 eV (R ~ 2000 at 6 keV); (2) Enhanced Main Array: FOV 1', pixel size 0.5", energy resolution ~2 eV (R ~ 3000 at 6 keV); (3) Ultra High Energy Resolution Array: FOV 1', pixel size 1", energy resolution ~0.3 eV (R ~ 2000 at 0.6 keV). PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-6

The Lynx X-Ray Grating Spectrometer (XGS) would provide an order of magnitude higher spectral resolving power (R ~ 7500 over the whole energy range) than the Chandra and XMM-Newton grating instruments, and greater than 500 times higher throughput at soft X-ray energies. In summary, Lynx would provide several orders of magnitude in sensitivity enhancements for X- ray imaging and spectroscopy over both Chandra and ESA’s (approved) Athena X-ray mission. While Athena matches the collecting area of Lynx, the 5" goal for Athena optics limits point source detection sensitivity (Figure J.2), which precludes the study of BH formation and evolution in the high-redshift universe. Athena’s combination of moderate angular and (microcalorimeter-like) energy resolution is sufficient for point and significantly extended sources, but the Lynx combination of high throughput, 10 times better angular resolution, and ultra-high spectral resolution, is crucial for detailed studies of galactic and circumgalactic environments. J.2.2 Technology Drivers and Associated Risks The primary Lynx technology driver is the development of the X-ray optics. These require angular resolution ~0.5" (HPD) and large effective area (2 m2 at 1 keV). The Lynx team selected the most mature technology, Silicon Meta-Shell Optics, for the Design Reference Mission (DRM), while maintaining full shell and adjustable segmented optics as risk reducing, potential “breakthrough” alternatives. The baseline approach exploits monocrystalline silicon segments and a highly modular design that achieves the requisite area and angular resolution by integrating tens of thousands of mirror segments into hundreds of mirror modules, all requiring alignments to a fraction of an arcsecond. The Silicon Meta-Shell approach has been demonstrated with mirror pairs mounted to 1.3" (HPD) at 4.5 keV, which, when corrected for gravity sag, suggests that sub-arcsecond performance is indeed achievable. Currently, this technology is at Technology Readiness Level (TRL) 3, with good documented progress on the fabrication, coating, alignment, and bonding tasks required to reach TRL 4. Nevertheless, the highly modular approaches required to fabricate a complete optic, while maintaining both very high angular resolution and very large area, pose a major challenge and add considerable risk to the program. The mirror segments and modules need to be manufactured on an unprecedentedly large scale, while utilizing rapid, reproducible precision fabrication and optical alignment processes. The Lynx team clearly recognizes this industrialization challenge and the need for tackling it early, as evidenced by publications and by a recently established contract with industry partners experienced in large-scale fabrication. J.2.3 TRACE Analysis of the Lynx Proposal A TRACE analysis of the Lynx program plan was performed by the Aerospace Corporation to provide an independent assessment of the cost, schedule, and risk baseline. The TRACE assessment indicated that the Lynx project team’s cost estimate of $6.2 billion ($ fiscal year [FY] 2020) has ~11 percent probability of not being exceeded. The TRACE cost estimate at 70 percent probability is $9.0 billion ($FY 2020), 45 percent higher. Additionally, the TRACE analysis found the Lynx schedule estimates for both Pre-Phase A to Phase A and for Phase B through launch to have a probability of not being exceeded of 22–25 percent. A 70 percent confidence schedule was estimated to require 6.75 years of technology development and Phase A formulation, and 11.5 years for Phase B–D development and launch, compared to the Lynx team estimates of 5 years for technology development and Phase A formulation, and 10 years for Phase B–D development and launch. The primary differences between the TRACE analysis and that of the Lynx team were associated with the technology development and the flight build and integration of the LMA, which was assessed to be the highest risk area in the TRACE report. This assessment of risk is consistent with the opinions of this panel. As discussed above, the Lynx team has proposed continuing development of three independent X-ray mirror technologies. The panel agrees that the Lynx team has underestimated the complexity of PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-7

industrialization and integration/alignment of the optics, but also believes some savings relative to the TRACE estimate could be achieved by downselecting among the available technologies earlier. The other driver for the difference between the TRACE and Lynx estimates is in the area of reserves and margin, both during the Pre-Phase A and Phase A technology development and formulation phases, and during Phases B–D. When excluding reserves from both estimates, the Lynx subsystem development estimates are consistent with 40 percent confidence in the TRACE analysis. Overall, the $2.8 billion increase in the total ($FY 2020) cost in the TRACE estimate contained an additional ~$800 million for technology development, including reserves, and $1.5 billion in Phase B– E reserves for potential schedule underscope and complexity growth, plus additional increases for management, systems engineering, and mission assurance. Based on the actuals used to perform the TRACE analysis, this panel believes the higher estimates for the Lynx cost and schedule are likely to be closer to reality than the Lynx team estimates. Additionally, implementing either the Lynx team $6.2 billion ($FY 2020), 15-year program or the TRACE $9 billion ($FY 2020), 19-year program, would require peak-year funding in excess of $1 billion ($ real year [RY]) for multiple years. Assuming the $500 million ($750 million) per year nominal (aspirational) NASA Astrophysics budget for new missions that was presented to the panel, an additional 3+ years would be required. J.3 PROPOSED FAR INFRARED FLAGSHIP MISSION CONCEPT: ORIGINS SPACE TELESCOPE The Origins Space Telescope is the second major Flagship mission concept that falls within the purview of the EOS2 panel.11 The mission concept is based on the Far-IR Surveyor notional mission envisioned in the NASA Astrophysics Roadmap Enduring Quests, Daring Visions.12 It would incorporate a 5.9 m cryogenic space telescope, operating from 2.8 to 588 μm, and deliver > 1000 times higher sensitivity than previous far IR/submillimeter missions. Origins science would address many of the key science questions and discovery areas identified by the Astro2020 Science Panels, especially the panels on Galaxies; the Interstellar Medium and Star and Planet Formation; Stars, the Sun, and Stellar Populations; and Exoplanets, Astrobiology, and the Solar System. Origins has three main science themes, described below, of broad interest to scientists and laypersons alike. The Origins mission core science cannot be addressed with complementary observations in other wavelength bands. Theme 1—How do galaxies form stars, make metals, and grow their central SMBHs from reionization to today? The cooling processes central to the earliest phases of galaxy formation are largely facilitated by line emission observable in the far IR. Origins would enable spectroscopic observations in this band that provide an especially powerful probe of both BH irradiation and star formation. Both processes are well traced by the rich collection of far-IR line emission from various atoms, ions, and molecules (see Figure J.3). Origins traces the assembly and growth of galaxies, stellar populations, and gas phase metallicities through far-infrared spectroscopy. It also measures the gas inflowing to and outflowing from the galaxy, driven primarily by supernovae and AGN activity. These velocity-resolved spectra of species, such as OH, thereby trace the link between the buildup of BH mass and stars in galaxies across cosmic time. Theme 2—How do the conditions for habitability develop during the process of planet formation? Stars form within molecular clouds through accretion-disk-like structures onto protostellar cores. Planets form from the residual protoplanetary disk. However, key questions remain about the role of hydrogen in gas giant formation, the role of water in the formation of habitable planets, and how the major building 11 NASA Goddard Space Flight Center, 2019, Origins Space Telescope Mission Concept Study Report, Astrophysics Science Division, Greenbelt, MD, https://asd.gsfc.nasa.gov/firs/docs/OriginsVolume1MissionConceptStudyReport25Aug2020.pdf. 12 NASA, Enduring Quests, Daring Visions: NASA Astrophysics in the Next Three Decades, 2013, NASA Astrophysics Subcommittee, Washington, DC, https://science.nasa.gov/science-red/s3fs-public/atoms/files/secure- Astrophysics_Roadmap_2013_0.pdf. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-8

blocks of life get delivered to habitable planets. Origins would address these questions again through far IR spectroscopy. Line strengths trace gas mass and excitation, and velocity resolved observations tomographically place the line emission radially in the protoplanetary disk assuming Keplerian orbits (see Figure J.3). Theme 3—Do planets orbiting M-dwarf stars support life? The search for life on exoplanets involves spectroscopy of the atmospheres in the “habitable zone” around main sequence stars. Measuring the abundances of species that can be linked with biology (e.g., O2, O3, CH4, CO2, H2O, N2O) that are clearly out of chemical equilibrium will be a signal for life. Origins would contribute to this field through transit spectroscopy in the mid-IR for Earth-like planets orbiting M or K dwarf stars. Transits by lower mass main sequence stars have greater depth, both owing to the relatively low flux from the parent star, and the relatively small radius of the habitable zone for M/K dwarf stars. J.3.1 Instrumentation Required for Origins The Origins Survey Spectrometer (OSS) would be a very broadband (25–588 μm) long slit grating spectrometer with resolving power R = 300, sufficient to separate line from continuum in the far-IR fine structure lines. Its sensitivity would allow the measurement of primary diagnostic lines for star formation from galaxies in the epoch of reionization at z > 6, as well as lines such as [OIV] 25.9 μm and [NeV] 14.3, 24.3 μm that measure both BH mass and accretion rates in the important 8–10 redshift interval when 104 –105 M primordial seed BHs grew by accretion to the >106 M◉ BH cores of the first galaxies. OSS would also include a Fourier transform spectrometer (FTS) placed in front of the grating to achieve resolving powers up to 43,000*(112 μm/λ), enabling both velocity-resolved spectra of galactic inflows and outflows (require R > 5000), and velocity resolved tomography of spectral lines from proto-planetary disks. A very-high-resolution “Etalon” stage (R ~325,000*[112 μm/λ]) would resolve protoplanetary disk spectral lines, enabling detailed studies of disk properties and the deposition of critical materials for the build-up of terrestrial planets. The Far-IR Imager Polarimeter (FIP) would be an imaging polarimeter operating at two bands, 50 and 250 μm. With 8000 pixels, it would be capable of wide field (> 1000 deg2), diffraction-imited imaging that would address a variety of important astrophysical topics from the evolution of star formation over cosmic time to transient follow-up and monitoring. The Mid-infrared Spectrometer Camera Transit Spectrometer (MISC-T) would be a low- resolution (R = 50 to 300) imaging spectrometer delivering simultaneous spectra from 2.8 to 20 μm. Its state-of-the-art detectors and pupil densification would achieve the 5 ppm precision and stability necessary to detect the spectral lines that are biosignatures from exoplanets transiting main-sequence M and K stars. In summary, Origins would provide unique or substantially enhanced capability with respect to all previous and planned FIR and submillimeter wave facilities. The OSS would be orders of magnitude more sensitive for spectral line surveys of high-redshift galaxies—1000 times more sensitive than Herschel. The only other proposed mission in this wavelength band was the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), a 2.5 m, cold telescope that was a candidate for ESA's M5 selection, but was canceled before selection. The Atacama Large Millimeter Array (ALMA) can directly resolve protoplanetary disks, but Earth’s atmosphere prevents ALMA observations of both HD and neutral oxygen, and limits water observations to three high excitation lines. In the area of transit spectroscopy of low-mass stars, the MISC-T would have significantly higher stability than JWST, in the crucial 5–10 μm band. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-9

FIGURE J.3 Top: Infrared Space Observatory (ISO) far-IR spectrum of the Circinus galaxy showing spectral features detectable by Origins OSS at redshifts up to z = 8. The color coding indicates lines that are sensitive to AGN activity, feedback, and star formation. Bottom: Line-tomography example from protoplanetary disks. Assuming Keplerian orbits, the velocity resolved spectra of protoplanetary disks (left) reveal the location of important gas phase building blocks for planets through their line profiles (right). SOURCE: NASA Goddard Space Flight Center, 2019, Origins Space Telescope Mission Concept Study Report, Astrophysics Science Division, Greenbelt, MD. Courtesy of M. Meixner et al., 2019, arXiv:1912.06213. Reproduced with permission. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-10

J.3.2 Technology Drivers and Associated Risks The Origins concept would incorporate a 5.9 m primary mirror, cooled to 4.5 K, the requisite low temperature for reducing the FIR thermal photon background that would otherwise limit system sensitivity. The FIR focal plane employs superconducting transition edge sensor (TES) bolometers with multiplexed SQUID readout. The mid and near IR employs Si:As and HgCdTe arrays with an integrated readout. The Origins team identified the primary mirror, the cryogenic system, and the detectors as the primary technical challenges in the mission, and the panel agrees with that assessment. The TES bolometer arrays are of principal concern, given the large number of pixels required for the OSS, and the high degree of multiplexing involved. While prototype arrays with close to the required characteristics have been fabricated, these are not available at the production rate and high yield required to limit cost risk. J.3.3 TRACE Analysis of the Origins Proposal A TRACE analysis of the Origins program plan was also performed by Aerospace Corp to provide an independent assessment of the cost, schedule, and risk baseline. The TRACE assessment indicated that the Origins project team’s cost estimate of $7.4 billion ($FY 2020) has ~25 percent probability of not being exceeded. The TRACE cost estimate at 70 percent probability is $10.6 billion ($FY 2020), 43 percent higher. Additionally, the TRACE analysis found that the Origins schedule estimates for both Pre-Phase A to Phase A and for Phase B through launch have a probability of not being exceeded of 42–47 percent. A 70 percent confidence schedule was estimated to require 6.25 years of technology development and Phase A formulation, and 9.3 years for Phase B–D development and launch, compared to the Origins estimates of 5.25 years for technology development and Phase A formulation, and 8.3 years for Phase B–D development and launch. The primary differences between the TRACE analysis and that of the Origins team were in threat and reserve estimates, driving $56 million ($FY 2020) of the $88 million ($FY 2020) increase of the TRACE Pre-Phase A/Phase A estimate, and $2.75 billion ($FY 2020) of the $3.1 billion ($FY 2020) increase of the TRACE Phase B–E estimate. The schedule discrepancy between the two analyses was driven by the OSS instrument development and integration time, that the TRACE team estimated would require an additional ~1.5 years. OSS development was also identified as the critical path by the Origins team. Although the allocation of reserves drove the TRACE analysis to predict a higher cost for Origins, the panel notes that the TRACE analysis predicted lower costs for the OSS and FIP instruments as compared to the Origins team estimates. The large difference in threat and reserve estimates is likely driven, at least in part, by the ~2000 kg difference in TRACE (higher) versus Origins team maximum expected value (MEV) mass. Overall, this panel expects that with careful management the cost to implement Origins is likely somewhere between the Origins team estimate and the TRACE estimate. As in the case of Lynx, implementing Origins within the schedules defined by either the project team or the TRACE analysis will require far in excess of the $500 million ($750 million) per year nominal (aspirational) NASA Astrophysics budget for new missions. The TRACE technically limited funding profile for Origins would require peak-year funding in excess of $1.45 billion ($RY) for multiple years. Executing this program would require reformulating the program plan to fit within a reasonable maximum annual funding limit. This would likely cause a significant increase to the duration of Phases B–D. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-11

J.4 THE COSMIC DANCE VISION: A JOINT X-RAY/FAR-IR FLAGSHIP PROGRAM As the panel reviewed the science of Lynx and Origins, it became clear that the missions complemented one another strongly. Neither mission by itself can address the full range of key science questions identified by the Astro2020 Science Panels, but together, they provide the required capabilities. When X-ray and FIR observations are both available, the whole is definitely more than the sum of its parts. However, as the TRACE analysis clearly demonstrated, if NASA were to pursue these two missions in series, the launches would be > 20 years apart, seriously delaying—if not inhibiting—the full science return. In the unanimous judgment of this panel, it is more important and more scientifically valuable to have contemporaneous advanced capability in both the X-ray and the FIR bands, than it is to have the full capability of either Lynx or Origins by itself. Inevitably, some reoptimization would be necessary. Such an effort would focus on a central scientific theme, to enable the difficult decisions as to which elements of the present observatory designs are essential to maintain. Clearly, fitting two Flagships into one program cannot be accomplished without narrowing of scope to maintain cost reality. The synergy between powerful X-ray and FIR observations is particularly strong in the study of the fundamental and intricate cosmic dance between star formation in galaxies, and the growth of their central BHs from the earliest times (see Section D-Q3). Each regulates the other, and their mutual evolution is an ecosystem that must be studied in its entirety from formation epoch to the present. This interplay is indeed one of the three pillars of Lynx, and one of the three themes of Origins. The X-rays detect the “fire,” the central source of energy produced by accretion onto the BH, while the FIR detects the “smoke,” the effect on the surrounding environment owing to the central irradiation, and its consequences on the galactic star formation and evolution. Because of the importance of this science, and the fact that it so strongly benefits from contemporaneous X-ray and FIR observations, the panel has chosen this to be the central defining theme of a joint X-ray/FIR program. The cosmic dance began in the first billion years of the universe with first light—the ignition of the very first stars, and the formation of galaxies from these stars along with their central BHs—via a process that still remains completely unknown. This epoch of reionization, or “cosmic dawn,” is presently at the very frontier of astronomical research, with the detection of starlight from a few z > 7 galaxies recently detected by Hubble and Spitzer. It is thought that these first galaxies form at local overdensities in the dark matter distribution by the accretion of almost pure hydrogen/helium gas, cooling through lines in the far-IR to sub-mm bands and finally forming stars or “seed” BHs, either from zero-metallicity Pop III stars (~100 M) or via some form of direct collapse (~104 M). The lack of heavy elements (Z/Zsol 10-3) during the first billion years (z > 6) appears essential to the formation of both Pop III stars and BH seeds. The growth of these seeds proceeds either by (gravitational-wave-emitting) mergers and/or by accretion. The ESA mission LISA will “hear” SMBH mergers out to cosmic dawn at low masses, providing a complementary multi-messenger insight into this process. Studying the first light epoch sets the bar for new key capabilities in the X-ray and FIR, well beyond what is available today, and even beyond all the planned missions for the next decade (Figure J.4). These are (1) the ultra-deep (10-19 ergs cm-2 s-1) X-ray sensitivity limit over a large FOV (~1 sq. deg in ~25 Msec); and (2) the ability to undertake FIR broadband, highly multiplexed, spectroscopic surveys. Two of the Probe concepts, the Advanced X-Ray Imaging Satellite (AXIS) and the Galaxy Evolution Probe (GEP), were proposed to pursue some of this science using similar technological approaches to Lynx and Origins, respectively, albeit in smaller implementations scaled to fit within the $1 billion cost cap for Probe missions. Neither meets the required capabilities outlined above: AXIS has only one-third the collecting area of Lynx, and it is proposed as a 5-year mission. Its deep survey is 2–5 times less sensitive than that proposed for Lynx, missing the required sensitivity target by factors of 3–4 (depending on exposure times). AXIS would fail, therefore, to detect BH seeds at z > 6 and discern subtle features in PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-12

the X-ray luminosity function that are essential to this science.13 GEP incorporates a smaller mirror than Origins, which reduces its sensitivity by a factor of 9, putting high-redshift (z > 2) galaxies and AGN out of reach. More importantly, its mapping speed is down by a factor of 50 compared to Origins, and the long wavelength cutoff adopted for its spectrometer (193 μm) means that important diagnostic lines cannot be detected for redshifts greater than 2.5, and its spectral resolution (R=200) is much too low to velocity-resolve the gas flowing into and out of galaxies. In summary, a Flagship program is clearly required to enable the cosmic dance science. The panel envisions reoptimized versions of Lynx and Origins that preserve the essential capabilities, which for the purposes of this report are called Fire and Smoke, respectively. Both missions would be developed together as a single program, and launched contemporaneously, with a common science team to enable evaluation of trades both within and between them. While optimized for studying cosmic dance science, Fire and Smoke would also enable a broad range of high-priority science in other fields (as illustrated in Table J.3).   FIGURE J.4 FIR to X-ray spectral energy distribution for a black hole seed at z = 9. The Origins sensitivity curve is the thick red line with arrows on the upper left, the Lynx sensitivity curve is the thick green line on the lower right. Lynx detects all stages of black hole growth from 5 Myrs (thin, solid red line) after accretion begins, while Origins detects the later stages (> 75 Myr, thin green line). The Origins OSS sensitivity plotted is binned to a resolving power (RP) of 3. The [OIV] 25.9 um line is detectable by Origins at the native resolving power of 300 and is an important diagnostic of black hole mass and accretion rates. SOURCE: Adapted from F. Pacucci et al., 2019, Detecting the birth of supermassive black holes formed from heavy seeds, Bulletin of the AAS, 51(3). Retrieved from https://baas.aas.org/pub/2020n3i117. Reproduced with permission. 13 A. Ricarte and P. Natarajan, 2018, The observational signatures of supermassive black hole seeds, Monthly Notices of the Royal Astronomical Society, 481(3):3278–3292. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-13

J.4.1 Achieving the Vision The Flagship Program of Fire and Smoke is scientifically compelling. Successful realization of its scientific promise will require careful formulation and management in a cost-constrained environment. Both the total cost and the funding profile are important, and related, as inability to meet a planned funding profile results in stretching of the schedule and increased cost. Based on the analyses performed in conjunction with the TRACE process, the execution of an Astro2020 Flagship at the $10 billion level requires funding authorization of at least $750 million for several years. In the assessment of this panel, a cost cap and annual funding peak in these ranges would enable launches of both Fire and Smoke by 2040. To do so, however, will require a different approach to the execution of Flagships than has generally been followed. The design and development of new capabilities and the execution of the program must be undertaken from beginning to end in a constrained environment. The panel envisions a single program office, with full integration of the science and engineering teams to make hard choices and trades, and to ensure that science per dollar is always maximized and program constraints are maintained. As the design is concurrently matured, special attention needs to be paid to identifying where performance could gracefully degrade and still allow scientific advancement, versus where potential technology or performance limitations would cause step- function degradation in observational sensitivities. With the critical science and key observations required to address that science identified, the program would proceed to advance the requisite technologies, including manufacturability, scalability, and industrialization as appropriate. As emphasized in the 2017 report, Powering Science,14 providing sufficient funding up front to enable such studies is the best way to retire risks prior to mission confirmation. Maintaining cost and schedule caps for the Pre-Phase A and Phase A periods, the program would then proceed into execution (Phases B–D) only if the results of the technology development and mission planning demonstrate a viable program, inclusive of sufficient cost and technical reserves. With the exception of the period of capability development, this is the approach now followed for competed missions that from inception to completion live in a constrained environment. That has been shown to achieve the desired effect—maximizing science per dollar while remaining within constraints. Applying the same philosophy to an initial period of capability development (technology, scalability, and manufacturability) would extend this well-demonstrated method to Flagships. There are indeed a few such success stories at this scale. AXAF (Chandra) was restructured to cost, and it came within a few percent of that cost 7 years later. SIRTF (Spitzer) is a more complete example, as it was reset as a cost- constrained mission, completely redesigned to maximize science in a constrained environment, and succeeded. To ensure the feasibility of this approach for Fire and Smoke, the panel examined the data received through the TRACE analysis of Lynx and Origins, as well as the reports for AXIS and GEP. Recognizing that this panel cannot and should not design the cosmic dance program, the top-level cost scaling exercise described below was performed to ensure the feasibility of executing both Fire and Smoke concurrently, and within the available NASA budget. Lower Bound. Although they are inadequate as proposed to fully explore the cosmic dance, the AXIS and GEP Probe concepts can be used to derive a reasonable lower bound for Fire and Smoke. The proposed costing for these missions, at ~$1 billion each as described in the concept reports and white papers, also assumed all requisite technology has achieved ~TRL 5 or 6 prior to mission start. Addressing the necessary technology development to achieve cosmic dance science by implementing the ~$1.1 billion technology development program defined for Lynx and Origins, and recognizing these Probe-class programs would still need to be scaled up somewhat following the incorporation of this new technology, this panel found that a reasonable lower bound for the total cosmic dance program cost is on the order of a few billion dollars. While this lower bound on the cosmic dance program cost might conceivably overlap 14 National Academies of Sciences, Engineering, and Medicine, 2017, Powering Science: NASA’s Large Strategic Science Missions, The National Academies Press, Washington, DC. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-14

the cost of AXIS and GEP combined, the broader scope of the science addressed, the need for coordinated development of Fire and Smoke to make science and instrument trades, and the longer mission life all argue that this is a Flagship-class mission. Upper Bound. The panel was able to perform a more comprehensive cost estimation for Fire and Smoke by utilizing the TRACE estimates for the individual instruments required to accomplish the cosmic dance science. For Fire, this would be a wide-field imaging detector, while for Smoke, it would be a spectroscopic survey instrument. In addition to directly using instrument costs, the remaining flight system cost estimates were scaled roughly by mass for the slightly smaller implementations that would be acceptable given the necessary sensitivity limits quoted above. Costs were scaled in this way based on discussions with the Aerospace Corporation, which indicated that mass is the primary driver in the cost models incorporated in the TRACE process. The main difference between the TRACE analysis of Lynx and Origins and the cost estimates contained within the program reports was the addition of reserves and uncertainty based on prior program actuals, representing an increase in potential cost. In this costing exercise for a potential Fire and Smoke implementation, the same wrap rates for reserves and uncertainty recommended in the TRACE reports were maintained. The panel estimates that Fire could be executed for $4.9 billion, and Smoke could be executed for $4.5 billion, inclusive of technology development. Because this process maintained full margin and uncertainty, this panel considers $9.4 billion to be a reasonable 70th percentile upper bound for this two-mission program. Further efficiencies in program cost and execution can likely be gained from additional optimization of the joint mission design in a cost constrained environment, and from potential international contributions. Notional time-phasing of the costs from this analysis are displayed in Figure J.5. Assuming Pre- Phase A technology development can begin in 2022, launch of both Fire and Smoke could be as early as 2038, ensuring the desired contemporaneous operations of the two missions required to address the cosmic dance science. FIGURE J.5 Notional cost profiles ($FY 2020) for the various elements of the Cosmic Dance Flagship Program, assuming a 2022 start to the technology development. Note that the peak-year funding level is consistent with a $750 million per year funding cap in $FY 2020. J.5 PROBE-CLASS MISSIONS The current NASA Astrophysics program includes missions ranging from multi-billion-dollar Flagships, to Medium-Class Explorers (MIDEX) up to ~$300 million, to Small Explorers (SMEX) up to PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-15

~$150 million, to Missions of Opportunity (MoOs) up to $75 million, to SmallSats around $20 million to $35 million, to the recently announced Astrophysics Pioneers up to $20 million, and last, suborbital balloon flights and CubeSats in the $1 million to $10 million range. This mission suite includes a striking “hole” in the cost range from $300 million to several billion. That hole can be filled with the proposed Probe class. Several groundbreaking missions have fallen in this cost range in the past (e.g., Compton, COBE, RXTE, Fermi, Spitzer, Kepler, with Compton and Spitzer being two of the four Great Observatories). Twelve white papers describing concepts for Probes were submitted to EOS2, encompassing the full range of the electromagnetic spectrum, as well as other messengers. They collectively demonstrate the great potential of Probe-class missions. Table J.2 briefly lists the Probe concepts that the EOS2 panel reviewed. Almost all of the science topics they cover have been highlighted in the Science Panel Key Questions. Assessing these inputs, it was evident to the panel that a wide range of strong mission concepts, including some that would achieve greater than order of magnitude leaps in performance, would likely be submitted in response to a new Probe competition. TABLE J.2 EOS2-Related Probe-Scale Mission Concepts Mission Lead Closest Science Capabilities Spectral Concept Author Predecessor Coverage FARSIDE Burns N/A z > 10 neutral hydrogen and SETI search 200 kHz–40 MHz on lunar far side; exoplanets; heliophysics PICO Hanany Planck CMB polarization anisotropy 21–799 GHz CMB Spectral Kogut FIRAS CMB spectral distortions 10–6000 GHz Distortions GEP Glenn Spitzer, Star formation and SMBH growth over 400–10 µm Herschel cosmic time TSO Grindlay N/A UV–mid-IR time domain astronomy 5.0–0.3 µm follow up AXIS Mushotzky Chandra, Growth and fueling of SMBHs; transient 0.3–10 keV Athena universe; galaxy formation and evolution STROBE-X Ray RXTE Compact objects; X-ray counterparts; time 0.2–50 keV domain astronomy HEX-P Madsen NuSTAR Accreting compact objects; extreme 2–200 keV environments around black holes; neutron stars TAP Camp Swift Time-domain astrophysics 0.4 keV–1 MeV AMEGO McEnery Compton, Multi-messenger; γ-ray studies of neutron 200 keV–10 GeV Fermi star mergers; supernovae; flaring AGN POEMMA Olinto N/A Ultra high-energy cosmic rays and cosmic Cosmic rays > 2 × neutrinos from space 1019 eV Neutrinos > 20 PeV MFB Michelson N/A Fills gaps in frequency coverage between Gravitational waves LIGO and LISA 10 mHz–1 Hz Institution of a Probe class of missions would enable a broader NASA Astrophysics program, more balanced in size, cost, wavelength, and messenger coverage, that would better address the extraordinary range and richness of 21st century astrophysics. A truly open competition would be most responsive to the community and would enable new opportunities. However, in the view of the panel, a more limited competition, focused on particular strategic areas, may be appropriate for the first Probes, given some pressing priorities in the field. Such a strategic competition approach has been invoked in the past for Planetary Science New Frontiers missions. Below, two high-priority areas for strategic Probe competitions are identified: Time Domain and Multi-Messenger Astrophysics, and Early Universe Cosmology. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-16

J.5.1 Time-Domain and Multi-Messenger Astrophysics In several of the science panels, time-domain astrophysics emerged as a key scientific priority for the next decade (see Table J.3, especially the entries for compact objects and energetic phenomena). With the first LIGO/VIRGO detections of gravitational wave events, and new exciting results from cosmic ray and neutrino detectors, it is now clear that astronomy is no longer restricted to the electromagnetic spectrum; it is the era of multi-messenger astrophysics. The current modest rate of transient events will increase dramatically in the near future with planned advancements to LIGO, and with the onset of operations of the Vera C. Rubin Observatory (former LSST), that will generate 10 million time-domain alerts per night. Sifting through that data stream to identify and follow up the most exciting transients will be a major challenge to the existing system of observatories, at all wavelengths. At present, the U.S. “workhorse” observatory for space-based time-domain studies is the Neil Gehrels Swift Observatory. Swift is an aging medium-size Explorer mission. It was launched in 2004, and although it has no expendables, its future longevity is uncertain. Outside the United States, a few other time-domain missions (see Table J.2) are now under development, notably the Gravitational Wave High- Energy Electromagnetic Counterpart All-Sky Monitor (GECAM), a Chinese mission (launched December 2020), and the Space Variable Objects Monitor (SVOM), a French-Chinese mission (2021 launch). The former combines two spacecraft 180 degrees apart, thus providing true 100 percent sky coverage in the 6 keV–5 MeV band. GECAM will immediately distribute 1-degree localizations of gamma-ray bursts (GRBs) to the community for follow up observations. SVOM, on the other hand, will be equipped with soft X-ray, gamma-ray, and optical instruments that will enable follow-up on board the mission. It will also promptly distribute GRB positions together with their main prompt gamma-ray properties, and the magnitude of their early afterglows. However, neither mission will have significant, if any, U.S. involvement, and the pair of them will not address all of the needs of the future time-domain program. While these two missions will certainly contribute to the study of time-domain phenomena in the universe, they will not adequately serve the U.S. community that has pioneered this field over the past few decades. Therefore, new NASA-led time-domain missions with enhanced capabilities are urgently needed, both to ensure long-term continuity in this developing core field and to successfully capitalize on the science that will come from advanced gravitational wave detectors and the Rubin Observatory. Space-based platforms provide access to those bands that are undetectable from the ground: gamma-rays, X-rays, ultraviolet, and the mid- to far-IR. Historically, these bands have proven crucial to transient event detection, as well as event characterization and classification. A future system needs to include the following features: (1) detection capability at X-ray/gamma-ray energies with near 4π sr coverage; (2) prompt event localization at the few arcsecond level or better; (3) rapid-slewing for follow- up imaging and spectroscopy at X-ray, ultraviolet, and IR wavelengths; (4) long-term monitoring in these same bands; and (5) a data system capable of issuing fast alerts to the community with all essential information. Rather than advocate for a specific mission in this field, the panel suggests instead that NASA create a coordinated strategic program in time-domain astrophysics that provides the capabilities described above, potentially capitalizing on the international missions that are operational. These could be achievable with a mix of different implementations: either a single Probe mission or a suite of medium- and small-size experiments that address some of the requirements. It is essential, however, that all of those capabilities are simultaneously available in space, so coordination in the mission developments and launches is of paramount importance. A competitive selection of the mission architecture based on targeted NASA research announcements that explicitly call for proposals to meet these objectives would harness the ability of the broad community to devise creative solutions to achieving the science at the lowest cost. That is a new paradigm for NASA Astrophysics, but the panel believes it is required to meet the science needs of this field. An appropriate total life-cycle cost of this coordinated program would be up to $1 billion over the decade. This estimate comes from the costing analyses of specific Probe-class mission studies that meet PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-17

these science goals, as well as from the panel members’ own knowledge of various Explorer-class proposals. During the course of the EOS2 panel’s deliberations, a time-domain astrophysics working group was established within Astro2020 to raise and address issues that are common to multiple program panels. The working group identified a suite of policy suggestions for this area of research to be most productive. Of particular interest to EOS2 are (1) the need for an open data policy for time-domain alerts on both NASA and NSF public facilities; (2) a suggestion that future NASA research announcements include requirements for time-domain capabilities; (3) an explicit recognition of the importance of simultaneous operation of multiple missions and facilities broadly covering multi-messenger astrophysics; and (4) consideration of the fact that time-domain investigations typically involve large collaborations, so that large author lists do not adversely affect the careers of promising young scientists in this field. J.5.2 Early Universe Cosmology and Fundamental Physics As detailed by the Panel on Cosmology, tremendous progress has been and continues to be made on observational and experimental data to study profound and fundamental questions about the universe on the grandest scale. The results have led to a simple empirical “concordance cosmological model” that unifies a wide range of cosmological phenomena, agreeing well with observational results that have improved by orders of magnitude over the past two decades. These results, however, do not obscure the fact that the key ingredients of that model—inflation, dark matter, neutrinos with nonzero mass, and dark energy—are not naturally explained by the “standard model of particle physics,” which has been equally successful at accounting for the properties of particle collisions at high energy accelerator facilities. The age-old quest to understand the Universe on the grand scale is far from over. To address the major science questions identified by the Panel on Cosmology, the cosmic microwave background (CMB) remains the single most important phenomenon that can be observed (see Table J.3). The CMB is the oldest light in the universe, emitted 13.8 billion years ago from an expanding spherical surface now 45 billion light years in radius. It is the cold, 2.7 K afterglow of the Big Bang. The CMB is a direct probe of physical conditions in the early universe, 370,000 years after the Big Bang. It is also a backlight to everything else observed in the intervening space and time. An enormous amount has already been learned from CMB measurements, and there is much that remains to be learned on topics ranging from fundamental physics to the formation and evolution of galaxies and clusters of galaxies, as highlighted as a Discovery Area by the Panel on Galaxies. At large angular scales, there is the prospect of detecting and characterizing relic gravitational waves from the Big Bang through their effect on CMB polarization. This has major implications for cosmology, since it provides insight into a critical phase when the infant Universe expanded by a factor of ~1026 in 10-32 s. It is also important for fundamental physics as it gives a new handle on particle interactions at energies forever unattainable in terrestrial laboratories. At smaller angular scales, precision measurements of temperature and polarization anisotropies of the CMB will determine the sum of neutrino masses, map the location of the dark matter in the Universe, find tens of thousands of galaxy clusters out to the highest redshifts, and, in combination with observations of galaxies at shorter wavelengths, (e.g., by DESI, Rubin Observatory, Euclid, the Roman Space Observatory, and SPHEREx) illuminate the evolution of the entire universe over cosmic time. The absolute temperature spectrum of the CMB also contains unique information about the universe. The history of the CMB field is that of continuously improving ground and sub-orbital experiments, punctuated by comprehensive measurements from satellite missions (COBE, WMAP, and Planck). This trend will continue into the next decade. CMB observations will ultimately be limited by the accuracy with which emission from all other astronomical sources in the universe can be separated out, and by systematic errors. Ground and space measurements are complementary. From the ground, CMB polarization observations offer better angular resolution, using large aperture telescopes that would PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-18

be expensive to fly in space, combined with excellent performance in a handful of atmospheric windows. Space, with its unrestricted access to all frequencies over the whole sky, its capability for uniquely accurate absolute calibration using the orbital motion of the spacecraft around the barycenter of the Solar system, and its freedom from atmospheric and other interfering signals, offers the lowest systematic errors and foreground residuals. Important progress in the next decade can be made by ambitious observations from the ground on angular scales of roughly ten degrees and smaller. Space observations will unquestionably be needed for the best foreground separation and lowest systematic errors on all angular scales, and especially on angular scales of greater than about ten degrees. A future dedicated CMB space mission, with a goal of measuring the polarized signal to the fundamental limits set by foregrounds, would realize these goals. This will require substantial early work to (1) improve all aspects of the detector systems (e.g., mm-wave filtering and coupling, CMB-noise limited detectors, low-noise stable readouts, low-noise cryogenics); (2) enhance the ability to simulate and separate foreground emission from the CMB; (3) simulate and mitigate the effects of systematic errors; and (4) use this knowledge to optimize the design of a flight system. This preparatory work could begin immediately, eventually resulting in a Probe-class mission, with a final design based on a targeted competition, selected near the end of the decade. The appropriate investment in technology development in advance of that selection is estimated by the panel to be ~$100 million, spent over the decade. J.6 PROGRAM BALANCE Science return, technological advancement, and effectiveness in training the next generation of researchers are maximized by a program comprising missions of many sizes, ranging from large Flagships down to the smallest scale where useful results can be obtained. Such a program is referred to as “balanced,” and the importance of balance has been strongly endorsed by previous decadal reviews. Recent developments have extended the small end of the range of space missions down to SmallSats, Pioneers, and CubeSats. The current NASA program is working well, although some improvements can be made. As emphasized in Section J.5, the addition of a Probe class will make the program even stronger. Below, the panel comments on the other classes of mission opportunities. Flagship Missions: Major advances in scientific capability will continue to require multi-billion- dollar missions. While such Flagship-class missions are scientifically compelling, maintaining a well- balanced portfolio requires that they be more accurately estimated and more tightly conceived and managed in a constrained environment. Past Flagship missions have focused on maximizing broad science return within a given wavelength band. Programs driven instead by a prioritized set of key science questions may make it easier to optimize design trades that control cost growth. Advances in technology designed to address such specific questions are also likely to enable unanticipated discoveries in other areas.  Explorers: Explorers have produced an incredible track record of exciting science, despite their modest size and cost. Explorers provide flexibility in the overall program not accessible to larger missions. While more narrowly focused in the scope of questions they can address, they allow for compressed time scales of development, permitting timely and rapid response to newly arising scientific questions, exploitation of the most recent innovations in instrumentation, and the opportunity for scientists and engineers to experience the end-to-end design and production of space missions. The EOS2 panel endorses a vigorous nontargeted Explorer program, maintaining a cadence of two new MIDEX, two new SMEX, and at least four MoO astrophysics missions per decade. A program budget adjustment to RY dollars is advisable. SmallSats, Pioneers, and CubeSats: SmallSats, the new Pioneers, and CubeSats offer opportunities in space at even smaller scales, which astronomy is just beginning to exploit. Important science is achievable with these platforms when angular resolution or collecting area are not the driving requirements. At present, the selection criteria for these small missions mirror those for larger missions, including the requirement for important and new science results. The panel’s view is that these missions PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-19

could take on added value and utility if the selection criteria were changed to allow for and encourage demonstration of new technology, as opposed to new science results, when such demonstrations would enable important science in subsequent larger missions and could not be done efficiently from the ground. Suborbital Programs: Suborbital experiments including sounding rockets have played a significant role in testing technologies and training researchers, leading to better space missions. The suborbital program offers flexibility and rapid scientific return, and supports technology development and testing of new concepts, particularly in the area of detectors. MeV gamma-ray missions, for example, are an exciting area that could take advantage of the capabilities of future suborbital programs. The number of successful balloon flights has dropped over the past years. Regaining and maintaining a high tempo of balloon launches is necessary to preserve the utility of this important suborbital platform. On-Orbit Servicing: Determination of whether on-orbit servicing is advantageous must be made carefully. Servicing requirements applied broadly can increase size, mass, schedule, and cost. Attempts to limit the extent of servicing with probabilistic analyses to identify likely failure candidates have had mixed success, as failures are generally not predictable. Missions advocated by this panel involving cryogenic components present special concerns for servicing, given the complex interfaces between cooled instruments and optics, and the need to maintain precise alignment. The panel identified three elements of successful servicing missions for consideration on future projects: provision for an attaching fixture to enable future servicing if needed; easy access for replenishing consumables, such as refueling; and detailed early planning for ground integration and test, as easy-to-make interfaces aid ground activity flow and ensure accessibility for servicing in space. J.7 INTERNATIONAL PROGRAMS The EOS2 panel was asked to evaluate NASA’s commitment to two large projects planned for development by ESA: Athena and LISA. J.7.1 Athena Athena is planned for launch in the early 2030s. It will address questions on the hot and energetic universe (e.g., the origin and evolution of large-scale structures, the physical processes that govern the growth of black holes, gamma-ray bursts, and more). Athena has two instruments: the X-ray Integral Field Unit (X-IFU; 0.2–12 keV), a cryogenic X-ray high-resolution spectrometer (2.5 eV at <7 keV and R = 2800 at E > 7 keV); and the Wide Field Imager (WFI; 0.2–15 keV). X-IFU provides spatially resolved spectroscopy with 5" pixels over a 5 arcmin field of view with 10 μs timing resolution. The WFI provides deep wide-field (40' × 40") X-ray spectral) imaging with a pixel size of 130 μm × 130 μm (2.2" × 2.2" pixels) with energy resolution of <170 eV at 7 keV. The full FOV can be read out in <5 μs, and a fast detector readout mode can obtain 80 μs time resolution to accommodate observations of the brightest X- ray sources. The mission will be placed in a large halo orbit at L2 with a baseline mission lifetime of 4 years. Athena is currently in an advanced stage—the configuration of its instruments is frozen, and it is currently scheduled for adoption in 2022. There is a non-exchange-of-funds agreement between NASA and ESA, with two main U.S. contributions, the detectors for X-IFU and the usage of the NASA/MSFC X-ray facility for testing the Athena mirrors. ESA will provide the spacecraft, ground segment, X-ray mirrors, SIM and service modules, launcher, and operations. The member states, in particular Germany (MPE) and France (CNES), will provide the WFI and the X-IFU, with contributions from the Netherlands (SRON), Japan (JAXA), and Spain. The panel finds Athena to be a compelling mission with an excellent return on investment for the U.S. contributions. The EOS2 panel strongly endorses this NASA commitment in its current form. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-20

J.7.2 LISA LISA will be the first gravitational-wave interferometric detector in space, with an expected launch date in the early 2030s. LISA will open up the millihertz-frequency band of gravitational waves, a band rich with sources ranging from white-dwarf binaries in the Milky Way to massive black holes throughout the entire universe. U.S. participation in LISA remains a high scientific priority. NASA’s planned level of contribution to both the LISA hardware and the analysis of the data are judged to be appropriate at present, although the detailed technical elements are still being worked out. It is important for the U.S. science community to play an active and prominent role in the scientific analysis of the data. The EOS2 panel strongly endorses the NASA commitment to LISA in its current form. J.8 CONCLUSIONS The next decade promises to be an extremely exciting period in the history of space astronomy, with the launches of JWST, the Roman Space Observatory, and other smaller missions. Many fields of astrophysics have reached a mature state, where attention is now focused on answering long-term key questions, rather than simply searching for new phenomena. It is clear to the EOS2 panel that advances are most likely to come from a truly panchromatic approach, one that makes full use of the unique opportunity provided by platforms in space—access to the entire electromagnetic spectrum, free of obscuration, distortion, and background contamination by Earth’s atmosphere. Opportunities for discovery space will also remain strong with an emphasis on panchromaticity. The panel was presented with a wide array of white papers presenting a plethora of exciting new ideas for future space experiments. The proposed Flagship missions, Lynx and Origins, were subjected to particular scrutiny. While each of these mission concepts is tremendously promising in its own right, the panel judged that the contemporaneous presence of both an advanced X-ray mission and an advanced far IR mission would be even more compelling, given the strong complementarity between X-ray and FIR observations. The panel suggested that Lynx and Origins could be jointly reoptimized to yield two smaller, but achievable missions, Fire and Smoke, that would together execute a single program focused on studying the cosmic dance between BHs and their host galaxies as the Universe evolved. The panel strongly endorses the proposal for a new Probe-class of missions filling the hole between Explorers and Flagships. While a fully competitive Probe class would harness the creativity of the community, the panel endorses two areas for new Probe science that can be highlighted for strategic competition: Time Domain and Multi-Messenger Astrophysics, and Early Universe Cosmology. These fields are ripe for discovery, address fundamental questions, and the requisite technology is either mature, or near-ready with modest additional investment. Last, the panel strongly reendorses the concept of “balance” within the NASA program. Smaller mission opportunities are essential to train the next generation of space astronomers, test out new technologies, and address specialized scientific questions. If this broad vision is realized, the vast majority of high-priority science questions and discovery areas highlighted by the science panel reports will be addressed and answered. Table J.3 illustrates the coupling between these questions and the mission concepts highlighted above, with brief comments on which specific new capabilities are key to addressing them. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-21

TABLE J.3 Science Panel Questions and Discovery Areas Mapped to the EOS2 Vision Science Panel Question or Discovery Which EOS2 Mission Addresses the Question or Discovery Area Area D—Science for which the mission is specifically designed. S, G—Additional science to which the mission can make a Strong or Good contribution, but for which the mission is not specifically designed. Panel on Compact Objects and Energetic Phenomena 1. What are the mass and spin distributions S, Firea: Continuum shapes of disk components, broad Fe-K line shapes. of neutron stars and stellar black holes? D, TDAb: Continuum shapes of disk components, broad Fe-K line shapes. 2. What powers the diversity of explosive S, Fire and Smokec: Late-time transients monitoring for many types. phenomena across the electromagnetic D, TDA: γ-, X-ray, IR discovery rapid follow-up of transients. spectrum? 3. Why do some compact objects eject S, Fire: Imaging of galactic and extragalactic jets. material in nearly light-speed jets, and what D, TDA: γ-, X-ray monitoring of jet spectral evolution. is that material made of? 4. What seeds supermassive black holes, D, Fire and Smoke: Detecting and characterizing the population of seed black and how do they grow? holes in the low-metallicity era at z > 8. DA. Transforming our view of the universe S, Fire and Smoke: Late-time behavior of GW events. by combining new information from light, D, TDA: γ-, X-ray, IR rapid follow-up and characterization of GW events; particles, and gravitational waves monitoring neutrino sources such as blazars. Detect short GRBs from jetted BH- BH mergers and NS-BH mergers in conjunction with GW observatories. Panel on Cosmology 1. What set the Hot Big Bang in motion? D, CMBd: Detection of primordial gravitational waves would significantly narrow models of the early universe and provide strong support for inflation. 2. What are the properties of dark matter S, Fire: Study of clusters of galaxies (with Roman Observatory, other lensing); and the dark sector? annihilation line searches. D, CMB: Lensing of CMB. 3. What physics drives the expansion and D, CMB: Measurements can test the change of expansion rate and equation of large-scale evolution of the Universe? state over cosmic time. 4. How will measurements of gravitational G, Fire: Gravitational wave counterparts. waves reshape our cosmological view? D, TDA: Gravitational wave counterparts. D, CMB: Through primordial gravitational waves, the CMB probes fundamental physics at energy scales unattainable on Earth and thereby informs how the universe began. DA. The Dark Ages as a cosmological — probe Panel on Exoplanets, Astrobiology and Solar System 1. What is the range of planetary system — architectures, and is the configuration of the solar system common? 2. What are the properties of individual — planets, and which processes lead to their diversity? 3. How do habitable environments arise and S, Fire: Planet atmosphere loss rates owing to host star irradiation, winds. evolve within the context of their planetary Coronal activity of planet-hosting stars, populations. systems? S, TDA: Observe flaring from magnetically active M-dwarfs to construct first full record of X-ray heating of exoplanet atmospheres. 4. How can signs of life be identified and — interpreted in the context of their planetary environments? DA. The search for life on exoplanets — Panel on Galaxies 1. How did the intergalactic medium and D, Fire and Smoke: Study of z > 6 star formation and black hole growth. the first sources of radiation evolve from D, TDA: Detect long GRBs at z > 6 to probe star formation rate in epoch of cosmic dawn through the epoch of reionization. High-z cutoff of long GRBs to probe era of population III stars. reionization? S, CMB: Reionization optical depth and redshift. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-22

2. How do gas, metals, and dust flow into, D, Fire and Smoke: Outflows driven by AGN and stars versus cosmic time through, and out of galaxies? using Fe-K, H2, OH absorption. D, Smoke: Metallicity and dust properties of galaxies as a function of cosmic time from z = 8 to z = 0 via IR lines and dust features. 3. How do supermassive black holes form, D, Fire: Measuring luminosity function of rapidly growing black holes in the and how is their growth coupled to the first Gyr. evolution of their host galaxies? D, Smoke: Measure star formation and black hole accretion rates since cosmic dawn. X-ray and IR light penetrates even heavily dust-obscured regions. S, TDA: Extreme hard X-ray flaring of highly beamed blazars at high z to probe formation and evolution of SMBHs. 4. How do the histories of galaxies and their S, Fire and Smoke: Measuring the properties of both the hot gas filling the dark matter halos shape their observable dark matter potential wells and the cold matter into which it cools to form stars properties? as a function of environment and galaxy properties. DA. Mapping the circumgalactic medium S, Fire and Smoke: Imaging and mapping CGM, IGM, and ICM metallicities and the intergalactic medium in emission as a function of environment and cosmic epoch, resolving out field sources. S, CMB: SZ studies of galaxy clusters. Panel on the Interstellar Medium and Star and Planet Formation 1. How do star-forming structures form, S, Smoke: Map star-forming regions. Measure fine-structure emission lines evolve, and interact with the diffuse ISM? from O, C, Ne, S, N, Fe, Ar, and Si to determine dynamics, physical conditions, and mass in different ISM phases. S, Fire: Measuring X-ray emission from young protostars in star-forming regions. 2. What regulates the structure and motions S, Smoke: Cloud energetics and structure mapped with O, C, and N fine- within molecular clouds? structure lines and molecular lines including mid- to high-J CO rotational lines, H2O, OH, and NH3 3. How does gas flow from parsec scales S, Fire: Effect of protostar activity on the surrounding disk. down to protostars and their disks? S, Smoke: Measure H2O emission lines tracing ice and water vapor from 10 K to 1000 K. 4. Is planet formation fast or slow? S, Smoke: Velocity-resolved tomography reviews the accretion rates of O, H2O, H2, and HD. DA. Detecting and characterizing forming — planets Panel on Stars, the Sun, and Stellar Populations 1. What are the most extreme stars and G, Fire: Magnetic field structures from coronal properties; coronal activity of stellar populations? populations. 2. How does multiplicity affect how a star G, Fire: Coronal activity; image “jets” of Miras. lives and dies? 3. What would stars look like if we could G, Fire: Coronal activity versus rotation; asymmetries in SN explosions. view them like we do the Sun? 4. How do the Sun and other stars create S, Fire: Trace coronal mass ejections from flaring stars, especially magnetically space weather? active M dwarfs and AGB stars. Detect, image supernovae, supernova remnants, determine expansion rates S, Smoke: Measure mass outflows and ISM enrichment from supernovae. DA. Industrial-scale spectroscopy — PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION J-23

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We live in a time of extraordinary discovery and progress in astronomy and astrophysics. The next decade will transform our understanding of the universe and humanity's place in it. Every decade the U.S. agencies that provide primary federal funding for astronomy and astrophysics request a survey to assess the status of, and opportunities for the Nation's efforts to forward our understanding of the cosmos. Pathways to Discovery in Astronomy and Astrophysics for the 2020s identifies the most compelling science goals and presents an ambitious program of ground- and space-based activities for future investment in the next decade and beyond. The decadal survey identifies three important science themes for the next decade aimed at investigating Earth-like extrasolar planets, the most energetic processes in the universe, and the evolution of galaxies. The Astro2020 report also recommends critical near-term actions to support the foundations of the profession as well as the technologies and tools needed to carry out the science.

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