X-Ray Science Interest Group
(XR SIG)

Decadal Survey White Paper Projects

List of X-ray science white papers submitted by community members via XRSIG

1. Physics of cosmic plasmas from high angular resolution X-ray imaging of galaxy clusters

Details

Lead Author	Maxim Markevitch
Lead Author email	maxim.markevitch@nasa.gov
Coauthors: Esra Bulbul, Eugene Churazov, Simona Giacintucci, Ralph Kraft, Matthew Kunz, Elke Roediger, Mateusz Ruszkowski, Alex Schekochihin, Reinout van Weeren, Alexey Vikhlinin, Stephen A. Walker, Qian Wang, Norbert Werner, Daniel Wik, Irina Zhuravleva, John ZuHone
Key words	Galaxy evolution, cosmology and fundamental physics
Link to white paper/draft	[PDF]
Abstract: Galaxy clusters are massive dark matter-dominated systems filled with X-ray emitting, optically thin plasma. Their large size and relative simplicity (at least as astrophysical objects go) make them a unique laboratory to measure some of the interesting plasma properties that are inaccessible by other means but fundamentally important for understanding and modeling many astrophysical phenomena — from solar flares to black hole accretion to galaxy formation and the emergence of the cosmological Large Scale Structure. While every cluster astrophysicist is eagerly anticipating the direct gas velocity measurements from the forthcoming microcalorimeters onboard XRISM, Athena and future missions such as Lynx, a number of those plasma properties can best be probed by high-resolution X-ray imaging of galaxy clusters. Chandra has obtained some trailblazing results, but only grazed the surface of such studies. In this white paper, we discuss why we need arcsecond-resolution, high collecting area, low relative background X-ray imagers (with modest spectral resolution), such as the proposed AXIS and the imaging detector of Lynx.

2. Probing Macro-Scale Gas Motions and Turbulence in Diffuse Cosmic Plasmas

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Lead Author	Esra Bulbul
Lead Author email	ebulbul@cfa.harvard.edu
Coauthors: Massimo Gaspari, Gabriella Alvarez, Camille Avestruz, Mark Bautz, Brad Benson, Veronica Biffi, Douglas Burke, Nicolas Clerc, Edoardo Cucchetti, Urmila Chadayammuri, Dominique Eckert, Stefano Ettori, Bill Forman, Fabio Gastaldello, Vittorio Ghirardini, Ralph Kraft, Maxim Markevitch, Mike McDonald, Eric Miller, Tony Mroczkowski, Daisuke Nagai, Paul Nulsen, Gabriel W. Pratt, Scott Randall, Thomas Reiprich, Mauro Roncarelli, Aurora Simionescu, Randall Smith, Grant Tremblay, Stephen Walker, John ZuHone, Irina Zhuravleva
Key words
Link to white paper/draft	[PDF]
Abstract: Clusters of galaxies, the largest collapsed structures in the Universe, are located at the intersection of extended filaments of baryons and dark matter. Cosmological accretion onto clusters through large scale filaments adds material at cluster outskirts. Kinetic energy in the form of bulk motions and turbulence due to this accretion provides a form of pressure support against gravity, supplemental to thermal pressure. Significant amount of non-thermal pressure support could bias cluster masses derived assuming hydrostatic equilibrium, the primary proxy for cluster cosmology studies. Sensitive measurements of Doppler broadening and shift of astrophysical lines, and the relative fluctuations in thermodynamical quantities (e.g., density, pressure, and entropy) are primary diagnostic tools. Forthcoming planned and proposed X-ray (with large etendue, throughput, and high spectral resolution) and SZ observatories will provide crucial information on the assembly and virialisation processes of clusters, involving turbulent eddies cascading at various spatial scales and larger gas bulk motions in their external regions to the depth or their potential wells.

3. Do Supermassive Black Hole Winds Impact Galaxy Evolution?

Details

Lead Author	Francesco Tombesi
Lead Author email	ftombesi@astro.umd.edu
Coauthors: M. Cappi, F. Carrera, G. Chartas, K. Fukumura, M. Guainazzi, D. Kazanas, G. Kriss, D. Proga, T. Turner, Y. Ueda, S. Veilleux, M. Brusa, M. Gaspari
Link to white paper/draft	37-462290b65c11cab04b53d0fddd619f1b_TombesiFrancesco.pdf
Abstract: Powerful winds driven by SMBHs are likely the main mechanism through which SMBHs regulate their own growth and influence the host galaxy evolution. However, their origin and their capability to impact the large-scale environment are still highly debated. Fundamental results will come from high-energy and spatial resolution X-ray observatories.

4. X-ray Studies of Supernova Remnants

Details

Lead Author	Brian Williams
Lead Author email	brian.j.williams@nasa.gov
Coauthors: Katie Auchettl, Carles Badenes, Daniel Castro, Oleg Kargaltse,v Laura A. Lopez, Koji Mori, Daniel J. Patnaude, Paul Plucinsky, John C. Raymond, Samar Safi-Harb, Patrick Slane, Takaaki Tanaka, Tea Temim, Jacco Vink, Hiroya Yamaguchi
Key words	supernovae; supernova remnants
Link to white paper/draft	[PDF]
Abstract: This white paper outlines the progress that can be made from the next generation of X-ray telescopes in the study of the remnants of supernovae, both galactic and extragalactic. We primarily focus on high spectral resolution instruments and the science they will enable

5. Density measurement of X-ray outflows (warm absorbers and ultra fast outflows)

Details

Lead Author	Sibasish Laha
Lead Author email	sib.laha@gmail.com
Coauthors: Randall Smith, Panayiotis Tzanavaris, Tim Kallman, Sylvain Veilleux, Francesco Tombesi, Gerard Kriss, Matteo Guainazzi, Massimo Gaspari, Jelle Kaastra, Alex Markowitz, Mike Crenshaw, Ehud Behar, Keigo Fukumura, Anna Lia Longinotti, Agata Rozanska, Jacobo Ebrero, Gary Ferland, Claudio Ricci, Chris Done, Daniel Proga, Mitchell Revalski, Andrey Vayner
Key words	X-ray outflows; density measurement; distance; kinetic energy of outflows
Link to white paper/draft	[PDF]
Abstract: The highly energetic outflows from Active Galactic Nuclei detected in X-rays are one of the most powerful mechanisms by which the central supermassive black hole (SMBH) interacts with the host galaxy. The last two decades of high resolution X-ray spectroscopy with XMM-Newton and Chandra have improved our understanding of the nature of these outflowing ionized absorbers and we are now poised to take the next giant leap with higher spectral resolution and higher throughput observatories to understand the physics and impact of these outflows on the host galaxy gas. The future studies on X-ray outflows not only have the potential to unravel some of the currently outstanding puzzles in astronomy, such as the physical basis behind the MBH−σ relation, the cooling flow problem in intra-cluster medium (ICM), and the evolution of the quasar luminosity function across cosmic timescales, but also provide rare insights into the dynamics and nature of matter in the immediate vicinity of the SMBH. Higher spectral resolution (≤ 0.5 eV at 1 keV) observations will be required to identify individual absorption lines and study the asymmetries and shifts in the line profiles revealing important information about outflow structures and their impact. Higher effective area (≥ 1000 cm2) will be required to study the outflows in distant quasars, particularly at the quasar peak era (redshift 1 ≤ z ≤ 3) when the AGN population was the brightest. Thus, it is imperative that we develop next generation X-ray telescopes with high spectral resolution and high throughput for unveiling the properties and impact of highly energetic X-ray outflows. A simultaneous high resolution UV + X-ray mission will encompass the crucial AGN ionizing continuum (1 eV–100 keV), and also characterize the simultaneous detections of UV and X-ray outflows, which map different spatial scales along the line of sight.

6. Unveiling the Galaxy Cluster—Cosmic Web Connection with X-ray observations in the Next Decade

Details

Lead Author	Stephen A. Walker, Daisuke Nagai
Lead Author email	stephen.a.walker@nasa.gov; daisuke.nagai@yale.edu
Coauthors: A. Simionescu, M. Markevitch, H. Akamatsu, M. Arnaud, C. Avestruz, M. Bautz, V. Biffi, S. Borgani, E. Bulbul, E. Churazov, K. Dolag, D. Eckert, S. Ettori, Y. Fujita, M. Gaspari, V. Ghirardini, R. Kraft, E. T. Lau, A. Mantz, K. Matsushita, M. McDonald, E. Miller, T. Mroczkowski, P. Nulsen, N. Okabe, N. Ota, E. Pointecouteau, G. Pratt, K. Sato, X. Shi, G. Tremblay, M. Tremmel, F. Vazza, I. Zhuravleva, E. Zinger, J. ZuHone
Key words	cluster outskirts; missing baryons; whim; metals; cosmic web; filaments;
Link to white paper/draft	[PDF]
Abstract: In recent years, the outskirts of galaxy clusters have emerged as one of the new frontiers and unique laboratories for studying the growth of large scale structure in the universe. Modern cosmological hydrodynamical simulations make firm and testable predictions of the thermodynamic and chemical evolution of the X-ray emitting intracluster medium. However, recent X-ray and Sunyaev-Zeldovich effect observations have revealed enigmatic disagreements with theoretical predictions, which have motivated deeper investigations of a plethora of astrophysical processes operating in the virialization region in the cluster outskirts. Much of the physics of cluster outskirts is fundamentally different from that of cluster cores, which has been the main focus of X-ray cluster science over the past several decades. A next-generation X-ray telescope, equipped with sub-arcsecond spatial resolution over a large field of view along with a low and stable instrumental background, is required in order to reveal the full story of the growth of galaxy clusters and the cosmic web and their applications for cosmology.

7. Unlocking the Capabilities of Future High-Resolution X-ray Spectroscopy Missions Through Laboratory Astrophysics

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Lead Author	G. Betancourt-Martinez
Lead Author email	gabriele.betancourt@irap.omp.eu
Coauthors: Hiroki Akamatsu, Didier Barret, Manuel Bautista, Sven Bernitt, Gabriele Betancourt, Stefano Bianchi, Dennis Bodewits, Nancy Brickhouse, Greg Brown, Marcello Coreno, Jose Crespo, Renata Cumbee, Monica De Simone, Jan-Willem den Herder, Megan Eckart, Gary Ferland, Fabrizio Fiore, Mike Fogle, Adam Foster, Javier Garcia, Tom Gorczyca, Victoria Grinberg, Nicolas Grosso, Liyi Gu, Ming-Feng Gu, Matteo Guainazzi, Natalie Hell, Jelle Kaastra, Tim Kallman, Julia Lee, Maurice Leutenegger, Joan Marler, Dan McCammon, Shinya Nakashima, Fabrizio Nicastro, Frits Paerels, Francois Pajot, Etienne Pointecouteau, Delphine Porquet, Scott Porter, Daniel Savin, Makoto Sawada, Chintan Shah, Aurora Simionescu, Chad Sosolik, Phil Stancil, Rene Steinbruegge, Hiroya Yamaguchi
Key words	laboratory astrophysics, AGN outflows, AGN feedback, galaxy evolution, missing baryons
Link to white paper/draft	[PDF]
Abstract: Thanks to high-resolution and non-dispersive spectrometers onboard future X-ray missions such as XRISM and Athena, we are finally poised to answer important questions about the formation and evolution of galaxies and large-scale structure. However, we currently lack an adequate understanding of many atomic processes behind the spectral features we will soon observe. Large error bars on parameters as critical as transition energies and atomic cross sections can lead to unacceptable uncertainties in the calculations of, e.g., elemental abundance, velocity, and temperature. Unless we address these issues, we risk limiting the full scientific potential of these missions. Laboratory astrophysics, which comprises theoretical and experimental studies of the underlying physics behind observable astrophysical processes, is therefore central to the success of these missions.

8. Supermassive Black Hole Spin and Reverberation

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Lead Author	Abdu Zoghbi
Lead Author email	abzoghbi@umich.edu
Coauthors: D. R. Wilkins, L. Brenneman, G. Miniutti, G. Matt, J. Garcia, E. Kara, E. Cackett, B. De Marco, M. Dovciak, P. Tzanavaris, A. Hornschemeier, E. Bulbul, J. Miller, R. Kraft, A. Ptak, R. Smith, R. Petre.
Key words	Black holes; AGN; Spin; Reverberation
Link to white paper/draft	[PDF]
Abstract: X-ray reverberation mapping has emerged as a powerful probe of microparsec scales around AGN, and with high sensitivity detectors, its full potential in echo-mapping the otherwise inaccessible disk-corona at the black hole horizon scale will be revealed

9. Cooking with X-rays: Can X-ray binaries heat the early Universe?

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Lead Author	A.R. Basu-Zych
Lead Author email	antara.r.basu-zych@nasa.gov
Coauthors: Andrei Mesinger, Bradley Greig, Bret Lehmer, Panayiotis Tzanavaris, Judd Bowman, Tassos Fragos, Steve Furlanetto, Ann Hornschemeier, Piero Madau, Kiyoshi Masui, Felix Mirabel, Jordan Mirocha, Andrew Ptak, Eli Visbal, Neven Vulic, Mihoko Yukita
Key words	X-ray binaries; galaxies; galaxy evolution; X-ray scaling relations; cosmology
Link to white paper/draft	[PDF]
Abstract: X-rays from high mass X-ray binaries (HMXBs) within the first primordial galaxies likely played a significant role in heating the early Universe at z ≳ 10. While X-ray observations of distant z ≳ 2 galaxies may be prohibitively expensive, studying a sample of relatively nearby (z ≤ 0.05) low metallicity galaxies is within reach and offers several advantages: (1) a complete and unbiased survey of the stochasticity in X-ray scaling relations and (2) measurements of the shape of their X-ray spectra. Combining observations from next generation X-ray telescopes with upcoming measurements of the cosmic 21-cm signal will have the power to reveal the interstellar medium (ISM) structure of the first primordial galaxies and their X-ray emission properties.

10. Accretion in Stellar-mass Black Holes at High X-ray Resolution

Details

Lead Author	Jon M. Miller
Lead Author email	jonmm@umich.edu
Coauthors: Didier Barret, Edward Cackett, Maria Diaz Trigo, Christine Done, Elena Gallo, Jelle Kaastra, Christian Motch, Ciro Pinto, Gabrielle Ponti, Natalie Webb, Abderahmen Zoghbi
Key words
Link to white paper/draft	[PDF]
Abstract: Accretion disks around stellar-mass black holes offer unique opportunities to study the fundamental physics of standard thin disks, super-Eddington disks, and structure that may be connected to flux variability. These local analogues of active galactic nuclei (AGN) are particularly attractive for their proximity, high flux, and peak emissivity in the X-ray band. X-ray calorimeter spectrometers, with energy resolutions of 2–5 eV, are ideally suited to study accretion in stellar-mass black holes. The results will make strong tests of seminal disk theory that applies in a broad range of circumstances, help to drive new numerical simulations, and will inform our understanding of AGN fueling, evolution, and feedback.

11. Cosmic evolution of supermassive black holes: A view into the next two decades

Details

Lead Author	Francesca Civano, Nico Cappelluti, Ryan Hickox, Rebecca Canning
Lead Author email	fcivano@cfa.harvard.edu
Coauthors: James Aird, Marco Ajello, Steve Allen, Eduardo Bañados, Laura Blecha, William N. Brandt, Marcella Brusa, Francisco Carrera, Massimo Cappi, Andrea Comastri, Klaus Dolag, Megan Donahue, Martin Elvis, Giuseppina Fabbiano, Francesca Fornasini, Poshak Gandhi, Antonis Georgakakis, Kelly Holley-Bockelmann, Anton Koekemoer, Andrew Goulding, Mackenzie Jones, Sibasish Laha, Stephanie LaMassa, Giorgio Lanzuisi, Lauranne Lanz, Adam Mantz, Stefano Marchesi, Mar Mezcua, Beatriz Mingo, Kirpal Nandra, Daniel Stern, Doug Swartz, Grant Tremblay, Panayiotis Tzanavaris, Alexey Vikhlinin, Fabio Vito, Belinda Wilkes
Key words	AGN, Cosmology
Link to white paper/draft
Abstract: The discoveries made over the past 20 years by Chandra and XMM-Newton surveys in conjunction with multiwavelength imaging and spectroscopic data available in the same fields have significantly changed the view of the supermassive black hole (SMBH) and galaxy connection. These discoveries have opened up several exciting questions that are beyond the capabilities of current X-ray telescopes and will need to be addressed by observatories in the next two decades. As new observatories peer into the early Universe, we will begin to understand the physics and demographics of SMBH infancy (at z > 6) and investigate the influence of their accretion on the formation of the first galaxies (§ 2.1). We will also be able to understand the accretion and evolution over the cosmic history (at z ~ 1–6) of the full population of black holes in galaxies, including low accretion rate, heavily obscured AGNs at luminosities beyond the reach of current X-ray surveys (§2.2 and §2.3), enabling us to resolve the connection between SMBH growth and their environment.

12. Time Domain Studies of Neutron Star and Black Hole Populations: X-ray Identification of Compact Object Types

Details

Lead Author	Neven Vulic
Lead Author email	neven.vulic@nasa.gov
Coauthors: Ann E. Hornschemeier, Vallia Antoniou, Antara Basu-Zych, Breanna Binder, Francesca M. Fornasini, Felix Fuerst, Frank Haberl, Marianne Heida, Bret D. Lehmer, Thomas J. Maccarone, Andrew F. Ptak, Gregory R. Sivakoff, Panayiotis Tzanavaris, Daniel R. Wik, Benjamin F. Williams, Joern Wilms, Mihoko Yukita, and Andreas Zezas
Key words	X-ray Binaries; Transients; Galaxies
Link to white paper/draft	[PDF]
Abstract: What are the most important conditions and processes governing the growth of stellar-origin compact objects? The identification of compact object type as either black hole (BH) or neutron star (NS) is fundamental to understanding their formation and evolution. To date, time-domain determination of compact object type remains a relatively untapped tool. Measurement of orbital periods, pulsations, and bursts will lead to a revolution in the study of the demographics of NS and BH populations, linking source phenomena to accretion and galaxy parameters (e.g., star formation, metallicity). To perform these measurements over sufficient parameter space, a combination of a wide-field (> 5000 deg2) transient X-ray monitor over a dynamic energy range (~ 1–100 keV) and an X-ray telescope for deep surveys with ≲5" PSF half-energy width (HEW) angular resolution are required. Synergy with multiwavelength data for characterizing the underlying stellar population will transform our understanding of the time domain properties of transient sources, helping to explain details of supernova explosions and gravitational wave event rates.

13. The Future Landscape of High-Redshift Galaxy Cluster Science

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Lead Author	Adam Mantz
Lead Author email	amantz@stanford.edu
Coauthors: Steven W. Allen, Nicholas Battaglia, Bradford Benson, Rebecca Canning, Stefano Ettori, August Evrard, Anja von der Linden, Michael McDonald
Key words	galaxy clusters; intracluster medium; X-rays; radio; optical/IR; AGN feedback; cosmology
Link to white paper/draft	[PDF]
Abstract: Modern galaxy cluster science is a multi-wavelength endeavor with cornerstones provided by X-ray, optical/IR, mm, and radio measurements. In combination, these observations enable the construction of large, clean, complete cluster catalogs, and provide precise redshifts and robust mass calibration. The complementary nature of these multi-wavelength data dramatically reduces the impact of systematic effects that limit the utility of measurements made in any single waveband. The future of multi-wavelength cluster science is compelling, with cluster catalogs set to expand by orders of magnitude in size, and extend, for the first time, into the high-redshift regime where massive, virialized structures first formed. Unlocking astrophysical and cosmological insight from the coming catalogs will require new observing facilities that combine high spatial and spectral resolution with large collecting areas, as well as concurrent advances in simulation modeling campaigns. Together, future multi-wavelength observations will resolve the thermodynamic structure in and around the first groups and clusters, distinguishing the signals from active and star-forming galaxies, and unveiling the interrelated stories of galaxy evolution and structure formation during the epoch of peak cosmic activity.

14. Probing the Black Hole Engine with Measurements of the Relativistic X-ray Reflection Component

Details

Lead Author	Javier A. García
Lead Author email	javier@caltech.edu
Coauthors: Matteo Bachetti, David R. Ballantyne, Laura Brenneman, Murray Brightman, Riley M. Connors, Thomas Dauser, Andrew Fabian, Felix Fuerst, Poshak Gandhi, Nikita Kamraj, Erin Kara, Kristin Madsen, Jon M. Miller, Michael Nowak, Michael L. Parker, Christopher Reynolds, James Steiner, Daniel Stern, Corbin Taylor, John Tomsick, Dominic Walton, Jörn Wilms, & Abderahmen Zoghbi
Key words	Black holes; accretion disks; X-ray reflection; spectral-timing
Link to white paper/draft	[PDF]
Abstract: We discuss the observational and theoretical challenges expected in the exploration, discovery, and study of astrophysical black holes in the next decade. We focus on the case of accreting black holes and their electromagnetic signatures, with particular emphasis on the measurement of the relativistic reflection component in their X-ray spectra.

15. Supermassive Black Hole Feedback

Details

Lead Author	Mateusz Ruszkowski
Lead Author email	mateuszr@umich.edu
Coauthors: Daisuke Nagai, Irina Zhuravleva, Corey Brummel-Smith, Yuan Li, Edmund Hodges-Kluck, Hsiang-Yi Karen Yang, Kaustuv Basu, Jens Chluba, Eugene Churazov, Megan Donahue, Andrew Fabian, Claude-André Faucher-Giguère, Massimo Gaspari, Julie Hlavacek-Larrondo, Michael McDonald, Brian McNamara, Paul Nulsen, Tony Mroczkowski, Richard Mushotzky, Christopher Reynolds, Alexey Vikhlinin, Mark Voit, Norbert Werner, John ZuHone, Ellen Zweibel
Key words	galaxy clusters; physics of black hole feedback; non-thermal particles in AGN bubbles; turbulence and velocity perturbations excited by AGN; ICM transport processes and coupling of the AGN energy to the ICM
Link to white paper/draft	[PDF]
Abstract: Understanding the processes that drive galaxy formation and shape the observed properties of galaxies is one of the most interesting and challenging frontier problems of modern astrophysics. We now know that the evolution of galaxies is critically shaped by the energy injection from accreting supermassive black holes (SMBHs). However, it is unclear how exactly the physics of this feedback process affects galaxy formation and evolution. In particular, a major challenge is unraveling how the energy released near the SMBHs is distributed over nine orders of magnitude in distance throughout galaxies and their immediate environments. The best place to study the impact of SMBH feedback is in the hot atmospheres of massive galaxies, groups, and galaxy clusters, which host the most massive black holes in the Universe, and where we can directly image the impact of black holes on their surroundings. We identify critical questions and potential measurements that will likely transform our understanding of the physics of SMBH feedback and how it shapes galaxies, through detailed measurements of (i) the thermodynamic and velocity fluctuations in the intracluster medium (ICM) as well as (ii) the composition of the bubbles inflated by SMBHs in the centers of galaxy clusters, and their influence on the cluster gas and galaxy growth, using the next generation of high spectral and spatial resolution X-ray and microwave telescopes.

16. X-rays Studies of the Solar System

Details

Lead Author	Bradford Snios
Lead Author email	bradford.snios@cfa.harvard.edu
Coauthors: William R. Dunn, Carey M. Lisse, Graziella Branduardi-Raymont, Konrad Dennerl, Anil Bhardwaj, G. Randall Gladstone, Susan Nulsen, Dennis Bodewits, Caitriona M. Jackman, Julián D. Alvarado Gómez, Emma J. Bunce, Michael R. Combi, Thomas E. Cravens, Renata S. Cumbee, Jeremy J. Drake, Ronald F. Elsner, Denis Grodent, Jae Sub Hong, Vasili Kharchenko, Ralph P. Kraft, Joan P. Marler, Sofia P. Moschou, Patrick D. Mullen, Scott J. Wolk, Zhonghua Yao
Key words	Planets; Minor bodies; Planetary satellites; Comets; Solar wind; Atmosphere
Link to white paper/draft	[PDF
Abstract: X-ray observatories contribute fundamental advances in Solar System studies by probing Sun-object interactions, developing planet and satellite surface composition maps, probing global magnetospheric dynamics, and tracking astrochemical reactions. Despite these crucial results, the technological limitations of current X-ray instruments hinder the overall scope and impact for broader scientific application of X-ray observations both now and in the coming decade. Implementation of modern advances in X-ray optics will provide improvements in effective area, spatial resolution, and spectral resolution for future instruments. These improvements will revolutionize Solar System studies in the following ways: Investigate early Solar System elemental and molecular composition via comet emissions, including rapid outflow events at the snow line Provide elemental composition maps of the surfaces of satellites throughout the solar system with clear implications for astrobiology at Europa Revolutionise magnetospheric studies of high energy transport and global dynamics of the space environments around planets Study effects of solar X-ray activity on planet atmosphere evolution with broad ramifications for the evolution of exoplanet atmospheres and their star-planet relationships These milestones will usher in a truly transformative era of Solar System science through the study of X-ray emission.

17. Detecting the Birth of Supermassive Black Holes Formed from Heavy Seeds

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Lead Author	Fabio Pacucci
Lead Author email	fabio.pacucci@yale.edu
Coauthors: Vivienne Baldassare, Nico Cappelluti, Xiaohui Fan, Andrea Ferrara, Zoltan Haiman, Priyamvada Natarajan, Feryal Ozel, Raffaella Schneider, Grant R. Tremblay, Megan C. Urry, Rosa Valiante, Alexey Vikhlinin, Marta Volonteri
Key words	black hole seeds; observational properties: IR, X-ray, GW; seeding models; JWST; Lynx; Athena; LISA; high-z survey planning
Link to white paper/draft	[PDF]
Abstract: We investigate the capabilities required to study supermassive black holes formed by heavy seeds in the early Universe. We show that detecting heavy seeds at z > 10 in the next decade will be feasible with upcoming and/or proposed facilities. Their detection will be fundamental to understand the early history of the Universe and its evolution.

18. X-ray follow-up of extragalactic transients

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Lead Author	Erin Kara
Lead Author email	ekara@astro.umd.edu
Coauthors: Raffaella Margutti, Azadeh Keivani, Wen-fai Fong, Brad Cenko, Eric Burns, Regina Caputo, Derek Fox, Scott Noble, Richard Mushotzky, John Ruan, Daryl Haggard, Geoffrey Ryan, David Burrows
Key words	Time Domain Astronomy; Multi-messanger astronomy; Tidal disruption events; black hole mergers; neutron star mergers; neutrino counterparts
Link to white paper/draft	[PDF]
Abstract: Most violent and energetic processes in our universe, including mergers of compact objects, explosions of massive stars and extreme accretion events, produce copious amounts of X-rays X-ray follow-up is an efficient tool for identifying transients: X-rays can quickly localize transients with large error circles X-rays reveal the nature of transients that may not have unique signatures at other wavelengths Here, we identify key science questions about several extragalactic multi-messenger and multi-wavelength transients, and demonstrate how X-ray follow-up helps answer these questions.

19. Black Hole Growth in Mergers and Dual AGN

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Lead Author	Michael Koss
Lead Author email	mike.koss@eurekasci.com
Coauthors: Vivian U, Edmund Hodges-Kluck, Ezequiel Treister, Laura Blecha, Claudio Ricci, Jeyhan Kartaltepe, Dale Kocevski, Julia M. Comerford, R. Scott Barrows, Claudia Cicone, Francisco Muller-Sanchez, Kayhan Gültekin, Adi Foord, and Jennifer Lotz
Key words	Mergers; AGN; X-rays; MIR; NIR;
Link to white paper/draft	[PDF]
Abstract: Hierarchical models of galaxy formation predict that galaxy mergers represent a significant transitional stage of rapid supermassive black hole (SMBH) growth. Yet, the connection between the merging process and enhanced active galactic nuclei (AGN) activity as well as the timescale of SMBH mergers remains highly uncertain. The breakthrough in reconciling the importance of galaxy mergers with black hole growth lies in a thoroughly-studied census of dual AGN across cosmic history, which will be enabled by next-generation observational capabilities, theoretical advances, and simulations. This white paper outlines the key questions in galaxy mergers, dual and offset AGN, and proposes multiwavelength solutions using future high-resolution observatories in the X-rays (AXIS, Lynx), near and mid-infrared (30 meter class telescopes, JWST), and submillimeter (ALMA).