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Report of the Panel on Compact Objects and Energetic Phenomena

INTRODUCTION

The stars shining in the sky have been familiar since the dawn of humanity, but their fates as they end their lives is a story understood only gradually over the past century. Compact objects—white dwarfs, neutron stars, and black holes—are the remnant cores of ordinary stars after their nuclear burning ends. These exotic objects are characterized by extremes of gravity and are often a source of high-energy radiation and particles. White dwarfs (WDs), the cores of the lightest stars, have masses similar to the Sun, but in a volume a million times smaller (the size of Earth). Neutron stars (NSs), the collapsed cores of some massive stars, again have masses similar to the Sun, but are only the size of a city, resulting in an extreme density comparable to atomic nuclei. Rapidly rotating and highly magnetized NSs, called pulsars—some spinning hundreds of times per second—emit regular pulses of radiation like exceptionally stable cosmic lighthouses. Magnetars are NSs that are so extremely magnetized that their magnetic fields can tear the NS crust and cause violent starquakes.

Other massive stars leave behind stellar-mass black holes (BHs), gravitational singularities in the spacetime of general relativity. Once viewed as a speculative hypothesis, their existence has been firmly established in multiple ways, with masses ranging from a few to many tens of solar masses (M_☉). In addition to stellar-mass BHs, supermassive black holes (SMBHs) in the 10^5–10¹⁰M_☉ range are observed at the centers of many galaxies. Accretion of matter into these SMBHs is understood to power emission from active galactic nuclei (AGN) and quasars, the most luminous objects in the universe. The existence of BHs between these extremes (intermediate-mass black holes, or IMBHs) remains an open question.

Many compact objects are members of binary star systems. If the binary companion is an ordinary star, then stellar and binary evolution can allow mass transfer and accretion from the donor companion onto the compact object, often mediated by an accretion disk. Accretion onto compact objects is an efficient power source for radiation, leading to systems called cataclysmic variables (accreting WDs) and X-ray binaries (accreting NSs and BHs). Until recently, X-ray binaries provided the only means to observe stellar-mass BHs. However, stellar evolution in an accreting binary eventually causes the donor companion to form a second compact object (a process that the binary itself may or may not survive). Binaries consisting of a pair

of compact objects can produce strong gravitational wave (GW) emission, resulting in angular momentum loss that eventually leads to coalescence (merger) of the binary and a GW transient. The successful detection of GWs from compact binary mergers in the past few years has opened a profoundly powerful new window on the study of compact object systems.

Some compact objects are sources of relativistic jets—collimated outflows of matter accelerated to nearly the speed of light. Compact objects are also closely linked to supernovae (Sne), gamma-ray bursts (GRBs), classical novae, and other explosive transients. Energetic phenomena associated with compact objects manifest through multiple messengers, including electromagnetic (EM) radiation ranging from low-frequency radio waves to the highest-energy gamma rays, GW radiation, high-energy neutrinos, and perhaps ultra-high-energy cosmic rays (UHECRs).

The past decade has seen extraordinary progress in the study of compact objects and energetic phenomena. Several of the major observational breakthroughs had been theoretically predicted in advance, including:

• The direct detection of GWs from merging compact binaries.
• The discovery of kilonovae (a new type of transient arising from merging NSs) and their associated ultra-relativistic jet outflows, through simultaneous detection of GWs and EM counterparts.
• The first imaging of a BH shadow, through radio interferometry of the SMBH in the nearby galaxy M87.

Other key breakthroughs included:

• The first detection of astrophysical high-energy neutrinos.
• The first discoveries of NSs more massive than 2 M_☉.
• The first discoveries of stellar-mass BHs substantially more massive than 15 M_☉ (the pre-2010 record).
• The discovery of ultraluminous X-ray pulsars (NSs apparently accreting matter at a rate many hundreds of times larger than the spherical Eddington limit).
• The detection of large numbers of tidal disruption events and the emergence of other important classes of astrophysical transients, including fast radio bursts and superluminous supernovae.

These observational breakthroughs have been matched by major advances in theoretical calculations and modeling. Advanced numerical simulations have also enabled substantial progress in understanding the physical processes that govern accretion disks, relativistic jets, particle acceleration, supernovae, stellar evolution, and compact binary coalescence.

Primed by this recent progress and attentive to the expectation of powerful new capabilities in the coming decades, this report is organized around four key science questions and one outstanding discovery area (see Table B.1 at the end of this appendix). Future progress in all these areas share two foundational needs. The first is support for a broad range of theoretical and computational studies, as well as next-generation computing facilities for multidimensional radiation hydrodynamic and particle-in-cell simulations and numerical relativity. The second is support for the next generation of observatories, public data access, public data products, and tools for data mining.

B-Q1. WHAT ARE THE MASS AND SPIN DISTRIBUTIONS OF NEUTRON STARS AND STELLAR-MASS BLACK HOLES?

Among the measurable properties of compact objects, two fundamental quantities are their mass and spin, which can constrain their birth and evolution. Significant advances in measuring these quantities have

been made recently, and more are expected in the coming decade. In particular, the GW detections of NS and BH binary mergers have opened new avenues for measuring masses and spins. Precision NS mass measurements are now possible for binary radio pulsars beyond NS-NS systems, and recent work has extended the NS mass range above 2 M_☉, approaching the theoretical upper mass limit near ~2.5 M_☉. GW detections of merging BH binaries have similarly proven the existence of >20 M_☉ BHs merging to form >40 M_☉ BHs, all substantially heavier than the BH population observed in X-ray binaries of the Milky Way Galaxy. Simultaneous measurements of NS mass and radius (or, equivalently, tidal deformability) through both X-ray pulsar timing and GW detection of NS-NS mergers has been achieved, pointing the way to eventual measurement of the full NS mass-radius relation and the equation of state of cold matter at supranuclear densities. New X-ray continuum surveys will more completely map the X-ray binary populations in the Milky Way and beyond, while next-generation radio surveys should find thousands of new radio pulsars, including binary systems where mass measurements are possible. Massive optical/infrared (IR) surveys for stellar radial velocity and photometric variations, astrometric motion, and microlensing will begin to probe the huge, previously unexplored population of isolated, free-floating, or non-interacting compact object/stellar binary systems. These efforts will directly inform the next generation of theoretical calculations of binary star evolution, simulations of massive star SN collapses and explosions, NS and BH formation, and GW-driven mergers.

B-Q1a. What Do the Mass and Spin Distributions Tell Us About Neutron Star and Black Hole Formation and Evolution?

Explaining the observed birth-mass and birth-spin distributions of NSs and BHs remains an important unsolved problem. A detailed understanding of the NS mass and spin distributions could map to different massive star progenitors; map to different binary evolution channels; or distinguish between iron-core collapse, accretion-induced collapse, and electron-capture SNe. For example, the lowest-mass NSs are currently expected to form from the lowest mass collapsing massive stars. Similarly, in some models of SNe, the lowest mass BHs should be ~5 M_☉. Whether or not there is a true “mass gap” in the distribution of NS and BH birth masses between 2.5 and 5 M_☉ constrains both the SN explosion mechanism and the potential for “fall back” during or after the explosion, with direct connection to the chemical enrichment of iron-peak elements in galaxies over cosmic time. (Note the new announcement in June 2020 of the compact binary merger GW190814, one of whose progenitor components was a 2.6 M_☉ compact object of unknown type—either a massive NS or a light BH.)

As with NSs, the BH mass and spin distributions trace the physical origin of isolated and binary BHs. The GW discovery of binary BHs with component masses >20 M_☉ was a major surprise to many astrophysicists, even though it had been predicted. The BH mass distribution, combined with the distribution and orientation of spins, may help reveal the physical origin of these BHs: do they arise from normal massive stellar binary evolution, triple/multiple star systems, or dynamical scattering in very dense stellar systems like globular clusters? That there might be gaps or breaks in the mass distribution at even higher masses may be understood as arising from known and hypothesized evolutionary pathways of single massive stars. For example, the theory of pair-instability SNe predicts the existence of a gap in the BH mass distribution in roughly the 50–140 M_☉ range. (Note the new announcement in September 2020 of the BH-BH merger GW190521: the masses of its product and one of its progenitors lie within, or near the edges of, the pair-instability gap.)

The rich array of questions that the panel is poised to explore includes the following: Are there features in the NS and BH mass distributions? Is there a sharp cutoff or a gradual decline in the BH mass function at low and high mass? What is the distribution of binary mass ratios? Is there a significant population of NS-BH binaries or of second-generation, hierarchically formed massive BHs? The question of whether most BHs are born spinning rapidly further constrains possible formation mechanisms. The observation

of a large sample of BH-BH mergers out to high redshift may reveal how these distributions evolve with redshift and metallicity.

B-Q1b. What Is the Population of Noninteracting or Isolated Neutron Stars and Stellar-Mass Black Holes?

More than 100 million NSs and perhaps more than 10 million BHs exist in the Milky Way Galaxy. Yet, our knowledge of these systems is mostly confined to pulsars and accreting systems, with just a handful of non-pulsar/non-accreting systems known. This may lead to a substantial bias in our understanding of the underlying population. Can we detect the huge expected population of free-floating BHs and “quiet” NSs? Can we identify a substantial population of massive dark companions in binaries? This represents a huge discovery space over the next decade and beyond. Because most massive stars occur in binary or higher order multiple systems, the mass distribution of NSs and BHs constrains critically uncertain aspects of interacting binary and massive star evolution. A substantially larger sample of NS- or BH-stellar binary systems could potentially allow for an understanding of the relative rate of SN success/failure as a function of metallicity because the companion stars can be characterized in detail.

B-Q1c. What Is the Equation of State of Ultradense Matter?

A more complete characterization of the NS and BH mass and spin distributions will also reveal fundamental physics. The equation of state of matter at supranuclear density cannot be probed in terrestrial laboratories, but it manifests through the NS mass-radius relation and is thus often referred to as the NS equation of state (NS-EOS). Constraints on the NS-EOS come from populating regions of the mass-radius plane with measurements from NSs, and from seeking out the extremes of the NS and BH mass distributions. In particular, pulsar searches have provided the most massive NSs, limiting the maximum NS mass from below, whereas the discovery and characterization of low-mass BHs limit the maximum NS mass from above. Double pulsar systems may provide the first measurement of an NS moment of inertia, providing a complementary constraint. The highest-spin NSs can, in principle, also constrain the NS-EOS through centrifugal limits; however, for reasons still not understood, the pulsars found to date all have spin frequencies significantly below the theoretical maximum.

Another approach to constraining the NS-EOS is to directly and simultaneously measure the masses and radii of a large sample of NSs. Precise radius measurements are particularly challenging. X-ray pulse profile modeling of NSs with the Neutron Star Interior Composition Explorer (NICER) and the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo detection of the NS-NS merger GW170817 have both demonstrated this approach and provided preliminary constraints on the NS-EOS. A large population of GW-detected NS-NS mergers will yield important independent constraints on the maximum NS mass and the tidal deformability (and hence the associated radius), and thereby a strong constraint on the NS-EOS. More sensitive X-ray timing of a large sample of millisecond pulsars (both rotation-powered and accretion-powered) may allow for a complete characterization of the NS mass-radius plane, mapping out the full NS mass-radius relation rather than relying on a few, isolated mass-radius points.

Relevant Measurements and Capabilities

Over the next decade and beyond, our understanding of the NS and BH mass and spin distributions will be revolutionized by new observations across the EM spectrum and by a host of other messengers. In particular, currently planned high-frequency (Hz/kHz) GW detectors will directly reveal the mass and spin distributions

of compact objects in merging BH-BH binaries to z ~1 and NS-NS binaries to ~1 Gpc. More advanced GW detectors could extend this range to z ~20 (BH-BH) and z ~5 (NS-NS), respectively, yielding an enormous sample of compact objects that would provide fundamental constraints on general relativity, the ultra dense matter equation of state, the diverse formation channels of merging BH and NS binaries, and the evolution of these channels with redshift and metallicity. Achieving this requires an order of magnitude improvement in the sensitivity of ground-based detectors beyond current design sensitivity, with even greater sensitivity improvement at lower frequencies (down to a few Hz). Multiple detectors will enable the localization of thousands of NS-NS binaries to better than 1 deg², well matched to synoptic EM surveys for prompt counterparts.

The next generation of wide-field pulsar searches will reveal new extreme pulsar systems, pulsar-WD binaries, more double-pulsar binaries, and perhaps the first pulsar-BH binary, all providing fundamental mass and spin measurements. However, this will require pulsar search/timing instrumentation for both single-dish (using multipixel receivers) and array observatories (where pulsar searches are extremely computationally expensive). It will also require observational investment for follow-up timing of each new pulsar to determine if they are scientifically interesting. Larger-area X-ray timing observatories can directly constrain the masses and radii of NSs in both accreting and bursting NS systems through pulse profile modeling, while more sensitive X-ray imaging and spectroscopy can more fully reveal the X-ray binary populations of our and other galaxies and allow for disk reflection line measurements of BH spins.

Missions like Gaia and its precision astrometric successors will probe the large population of stellar binaries with dark compact-object companions. Combined with current and forthcoming massive spectroscopic surveys like the Dark Energy Spectroscopic Instrument (DESI), Sloan Digital Sky Survey (SDSS)-V, and the next generation of massively multiplexed stellar spectroscopy, the very large (but still uncharacterized) population of NS- and BH-stellar binaries of the Milky Way will be revealed. The population of free-floating and otherwise undetectable NSs and BHs will be explored for the first time with the Nancy Grace Roman Space Telescope (formerly Wide Field Infrared Survey Telescope) gravitational microlensing survey. Follow-up observations with 8 m-class telescopes and 30 m-class telescopes (extremely large telescopes, or ELTs) will be required for more complete characterization of individual events. The Laser Interferometer Space Antenna (LISA) and other mHz GW detectors will open up the currently inaccessible regime of the numerous quiescent and otherwise EM-dark GW lighthouses of NS and BH binaries that are destined to merge on gigayear time scales in our own galaxy.

B-Q2. WHAT POWERS THE DIVERSITY OF EXPLOSIVE PHENOMENA ACROSS THE ELECTROMAGNETIC SPECTRUM?

Astrophysical transients (energetic events that appear in the sky only briefly) are signposts of the most catastrophic events in spacetime. Known classes include stellar explosions, stellar disruptions by supermassive BHs, stellar eruptions, and mergers of stars or compact objects, as well as very short bursts of radio emission with uncertain origin. Transients lie at the intersection of several critical areas of modern astrophysics and cosmology. Stellar explosions create dust, help trigger the formation of new stars, and produce BHs and NSs. Some transients are valuable “standard candles” used to trace the acceleration of the universe. The death throes of massive stars deposit radiative and mechanical energy into the interstellar medium (ISM) and drive the chemical enrichment and evolution of their host galaxies. Shocks from massive stellar explosions provide a key way to constrain the still-mysterious mass-loss history of massive stars before core collapse. Fast radio bursts offer a completely new probe of the intergalactic medium (IGM) and large-scale structure, while NS mergers play an important role in the synthesis of the heaviest elements of the periodic table. Additionally, shocks launched by a variety of astronomical transients constitute unique laboratories for relativistic particle acceleration under extreme physical conditions.

Recent technological advances have led to a revolution in the investigative power of astronomical time-domain surveys, which in turn have led to the discovery of new classes of transients (e.g., fast radio bursts, superluminous SNe, stellar mergers) and enabled the exploration of new parameter spaces. Upcoming optical surveys like the Legacy Survey of Space and Time (LSST) with the Vera C. Rubin Observatory will take this effort to the next stage, and this revolution will soon encompass a broader range of wavelengths outside the optical and gamma-ray bands—for example, radio with Square Kilometer Array precursors; X-rays with Extended Roentgen Survey with an Imaging Telescope Array (eROSITA); and near-IR with the Roman Space Telescope. Additionally, the recent discovery of GWs and light from the NS merger event GW170817, and the possible association of the high-energy neutrino event IceCube-170922A with blazar TXS 0506+056, clearly demonstrate the rich connection between EM transients and other astronomical messengers. The time is ripe to fully realize the discovery potential of this multi-messenger data stream and develop a complete physical understanding of the rich phenomenology of the transients that are observed.

B-Q2a. When and How Are Transients Powered by Neutron Stars or Black Holes?

Understanding the central engines (newly formed compact objects like magnetars and BHs) that power many explosive transients continues to be a fundamental astrophysical challenge. For example, superluminous SNe (SLSNe) have peak luminosities ~10–100× those of normal core-collapse SNe, requiring an additional source of energy beyond the traditional neutrino-powered SN mechanism. Possibilities include rotational energy from magnetars, or gravitational binding energy liberated during BH accretion. Central engines can also manifest through relativistic collimated outflows (i.e., jets in long-duration GRBs, tidal disruption events, or compact binary mergers). Which unique physical conditions enable some transients to launch ultra-relativistic jets? What are the nature and properties (e.g., mass, spin) of the newly formed compact objects? Is there EM emission before compact binary mergers, and what can it tell us about the properties of the progenitor systems? Current speculation is that fast radio bursts are also manifestations of cataclysmic events involving NSs or BHs. What actually triggers fast radio bursts? (The panel notes new observations in 2020 associating some fast radio bursts with magnetars.)

B-Q2b. When and How Are Transients Powered by Shocks?

Transient mass ejections produce shocks, either with an external medium or internal to the outflow itself. Shocks accelerate particles that subsequently radiate photons through thermal and nonthermal processes and allow for an efficient conversion of shock kinetic energy into radiation. For which transient phenomena does the efficient conversion of kinetic shock energy to radiation represent the dominant source of energy, and why? What determines the radiative efficiency of thermal and nonthermal processes?

Remarkably, classical novae (thermonuclear outbursts from accreting WDs) have recently been discovered to produce detectable gamma rays, something not theoretically predicted. This finding suggests an unexpectedly important role for strong shocks in nova phenomenology. Observations and better understanding of these nearby common transients may help test the hypothesis that SLSNe and other stellar explosions are also shock-powered. In stellar explosions, the breakout of shock radiation is the very first EM signal that reaches the observer, and it carries a wealth of information about the exploding star. What can we learn about the largely unconstrained population of SN progenitors from systematic observations of shock breakouts across the EM spectrum?

B-Q2c. When and How Are Transients Powered by Radioactivity?

The radioactive decay of newly formed nucleosynthetic products is a known source of energy powering the optical light-curves of both ordinary core-collapse (type II) and thermonuclear (type Ia) SNe. The thermalization of gamma rays originating from the β-decay of ⁵⁶Ni in the ejecta of type Ia SNe provides a key energy input to their early light curves. While it is clear the type-Ia SNe originate from carbon-oxygen WDs in binaries, the mechanism that triggers the explosion and the mapping between type-Ia SN observables and progenitor types remain unclear. Is the progenitor a WD-WD merger in a double-degenerate binary, or an accreting WD in a single-degenerate binary? Although type-Ia SNe were traditionally understood to arise from a WD exceeding the 1.4 M_☉ Chandrasekhar limit, recent theoretical and observational progress has highlighted that WDs over a relatively large range of masses can explode as a type-Ia SN, including both sub- and super-Chandrasekhar progenitors (the latter arising either from rapid rotation or from a WD-WD merger). Multiple channels may be possible. A critical unknown in type-Ia SN studies is to determine what fraction of explosions is produced by each channel.

Recent observations demonstrate that heavy elements were produced through r-process nucleosynthesis in the neutron-rich ejecta of the NS merger event GW170817 and that the subsequent radioactive decay powered a transient known as a kilonova, which evolved on a week-long time scale. These observations establish NS mergers as one of the sites of r-process nucleosynthesis. After this landmark discovery, the frontier is now to answer the question: Are NS mergers the main site of r-process nucleosynthesis and the main source of heavy chemical elements in the universe, or are there supernovae and other core-collapse events that contribute significantly to the heavy r-process budget of the universe?

B-Q2d. What Are the Unexplored Frontiers in Transient Phenomena?

During the past decade, we have witnessed the proliferation of discoveries of unanticipated classes of transients with observed properties that challenge traditional classification schemes or paradigms. The most prominent examples are fast radio bursts and gamma-ray transients associated with classical nova outbursts. Other examples include peculiar thermonuclear SNe (e.g., type Iax), stellar eruptions preceding core collapse, transients in the luminosity gap between classical novae and SNe, the very rapid time scales of fast and blue optical transients (FBOTs), and the extreme luminosities of SLSNe.

At the same time, there are solid theoretical predictions of astrophysical transients that are still lacking an uncontroversial observational example. These include the accretion-induced collapse of a WD into an NS, pair-instability SNe, and the merger of an NS-BH binary. Which other transient classes have yet to be revealed? Which theoretical models will find observational confirmation? What is the role of other sources of energy (like magnetic reconnection, free neutron decay, recombination) to power astrophysical transients? How do the explosions of the first stars appear?

Relevant Measurements and Capabilities

Progress in the field of astrophysical transients critically depends on two key capabilities: discovery power and understanding. Discovery power is effectively enabled by observing facilities with large fields of view, able to monitor the sky in real time. A healthy ecosystem of optical/IR transient surveys with a range of sensitivities and temporal cadences, combined with prompt public release of discoveries and data, is required to find and characterize transients over the entire range of time scales, distances, and luminosities produced by the cosmos. Wide-field monitors in the ultraviolet, X-ray, and low/medium-energy gamma-ray bands are needed to open the fields of SN shock breakout, to enable systematic exploration of tidal disrup-

tion events, and to maintain detection capabilities of GRBs in conjunction with GW events. Wide-field MeV gamma-ray spectroscopy is needed to detect nuclear line emission from Sne. High-frequency (Hz/kHz) GW observations with better localization and sensitivity are needed to enable larger samples of NS mergers with EM counterparts. An MeV neutrino observatory with an order-of-magnitude improvement in sensitivity relative to hyper-Kamiokande would allow detection of ~1 core-collapse SN per year. A wide-field radio time-domain survey with arcsecond-localization capabilities (like the Canadian Hydrogen Intensity Mapping Experiment [CHIME], but with sufficiently precise positioning for multiwavelength follow-up) is needed to map the radio transient sky and enable fast radio burst detection and characterization.

To understand the physics powering these transients, a variety of follow-up observing machines is required. Massively multiplexed optical spectroscopy over wide fields is needed for transient classification and characterization. Specifically, a massively multiplexed spectrograph with rapid repointing capabilities (able to promptly slew to the location of the rarest and most interesting transients while also acquiring large samples of spectra of known classes of transients) would be uniquely positioned to maximize the scientific return from time-domain astronomy in the next decade. Radio facilities with good (~μJy) flux sensitivity, sub-milliarcsecond (mas) angular resolution, and 1–100 GHz coverage are needed to map the kinetic energy structure of the explosion ejecta, look for the presence of relativistic jets, and constrain the onset of pulsar wind nebula-like emission. Broadband X-ray capabilities with high sensitivity (10^–19 erg/s/cm² at <10 keV; 10^–14 erg/s/cm² at 10–100 keV) are needed to map the kinetic energy structure of the ejecta, uncover the presence of relativistic jets, and map the media surrounding SNe, tidal disruption events, and NS mergers. Deep ultraviolet/optical/near-IR spectroscopy is needed to constrain the chemical composition of the ejecta, with particular emphasis on nebular phase spectroscopy and spectroscopy of distant kilonovae discovered by GW detectors. Except for the very nearest events, ELTs will be required for late-time kilonova spectroscopy. An additional crucial need is for co-observing and rapid response capabilities across all observatories.

B-Q3. WHY DO SOME COMPACT OBJECTS EJECT MATERIAL IN NEARLY LIGHT-SPEED JETS AND WHAT IS THAT MATERIAL MADE OF?

Relativistic jets—collimated beams of ejected material moving at nearly light-speed—are observed in a variety of systems: SMBHs in AGN, stellar-mass BHs and NSs in X-ray binaries, GRBs, and tidal disruption events (TDEs). Although jets have been intensively studied for many years with a variety of observational and theoretical techniques, they are still poorly understood. Several recent developments make this area ripe for progress. The Event Horizon Telescope (EHT) imaged the base of the jet in the nearby AGN M87 at an angular resolution comparable to the projected Schwarzschild radius. The panchromatic detection (radio to gamma ray) of an off-axis relativistic jet in the NS-NS merger event GW170817 discovered by LIGO/Virgo enabled the first constraints on jet structure in a transient (how energy is distributed within the jet). Atmospheric Cherenkov telescopes discovered TeV gamma rays from GRB jets. IceCube discovered extragalactic TeV-PeV neutrinos, which plausibly originate in powerful relativistic jets. Numerical simulation codes are now capable of first-principles investigation of magnetohydrodynamical jets and particle acceleration. Computer hardware (CPU/GPU) is approaching the power required to simulate problems in three dimensions. All of these nascent developments are poised for explosive growth.

B-Q3a. How Do Jets Launch and Accelerate?

At its base, a relativistic jet may be initiated and powered by rotating gas in the accretion flow via magnetic fields, gas pressure, or radiation pressure. Alternatively, jets may draw power directly from the

spin energy of the BH via frame-dragged magnetic fields. At present, there is no robust observational evidence favoring any proposed jet-launching mechanisms. If we could find evidence that relativistic jets extract power directly from spinning BHs through some combination of theory, simulation, and ultra-high-resolution observations of jet-launching regions, it would be a spectacular demonstration that BHs do not just consume mass and energy—that they sometimes also return energy to the world outside.

Accreting NSs sometimes produce relativistic jets (usually during a hard-to-soft spectral state transition), and there are indications that accreting WDs can also have jet-like activity. How similar are the observational properties of jets from different types of compact objects? This could provide clues to the jet-launching mechanism. Can we numerically simulate the launching of NS jets? Relativistic jets in both long- and short-duration GRBs may be launched from either BHs or highly magnetized NSs (magnetars). Can we combine theory and observations to discriminate between these possibilities? Do WD jets move relativistically—and if so, what physical process drives such rapid ejection from such shallow gravitational potentials?

The Lorentz factor of the jet in M87 has been mapped as a function of distance from the central SMBH. The acceleration appears to be gradual, and the peak Lorentz factor is reached in only ~10⁶ Schwarzschild radii. Is this behavior universal? What is the physics that controls the rate of acceleration? What determines the maximum Lorentz factor of the jet? What role do the available power, the amount of baryon loading, and drag from an external medium (ISM, stellar envelope, AGN cocoon) play? Why do stellar-mass BH jets reach Lorentz factors of only a few, while AGN jets (especially in luminous blazars) reach up to a few tens, and GRB jets reach up to a few hundreds?

B-Q3b. What Are Jets Composed of and How Are Particles Accelerated Within Them?

Leptons (electrons/positrons), baryons (protons/nuclei), and magnetic fields are all believed to play a role in determining jet dynamics. But the relative importance of these components is not known. Is the jet launched as an electron-positron pair plasma or as an electron-ion plasma? How much baryon contamination does the jet subsequently experience, and where does most of it happen? What fraction of the power is carried by the magnetic field, and how does it vary with distance from the central object? The fraction of EM emission in jets that is hadronic versus leptonic in origin has important implications for energy requirements (by several orders of magnitude) and for AGN feedback.

Relativistic jets produce nonthermal EM radiation, indicating the presence of electrons and positrons with power-law energy distributions and emitting synchrotron and inverse-Compton radiation. How are these particles accelerated, and what determines their energy distribution (minimum and maximum Lorentz factor, slope of the energy distribution)? Theory suggests several candidate acceleration mechanisms: Fermi acceleration in shocks, magnetic reconnection, and shear acceleration. Which of these dominates in any particular system? Could multiple mechanisms operate in the same system, either co-spatially or at different locations along the jet? The particle acceleration processes studied in jets are broadly relevant to many other areas (solar and planetary physics, space physics, plasma astrophysics, etc.) as well.

B-Q3c. Are TeV-PeV Neutrinos and Ultra-High-Energy Cosmic Rays Produced in Relativistic Jets?

Ultra-high-energy cosmic rays (UHECRs, in the ~EeV range) can contribute significantly to the reionization of the IGM at the epoch of formation of the first AGNs and GRBs. They are also an important pressure component in the IGM of galaxy clusters as they form and virialize. UHECRs are widely surmised to be accelerated by shocks in the relativistic jets of AGNs or GRBs, with other sources also possibly coming into play. Can we verify that this idea is correct? Direct identification is difficult, because UHECRs are

charged particles and thus lose directionality as they diffuse through magnetic fields. However, a clear directional signature would be provided by the high-energy neutrinos that UHECRs produce via interaction with the ambient photons in jets. Is this the origin of astrophysical high-energy neutrinos? An isotropic TeV/PeV neutrino background of astrophysical origin has been identified, but its origin in specific sources remains uncertain. Attempts at positional and temporal correlations with EM-detected bright GRBs have so far yielded negative results, but a 3σ positional and temporal correlation between gamma-ray flares and a high-energy neutrino associated with the blazar TXS 0506+056 is suggestive. Stacking analyses on other similar AGNs, however, indicate that additional types of sources may also need to be considered in order to account for the entire TeV-PeV neutrino background. Increased angular resolution and sensitivity of neutrino detectors are needed to address this.

Hadronic interactions between UHECRs and photons produce a comparable amount of energy in secondary gamma rays and neutrinos in the GeV-TeV range or above. Most of the higher-energy gamma rays will cascade down to lower energies via γ–γ interactions so that some of the observed GeV or lower-energy photons may be owing to this. In AGNs, would such hadronic cascades fill in the saddle point between the high- and low-energy humps of the EM spectral energy distribution? Does the lack of such radiation rule out significant UHECR acceleration in jets, or does it instead imply a low photo-hadronic optical depth on the target photons? These are questions for both theorists and observers.

Relevant Measurements and Capabilities

High-angular-resolution radio/millimeter imaging will address critical questions of jet launching and acceleration. With better (sub-mas to μas) angular resolution, polarization data for monitoring the magnetic field, and multiple observations to explore dynamics, one will be able to answer many key questions on jet launching. Improving angular resolution to a few μas and sensitivity to 1 mJy would increase the number of SMBH targets where the base of the jet can be imaged beyond the two or three currently accessible. Studying structure and acceleration along the length of a relativistic jet calls for high-angular-resolution imaging in several bands. Sub-mas angular resolution at cm wavelengths, with full polarization, would provide information on the jet inclination, lateral structure, magnetic field strength, and variation of Lorentz factor with distance for many objects. High-angular-resolution imaging in the optical and infrared with <10 mas resolution and in X rays with sub-arcsec resolution would provide substantial new empirical information on the jet acceleration zone. Significantly improved capabilities for radio, optical, X-ray, and gamma-ray polarimetry will be invaluable for exploring the magnetic fields and composition of jets, as well as the presence of shocks and the field orientation with respect to the shock normal, which impacts the particle acceleration efficiency. High-angular-resolution Faraday rotation measurement in radio will be particularly critical for these goals.

General-relativistic magnetohydrodynamics (GR-MHD) codes are now able to follow jets from launch out to many orders of magnitude in distance, precisely the region where the above instrumental capabilities will provide observational data. Codes running on fast GPUs are seeing large speed increases over CPU-based codes. Training young scientists in the use of these specialized techniques, as well as investments in the relevant hardware, will be critical in this growth area.

High-frequency (Hz/kHz) GW observations of NS mergers and low/medium-energy gamma-ray followup will be able to measure the time between initial energy release and the first EM emission from jets in short-duration GRBs. Combining the information on jet inclination angle from GW and EM observations is required to explore the nature and evolution of the jet structure.

The energy distribution of nonthermal electrons in a jet can be deduced from observations of the broadband synchrotron spectrum. Spatially resolved spectra will show where particles are accelerated and how

their energy distributions evolve, providing key constraints for theoretical models. The particle acceleration process itself, whether by shocks, reconnection, or shear, is best studied via numerical particle-in-cell (PIC) simulations, with 3D simulations about to become more routine. We can expect to understand the slope of the particle energy distribution for each acceleration mechanism, and the nature of the lower cutoff in the particle Lorentz factor. Determining the maximum Lorentz factor is more challenging. Support for theoretical and computational work and investment in computer hardware are essential for progress in this field.

For high-energy neutrinos, an effective detector volume an order of magnitude larger than IceCube and increased support for data analysis capabilities are required for detecting and localizing neutrinos from individual sources like TXS 0506+056 and to significantly increase the neutrino sample to enable studies of clustering and spectra. For rapid follow-up and correlation studies, continued support of Swift and Fermi spacecraft operations will be crucial until newer missions replace them. Also important are observations to study UHECR clusterings, anisotropies, composition, and spectra. Last, wide-field MeV-GeV gamma-ray facilities will be essential for identifying counterparts to high-energy neutrino and UHECR sources, along with wide-field and follow-up capabilities in the GeV-TeV band to extend these observations to higher energies for nearby sources.

B-Q4. WHAT SEEDS SUPERMASSIVE BLACK HOLES AND HOW DO THEY GROW?

While it is well established that SMBHs and galaxies grow together over cosmic time, the physics of both what seeded SMBHs in the first place and the processes that govern their growth remain poorly understood. This issue has couplings across astrophysics. Questions of accretion physics connect to AGN phenomenology and feedback as well as to X-ray binaries and other accreting stellar-mass compact objects. The nature of BH seeds—and the rate and timing of their growth—is also an important factor in understanding the sources of cosmic reionization.

There are unprecedented opportunities ahead for determining the nature of the seeds of SMBHs and understanding the physics of how they have grown to the present-day population. Gravitational wave observatories like LISA and pulsar timing arrays will detect merging BHs over the 10⁴–10¹⁰M_☉ range. Observatories are proposed with high sensitivity and high angular resolution across the EM spectrum, offering dramatic leaps in our understanding of the IMBH/SMBH population and growth mechanisms. Currently, our knowledge of super-Eddington accretion is rapidly growing on both the theoretical and observational fronts with the discovery of high-redshift massive quasars and NS ultra-luminous X-ray sources, as well as the first global 3D radiation magnetohydrodynamics simulations with realistic radiative transfer. In the next decade, a new era of 3D models of BH accretion—fully accounting for general relativity, radiation, and magnetohydrodynamics—will extend our understanding to the radiatively efficient accretion regime and provide physical calculations of radiative efficiency, accretion efficiency, and the launching of outflows and jets.

B-Q4a. How Are the Seeds of Supermassive Black Holes Formed?

At high redshift, SMBHs may originate with light seeds (~10²M_☉; the compact remnants of the first generation of stars), intermediate-mass seeds (~10³–10⁴M_☉; from gravitational runaway in dense star clusters), or heavy seeds (~10⁴–10⁶M_☉; from direct collapse of gas in high-redshift halos). We will soon have the potential to discriminate between these models and determine the primary channel of SMBH seeding. One important diagnostic is the mass distribution of BHs at high redshift. Will we find evidence for a very large population of ~100 M_☉ BHs at z ≳10, or will observations imply a smaller number of ~10⁵M_☉ BHs? Another powerful test of seed models is the population of IMBHs (~10²–10⁴M_☉) in local galaxies. How many off-center IMBHs are there, and what is their mass distribution? If SMBHs were formed from light

seeds, there should be many IMBHs that failed to merge into a galaxy’s central SMBH and survive today as wandering BHs.

B-Q4b. How Do Central Black Holes Grow?

In order to understand what seeded SMBHs at high redshift, it is important to also understand the physics of SMBH growth. Because we can observe BHs only when they are growing, we need to understand which BHs are growing and why in order to extrapolate to the broader population. Furthermore, if the seeds of SMBHs are light, then BH growth is by necessity more efficient at high redshift, so the rates and efficiency at which BHs gain mass provide an additional test of seed models.

In the coming decades, important progress can be made in understanding the role of BH-BH mergers in forming the population of SMBHs seen today. What is the rate of SMBH mergers as a function of mass (~10²–10¹⁰M_☉) and redshift (out to z ~20)? What fraction of binary (bound) and dual (neighboring but unbound) SMBHs merge? By answering these questions and measuring the spin distribution of IMBHs and SMBHs, the role of mergers in growing the SMBH population can be determined.

We are also poised to make strides in understanding BH accretion, which is also essential for understanding the physics of BH growth. Is super-Eddington accretion important in growing SMBH seeds? New sensitive facilities will be able to measure the accretion signatures of ~10⁵M_☉ BHs at z ~10. Meanwhile, theoretical work and observational studies in the more local universe will shed light on the physics of super-Eddington accretion and its EM signatures. Lower accretion rates are also important for SMBH growth, so we must understand the structure and stability of sub-Eddington quasar accretion disks as well as their radiative and accretion efficiencies. Last, a better understanding of the population of tidal disruption events will determine how often SMBHs swallow stars, and whether stellar tidal disruption is a significant contributor to SMBH growth.

Relevant Measurements and Capabilities

A range of planned facilities will be instrumental in enabling this science. LISA will detect mHz GWs from mergers of 10^4–10⁷M_☉ BHs out to z ~20, yielding measurements of SMBH mass, spin, and merger rate. It will also reveal IMBHs through extreme mass-ratio inspirals. In the coming decade, pulsar timing arrays will detect a background of nHz GWs from the population of more massive (≳10⁹M_☉) SMBH binaries and mergers throughout the universe, and potentially individual SMBH binaries out to z ~1. High-frequency (Hz-kHz) ground-based GW observatories will detect mergers of BHs straddling the stellar-mass to intermediate-mass divide (~10–10³M_☉). Currently planned detectors will reach to z ~1, while more advanced detectors could reach to z ~20. With this full-spectrum GW coverage, we will understand the population of merging BHs more than 10 orders of magnitude in mass. Planned time-domain optical surveys (including Rubin Observatory/LSST and other less-sensitive but higher-cadence facilities) will probe the population of tidal disruption events and find binary AGNs through periodicity searches. The Roman Space Telescope will constrain the presence of IMBHs in the Local Group by discovering hyper-velocity stars. The James Webb Space Telescope (JWST) will be able to efficiently establish a sample of high-redshift AGNs.

Further in the future, this science requires sensitive (~μJy), high-angular-resolution (sub-mas) radio imaging for finding accreting IMBHs and imaging binary AGNs. Sensitive (~10^–19 erg/s/cm²) X-ray observations with sub-arcsec angular resolution will also find accreting IMBHs and will additionally enable imaging of more widely separated dual AGN, measurements of SMBH spins, and—in concert with JWST spectra—measurement of the luminosity function of high-redshift AGN. Last, enhanced support for theoretical efforts is needed to enable accretion simulations and models of seed formation.

Discovery Area: Transforming Our View of the Universe by Combining New Information from Light, Particles, and Gravitational Waves

Astrophysical observations with non-EM messengers such as GWs, neutrinos, and UHECRs provide a new way to view the universe. Multi-messenger astrophysics, where these new observations are combined with more traditional data across the EM spectrum, opens enormous discovery space for understanding high-energy astrophysical sources, and provides new cosmological tools and tests of fundamental physics. For decades, there were only two examples of source-specific multi-messenger detections, both in MeV neutrinos: from the solar interior (starting in the 1960s), and from the nearby core-collapse supernova SN 1987A.

In the past decade, however, multi-messenger astrophysics has come of age. We have seen the advent of GW astronomy and the first detection of GWs and photons from the same astrophysical source. We have also seen the discovery of astrophysical high-energy neutrinos and a potential association with a specific astrophysical source. Last, we have obtained new constraints on cosmic rays within the Milky Way and extragalactic cosmic rays, including more precise measures of the spectrum and composition of UHECRs, the discovery of TeV halos, and the discovery of pevatrons (PeV cosmic ray sources) in our galaxy. There is enormous potential in multi-messenger astrophysics in the next decades, driven by improvements in ground-based GW detectors and neutrino observatories, by the advent of space-based GW observatories, and by the maturation of pulsar timing arrays. Multi-messenger astrophysics with these new messengers will be enabled by wide-field and rapid-response facilities across the EM spectrum for identification of EM counterparts and detailed follow-up studies. A few examples of this potential are given below.

B-DA1. Compact Binary (NS-NS and BH-NS) Mergers

Coordinated observation of compact binary mergers in both GWs and EM radiation is still in its infancy. However, improvements in ground-based GW interferometers combined with aggressive EM follow-up with existing and future facilities will usher in an era of population studies of NS mergers. As shown by our experience with GW170817, the combination of near-simultaneous gamma-ray and GW detections with rapid optical/infrared follow-up and gamma-ray/X-ray/radio monitoring can yield critical information. The next generation of instruments will enable detailed mapping between initial merger conditions (as determined by GWs and gamma-ray emission onset) and the merger outcome (e.g., BH or massive NS, jet/no-jet, jet physics/profiles), ejecta mass, ejecta chemical composition and r-process nucleosynthesis, and environment, as determined by kilonovae and long-term afterglows. We can hope for even more information from future nearby and favorably oriented events. In particular, had GW170817 been observed on-axis, the predicted TeV-PeV neutrino flux might have been detectable by current facilities, providing a new direct probe of particle acceleration and jet conditions for the first time. The detection of even a single NS merger in both high-energy gamma rays and neutrinos would provide unprecedented data, emphasizing the importance of having sufficiently sensitive instruments to enable this. The combination of GW and electromagnetic measurements will also provide standard siren probes of cosmology.

B-DA2. Astrophysical TeV-PeV Neutrino Sources

Neutrino astronomy has begun. Ongoing observations and improvements in TeV-PeV neutrino observatories will yield a large sample of astrophysical neutrinos over the next decade. This may enable identification of EM counterparts of neutrino sources, thus clarifying their origin. Are there multiple neutrino source classes? For known source classes, what characteristics determine neutrino intensity? Another exciting

possibility would be the discovery of positional coincidences between the highest-energy UHECRs and high-energy neutrinos. The prospects for such an identification will be greatly enhanced by future space-based UHECR observatories. Last, spatial, spectral, and composition measurements of UHECRs, combined with measurements of gamma-ray and both high-energy (TeV-PeV) and ultra-high-energy (EeV) neutrino diffuse emission, will establish the relationship between these quantities and possibly lead to a unified model to explain their origin.

B-DA3. Binary SMBHs

Groundbreaking near-future observations will be provided by low-frequency GWs with pulsar timing arrays (PTAs) and LISA. PTAs will likely detect the nHz GW stochastic background from the ensemble of >10⁸M_☉ SMBH binaries in the universe within the next few years, providing information about how SMBHs grow and evolve. By the end of the decade, PTAs could resolve multiple individual sources from either the closest SMBH binaries or the most massive (>10⁹M_☉). Those individual GW sources have the potential for exquisite EM follow-up, as the years-to-decades long periodicities of the SMBH binaries may manifest as variability across the EM spectrum. All-sky EM surveys may even facilitate PTA GW detections by initially identifying the most compact and nearby SMBH binaries, dramatically decreasing the size of the usual blind-search GW parameter space.

On a somewhat longer time scale, LISA measurements of mHz GWs will detect essentially every 10⁴–10⁸M_☉ SMBH merger in the universe, providing fundamental information on SMBH evolution. These sources may be accompanied by counterparts covering a broad range of the EM spectrum and a wide span of time scales, including transients before, during, or after the merger as well as persistent counterparts. LISA localizations will be ~10 arcmin², offering the opportunity to find these EM counterparts and enable multi-messenger studies of sources and their host galaxies. Combining LISA data with the highly complementary measurements provided by photons and particles will enable transformative multi-messenger science, including the birth and growth of supermassive BHs, and the expansion rate of the universe through standard siren measurements.

B-DA4. Galactic Ultracompact WD Binaries

LISA will detect thousands of ultracompact WD-WD binaries (and potentially many WD-NS binaries) in our galaxy that may be otherwise unidentifiable as such, or undetectable. Combining LISA detections with optical photometric, spectroscopic, and astrometric surveys (and with X-ray surveys for WD-NS binaries) will allow for a much more complete census of all types of WD binaries in our galaxy, including their mass distribution. It will also provide crucial constraints on progenitor models for type-Ia SNe and other transients and merger products. The WD-WD systems already identified as strong LISA candidates through optical studies show that this will be a powerful multi-messenger combination.

B-DA5. Diffuse Thermal Background from Core-Collapse Supernovae

In this decade, Super-Kamiokande (with gadolinium [Gd] loading added for improved sensitivity) may provide the first detection of the diffuse thermal neutrino background expected from the cosmic history of core-collapse SNe. The inclusion of Gd loading in its successor, Hyper-Kamiokande (an MeV neutrino experiment currently under construction in Japan), will provide significantly better sensitivity to this background with tens of events expected per year, ushering in a new era in neutrino astronomy. Combined with EM surveys of star formation and SNe, detection of the MeV neutrino background will provide a multi-

messenger connection to other tracers of core-collapse supernovae throughout the universe. The measured flux and spectrum of the neutrino background will provide information on the fraction of optically dark/unseen SNe, the fraction of core collapses that produce BHs, and important integrated constraints on the cosmic star formation history and the chemical enrichment of the universe from massive-star and SN nucleosynthesis.

B-DA6. A Supernova Within Our Own Galaxy

The ultimate multi-messenger event would be a core-collapse SN within our galaxy. This event would produce a large flux of neutrinos, nuclear MeV gamma-ray line emission, and broadband emission across the EM spectrum, and perhaps high-frequency GWs as well. Note that, conservatively assuming a supernova rate within our galaxy of only one per century (most published estimates are higher than this), the Poisson probability of at least one Milky Way massive star supernova occurring in the next 20 years is 18 percent. Given the transformational science return expected, it is worth planning seriously for this possibility.

MeV neutrino (and possibly GW detectors) would see such an event first, with many thousands of neutrinos detected over a few seconds in current and near-future detectors. These neutrino and GW detections would provide early warning and degree-scale localization for the full suite of humanity’s follow-up facilities to be deployed hours before shock breakout from a red supergiant progenitor (or perhaps just minutes before for compact stripped-envelope stellar progenitors). As in GW follow-up for NS-NS mergers, very wide-field monitors are necessary for quick identification, particularly in the IR given the large optical extinction along lines of sight in the galactic plane. Direct diagnostics of the explosion mechanism and the properties of the neutron star in formation (e.g., rotation, convection) could be gleaned from simultaneous neutrino and GW detections in the first seconds after collapse. Neutrino flavor information would inform neutrino physics and our understanding of SN nucleosynthesis. If it occurred, the transition from NS to BH would be imprinted on these signals, with profound implications for our understanding of this process. The late-time neutrino emission would directly inform our incomplete understanding of the birth of NSs. As evidenced by the famous few previous Milky Way core-collapse SNe (the Crab, Cas A, and SN 1987A), such an event would be studied for centuries.

The Kepler and Tycho SN remnants demonstrate the need to be equally prepared for the next Milky Way type Ia SN. Indeed, the expected rate is of the same order as that for core-collapse SNe. Here, the primary overlap in messengers is between the potential LISA detection of mHz GWs from the compact WD binary progenitor before explosion and information from the EM regime: MeV gamma-ray line emission from ejected nuclear products, multiwavelength continuum radiation from the SN, including nonthermal emission from shock acceleration, and possibly a direct connection to the EM progenitor, which may be identified in existing catalogues. Without neutrino or high-frequency GW triggers to provide real-time advance information, it will be LISA, ultra-wide-field all-sky EM monitors, amateur astronomers, and our own eyes that may alert us to an event as it begins.

B-DA7. Fundamental Physics

The science return from multi-messenger astrophysics extends beyond astrophysics. The extreme energies involved in interactions of UHECRs (and production of associated neutrinos and EM radiation) explores particle interaction cross sections at energies well beyond those achievable in terrestrial particle accelerators, allowing the study of exotic particle physics models. The relative arrival times of GWs and gamma rays from NS mergers provides the best measurement of the speed of gravity and a test of the weak equivalence principle. The relative arrival times of neutrinos and gamma rays from the same astrophysi-

cal event would provide a complementary test of the weak equivalence principle. NS formation, NS-NS mergers, and NS mass and radius measurements all constrain the equation of state of ultradense matter and the phase diagram of quantum chromodynamics (QCD).

B-DA8. Other Possibilities

More speculatively, there is a wealth of additional phenomena that may emerge with multi-messenger observations in the next two decades. EM emission from stellar-mass BH mergers may be seen. With improvements in the sensitivity of ground-based GW interferometers, continuous GWs from a known radio or X-ray pulsar may be detected, providing the first measurement of an NS quadrupole moment. We may find new sources of both high-energy and thermal neutrinos from sources such as SN shock interactions, long-duration GRBs, tidal disruption events, and classical novae. We may be able to use heavy cosmic-ray abundance measurements in the Milky Way to constrain sites of r-process nucleosynthesis. Last, with ongoing and improved GW observations at all frequencies, we may find anomalous GW events that challenge general relativity, such as violation of the no-hair theorem, non-GR ringdown, or entirely new and unanticipated classes of events. With current and upcoming facilities for multi-messenger astrophysics, we are opening a vast new discovery space. This virtually ensures that the most exciting new results will be in entirely unexpected areas.

Relevant Measurements and Capabilities

The key requirement to maximize the science return in multi-messenger astrophysics is a broad range of facilities operating contemporaneously. The specifics of the needed capabilities for the individual messengers and EM bands are discussed in previous sections and are summarized in Table B.2.

TABLE B.1 Key Science Questions and Discovery Area

TABLE B.2 Required Capabilities

NOTE: See Appendix P for the definitions of the acronyms and abbreviations in the table.