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Appendix L: Report of the Panel on Particle Astrophysics and Gravitation
Pages 440-463

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From page 440...
... Astronomy is now also being revolutionized by observations with new messengers -- gravitational waves, neutrinos, gamma rays, and cosmic rays -- that greatly complement and are leveraged by observations in conventional astronomy. The hallmark of this work, for which the projects have largely been developed, funded, and carried out as part of physics programs, is its ability to probe extremes of energy, fields, and density.
From page 441...
...  Gravitational-wave program: Medium-scale investments in three bands (kHz, nHz, and mHz) to develop a rich observational program: Cosmic Explorer, with NSF support for technology development to set the stage for large-scale investments and huge detection rates in the 2030s; the North American Nanohertz Observatory for Gravitational Waves (NANOGrav)
From page 442...
... L.2 DREAMS OF NEW ASTRONOMIES The fundamental goal of astronomy is to observe and understand the universe and its constituents. In 2015, the detection of gravitational waves from the collision of two black holes (GW150914)
From page 443...
...  Extreme accelerators, with huge luminosities of charged particles and accompanying gamma rays and neutrinos, and with per-particle energies ranging up to the TeV–PeV range and sometimes much higher. IceCube observations of the diffuse neutrino flux suggest a dominant population of sources that are gamma-ray obscured, showing that neutrino observations are essential to astronomy.
From page 444...
... For gamma rays and gravitational waves, this has been attained; for high-energy neutrinos, it is within close reach; and for cosmic rays, it is a hope to be nurtured. Sources that appear only in gravitational waves and/or neutrinos are especially interesting, as they reveal the universe's dark processes.
From page 445...
... Importantly, the panel considered how to maximize the scientific return by developing diverse fields as a coherent whole on a viable, coordinated timeline. The fields in the scope of the panel -- gravitational waves, neutrinos, gamma rays, and cosmic rays -- need to be considered coherently with each other and with the conventional astronomy program to maximize the value of research in each field and for astronomy as a whole.
From page 447...
... Other examples include a Milky Way supernova, a nearby long GRB, and the tidal disruption of stars by black holes. L.4 MISSIONS AND PROJECTS ENDORSED FOR THE SURVEY A successful new-messenger program requires investments in this decade in new observatories for gravitational waves, neutrinos, and gamma rays.
From page 448...
... From observations of high-energy gamma-ray sources, there are PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION L-9
From page 449...
... More generally, the first definitive source detections in high-energy neutrinos would have a huge, broad impact, much like that for the first detection of a binary neutron star merger in gravitational waves. Contemporaneous observations of IceCube-Gen2 and other facilities -- including those for gamma rays (CTA, based in Europe, and the Large High Altitude Air Shower Observatory [LHAASO]
From page 451...
... By lowering the low-frequency sensitivity limit of LIGO from 10 to 5 Hz and reducing the noise by a factor of 10, Cosmic Explorer will reach gravitational-wave signals all the way back in cosmic time, for a powerful and diversified science program. The science goals for Cosmic Explorer include determining the nature of the densest matter in the universe, enabling multi-messenger observations of binary neutron star systems, and PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION L-12
From page 452...
... Cosmic Explorer will provide critical input to the Astro2020 science questions COEPD, COEP1, COEP2, COEP3, COEP4, COS3, and COS4. Implementation -- Cosmic Explorer will be an L-shaped laser interferometer built on the surface of geologically appropriate and seismically quiet land in a U.S.
From page 453...
... This expansion is crucial to achieving the daily cadence of observations that NANOGrav requires to detect and characterize single continuouswave sources of gravitational waves. With the Arecibo and Green Bank observatories alone, NANOGrav has sufficient collecting area but insufficient overall observing time for detecting single sources.
From page 454...
... and new analysis techniques. Just as an example, space-based detectors will routinely observe gravitational waves emitted by binary systems with intermediate and sometimes extreme mass ratios that inspiral in generic (eccentric and double spin-precessing)
From page 455...
... Scientific Context -- Space-based gamma-ray observations are needed to probe astrophysical sources at high energies, where nonthermal activity is easily distinguished and indicates extreme physical conditions and possibly cosmic-ray acceleration. Continuum gamma-ray observations provide unique information on the structure and composition of relativistic winds and jets in sources such as pulsar wind nebulae, active galactic nuclei, supernovae, and gamma-ray bursts.
From page 456...
... . The CTA observatory, consisting of both Northern and Southern Hemisphere sites, provides access to the highest-sensitivity pointed observations across the full astronomical sky, with the highest PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION L-17
From page 457...
... L.4.4.1 Future Observatories for Neutrinos and Cosmic Rays Highly sensitive neutrino observatories will be needed at ultra-high energies to probe of the origins and composition of ultra-high-energy cosmic rays, building on a possible first detection of the PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION L-18
From page 458...
... Closely related to the above, technology development is needed to work toward dramatic improvements in ultra-high-energy cosmic ray sensitivity, as an order-of-magnitude expansion of existing arrays would require deployment over greater than tens of thousands of square kilometers. Such detectors could also be sensitive to ultra-high-energy gamma rays from the nearest sources of ultra-high-energy cosmic rays.
From page 459...
... But the United States must not cede its leadership in particle astrophysics and gravitation, which is of central and growing importance to both astronomy and physics. The projects considered by the panel span a wide range of topics -- gravitational waves, neutrinos, gamma rays, and cosmic rays -- where continued investment in each area nurtures unique power to address multiple science-panel questions.
From page 461...
... The panel endorses this. Longer-term, the MREFC budget is not likely to be sufficient to support large projects such as Cosmic Explorer.


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