Pathways to Discovery in Astronomy and Astrophysics for the 2020s (2023) / Chapter Skim
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Appendix L: Report of the Panel on Particle Astrophysics and Gravitation
Appendix L: Report of the Panel on Particle Astrophysics and Gravitation
Pages 448-471
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From page 448... ...
For gamma rays, there have been dramatic advances in the energy range, angular resolution, and number of sources. For cosmic rays, there have been dramatic advances in the precision and composition of the spectra, plus hints of sources.
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From page 449... ...
; • Extreme accelerators, with total power and per-particle energy far beyond laboratory experiments (including neutrino sources that are currently unknown and that may be hidden from electromagnetic observations) ; and • Multi-messenger sources, where some of these processes are also revealed by electromagnetic radiation, especially gamma rays (including mergers of neutron stars, gamma-ray bursts, flares of active galactic nuclei, and more)
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From page 450... ...
of these projects will be valuable themselves -- gamma rays reveal processes that longer-wavelength photons cannot -- and will greatly enhance the returns of neutrino and gravitational-wave observatories. In cosmic rays, the scientific opportunities are also outstanding, including the possibility of eventual directional astronomy with charged particles, but continued science and technology development is needed to drive sufficient advances over current and planned experiments.
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From page 451... ...
• 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.
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From page 452... ...
NOTE: EOS: equation of state; GR: general relativity; UHECR: ultra-high-energy cosmic ray; MW: Milky Way; AGN: active galactic nuclei. • 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.
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From page 453... ...
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. The most compelling programs for new investment thus strongly depend on the landscape of existing and planned experiments, in the United States and abroad, and its gaps.
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From page 454... ...
In the future, it will be possible to expand coverage to the 0.1–1 Hz band, which would be important for observing the merger of intermediate-mass black holes with 103–104 M⊙ masses. • Neutrinos: To probe extreme accelerators, neutrino observatories need greater sensitivity across a range of energies.
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From page 455... ...
support for and involvement in these activities, the United States will lose its leadership role. • Cosmic rays: Although cosmic rays do not point back to their sources, owing to magnetic deflections, observations with the Pierre Auger Observatory, Telescope Array, and other facilities show the existence and high power of extreme accelerators.
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From page 456... ...
Scientific Context: More than a century since the discovery of charged cosmic rays, their origins are still unknown, owing to magnetic deflections that obfuscate their sources. The dominant cosmic-ray component is nuclei, principally protons.
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From page 457... ...
A light composition could open a window for future directional astronomy with charged cosmic rays. Encouraging the development of new neutrino observatories, with much larger statistics than IceCube-Gen2, to probe the most extreme accelerators and the cosmic-ray composition through measuring the neutrino spectrum over a broad energy range.
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From page 458... ...
In addition to their dedicated physics programs and goals, the IceCube Upgrade at the South Pole and the Radio Neutrino Observatory in Greenland (RNO-G) will prototype a significant fraction of the hardware planned for IceCube-Gen2.
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From page 459... ...
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.
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From page 460... ...
Further measurements -- the spectrum of this background, in combination with the detection of individual 108–109 M⊙ binary mergers -- will yield a definitive picture of how these supermassive black holes evolve in their galactic environments. With this, NANOGrav will provide crucial input on the Astro2020 science questions COEPD, COEP4, and COS4.
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From page 461... ...
LISA will be able to observe all merging supermassive black holes in the universe (105–107 M⊙ masses) , the inspiral of small compact objects into supermassive black holes to redshifts of order one, white dwarf and neutron star binaries in the Milky Way, and stochastic backgrounds from the early universe.
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From page 462... ...
The suggested increase in LISA support from NASA would be $100 million for the decade, with $30 million to $40 million for sponsored science funding and $50 million to $60 million for the U.S. LISA Science Facility.
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From page 463... ...
The combination of gravitational-wave and gamma-ray observations for GRB 170817A bracketed the inspiral of the binary neutron stars and the first emergence of light from the resulting burst, enabling constraints on theories of gravity and initiating a massive campaign of groundbreaking follow-up observations that probed aspects such as heavy-element formation. The association of a neutrino, X rays, and gamma rays from the TXS 0506+056 blazar demonstrated the combined use of these observations to peer into the workings of relativistic jets.
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From page 464... ...
. CTA's low energy threshold extends the cosmological horizon for VHE astronomy beyond a redshift of one, thereby enabling VHE observations into the peak epoch of activity for active galactic nuclei and gamma-ray bursts.
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From page 465... ...
• Medium-Scale, Gravitational-Wave Program: Investment in three gravitational-wave bands, with support for technology developments toward Cosmic Explorer (NSF) , $66 million; increased sensitivity for NANOGrav (NSF)
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From page 466... ...
Such detectors could also be sensitive to ultra-high-energy gamma rays from the nearest sources of ultra-high-energy cosmic rays. Modest funding for small development efforts may be available through NSF PI programs, or the NSF MRI program, but significantly increased development opportunities are needed.
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From page 467... ...
The shorter implementation time for these programs is also important to support imminent multi-messenger and particle astrophysics advances and allow responsiveness to emerging discoveries. The panel reviewed a rich array of concepts for new space-based gamma-ray observatories.
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From page 468... ...
There is a high priority for observations of extreme gravitators, which include electromagnetically dark mergers of black holes, as well as of extreme accelerators, which include gamma-ray–obscured sources of high-energy neutrinos. But this is not the whole story, as emphasized through the COEP panel's discovery area of multi-messenger astronomy.
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From page 469... ...
As discussed in the text, even small numbers of new sources can have powerful impacts in terms of science results and in laying the groundwork for higher-statistics observations in the 2030s. BBH: Binary black hole; NSBH: neutron star–black hole.
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From page 470... ...
Several of the white papers relevant to the panel describe large, bold projects that would open new areas of observation space, such as localized cosmic ray sources, for example, but these projects are only at the early stages of conception. NASA and NSF support for technology development and small precursor missions is essential so that by the 2030s new capabilities will be available and ready.
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From page 471... ...
Last, the panel endorses the LISA Science Support Center as a mechanism to connect the U.S. and international communities; this may also be an example to other projects.
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