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# Multi-messenger astrophysics

## Abstract

Multi-messenger astrophysics, a long-anticipated extension to traditional multiwavelength astronomy, has emerged over the past decade as a distinct discipline providing unique and valuable insights into the properties and processes of the physical Universe. These insights arise from the inherently complementary information carried by photons, gravitational waves, neutrinos and cosmic rays about individual cosmic sources and source populations. This complementarity is the reason why multi-messenger astrophysics is much more than just the sum of the parts. In this Review article, we survey the current status of multi-messenger astrophysics, highlighting some exciting results, and discussing the major follow-up questions they have raised. Key recent achievements include the measurement of the spectrum of ultrahigh-energy cosmic rays out to the highest observable energies; the discovery of the diffuse high-energy neutrino background; the first direct detections of gravitational waves and the use of gravitational waves to characterize merging black holes and neutron stars in strong-field gravity; and the identification of the first joint electromagnetic plus gravitational wave and electromagnetic plus high-energy neutrino multi-messenger sources. We discuss the rationales for the next generation of multi-messenger observatories, and outline a vision of the most likely future directions for this exciting and rapidly growing field.

## Key points

• Besides the traditional electromagnetic observations, multi-messenger astrophysics uses the information about the astrophysical Universe provided by the gravitational, weak and strong forces. These new channels provide untapped, qualitatively different and complementary types of information, making previously hidden objects visible.

• Diffuse backgrounds of high-energy neutrinos (HENs) with energies from ~10 TeV to PeV, ultrahigh-energy cosmic rays (UHECRs) at energies up to ~1020 eV and γ-rays with energies between MeV and ~TeV have been measured, or upper limits have been provided, by Cherenkov detectors, satellites and ground-based air shower arrays.

• Gravitational waves from merging stellar mass black hole and neutron star binaries have been detected at frequencies in the ~10 Hz to ~1 kHz range with laser interferometric gravitational wave detectors.

• The sources of the diffuse UHECR and HEN backgrounds remain unknown, although a γ-ray-flaring blazar has been tentatively identified with the observed HENs. Although up to ~85% of the γ-ray background can be attributed to blazars, it appears that at most 30% of the HEN background has the same origin.

• The natural physical connection between high-energy cosmic ray interactions and the resulting very-high-energy neutrinos and γ-rays can provide clues about their unknown astrophysical sources. Although less direct, the connection with gravitational wave emission is expected to provide important information about supermassive black hole populations and dynamics.

• The advanced gravitational wave detectors will soon be able to detect hundreds of binary mergers up to ~Gpc distances, but electromagnetic counterpart searches rely primarily on the aging space-based facilities Swift and Fermi, currently operating well beyond their design lifetimes. There is an urgent need for a new generation of electromagnetic detectors, extending the range of frequencies.

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## Acknowledgements

The authors thank S. Coutu, D. Cowen, M. Mostafá and B. Sathyaprakash for useful discussions and comments.

## Author information

Authors

### Contributions

The authors contributed equally to all aspects of the article.

### Corresponding author

Correspondence to Péter Mészáros.

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## Glossary

Neutron stars

The remnant of a core-collapse supernova from a star in the ~8–25 M mass range, whose central core of mass ~0.5–2.0 M collapsed to a radius of ~10 km consisting mainly of neutrons.

Stellar mass black holes

Thought to arise in core-collapse supernova from stars 25 M, whose collapsed core has a mass greater than the maximum allowed for stable neutron star, resulting instead in a black hole of a neutron star.

Active galactic nuclei

A type of galaxies whose nuclear region, harbouring an accreting massive black hole, is so bright that it outshines the rest of the galaxy.

Gamma-ray bursts

A sudden, brief, extremely luminous sources of mainly γ-rays.

Supernova

An intense stellar explosion, leading to a rapid brightening of the optical emission by more than ten orders of magnitude, followed by a gradual dimming. There are two basic subtypes, core-collapse supernovae and type Ia (nuclear deflagration) supernovae.

Blazars

A type of active galactic nucleus where accretion to the central massive black hole leads to ejection of relativistic plasma jet pointing close to the line of sight to the external observer.

Air Cherenkov imaging telescopes

A steerable telescope measuring secondary optical photons produced by high-energy γ-rays impacting the upper Earth atmosphere.

Core-collapse supernova

The end result of the evolution of a star of mass 8 that has exhausted its nuclear fuel burning capacity, leading to the gravitational collapse of its inner core and the ejection of its outer envelope.

Short GRB

A short gamma-ray burst, confirmed recently to be due to the merger of a binary neutron star; also expected from neutron star–black hole binary mergers.

r-Process

The abbreviation of ‘rapid neutron capture nuclear process’, whereby a nucleus rapidly increases its atomic number by repeatedly capturing neutrons.

Air shower array

An array of detectors measuring the secondary particles or photons produced by a primary cosmic ray hitting Earth’s atmosphere.

Supermassive black holes

A black hole in the range 105 to 1010M, usually at the centre of a galaxy.

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Mészáros, P., Fox, D.B., Hanna, C. et al. Multi-messenger astrophysics. Nat Rev Phys 1, 585–599 (2019). https://doi.org/10.1038/s42254-019-0101-z

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