Multi-messenger astrophysics. Péter Mészáros, Derek B. Fox, Chad Hanna & Kohta Murase. Nature Reviews Physics, volume 1, pages 585–599 (2019). October 3 2019. https://www.nature.com/articles/s42254-019-0101-z
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.
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.
Introduction
Until the mid-twentieth century, of the four fundamental forces in nature — the electromagnetic (EM), gravitational, weak and strong nuclear forces — it was only messengers of the EM force, in the form of optical photons, that astronomers could use to study the distant Universe. Technological advances provided access to radio, infrared, ultraviolet, X-ray and γ-ray photons. But only in the past few decades, the messengers of the other three forces, namely gravitational waves (GWs), neutrinos and cosmic rays, could be used in astronomical observations. Thus, we are now finally using the complete set (as far as known) of the forces of nature, which are revealing exciting and hitherto unknown facts about the cosmos and its denizens.
Compared with most EM emissions, these new non-photonic messengers are generally more challenging to detect and to trace back to their cosmic sources. When detected, they are usually associated with extremely high-mass or high-energy-density configurations (for example, the dense core of normal stars, stellar explosions at the end of the life of massive stars, the surface neighbourhood of extremely compact stellar remnants such as white dwarfs, neutron stars or black holes, the strong and fast-varying gravitational field near either stellar mass black holes or the more massive black holes in the core of galaxies, or in energetic shocks in high-velocity plasmas associated with compact astrophysical sources). This association with the most violent astrophysical phenomena known means that the interpretation of observations enabled by multiple messengers requires, and can have implications for, our theories of fundamental physics, including strong-field gravity, nuclear physics and particle interactions.
The study of such high-energy compact objects started in the 1950s, after decades of a slow build-up with increasingly larger ground-based optical telescopes. The first major breakthrough came from the deployment of large radio-telescopes, followed by the launch of satellites equipped with X-ray and later γ-ray detectors, which established the existence of active galactic nuclei (AGN), neutron stars and black holes, and revealed dramatic high-energy transient phenomena, including X-ray novae, X-ray bursts and gamma-ray bursts (GRBs).
In the late 1960s, large underground neutrino detectors were built, first measuring the neutrinos produced in the Sun and later those arising from a supernova explosion (see joint multi-messenger results section below). It was only in the current decade that extragalactic neutrinos in the TeV to PeV range were discovered1,2. Cosmic rays in the GeV energy range started to be measured in the 1910s, but it was only in the 1960s that large detectors started measuring higher energies, suggesting an extragalactic origin, and only in the past decade has it become practical to start investigating the spectrum and composition in the 1018–1020 eV energy range, for example, see ref.3. GW detectors were first built in the 1970s, but it was not until the 1990s that the sensitivity required for detection was achieved owing to new technologies and sufficiently large arrays. The first GW detections4 came in 2015.
Until the mid-twentieth century, of the four fundamental forces in nature — the electromagnetic (EM), gravitational, weak and strong nuclear forces — it was only messengers of the EM force, in the form of optical photons, that astronomers could use to study the distant Universe. Technological advances provided access to radio, infrared, ultraviolet, X-ray and γ-ray photons. But only in the past few decades, the messengers of the other three forces, namely gravitational waves (GWs), neutrinos and cosmic rays, could be used in astronomical observations. Thus, we are now finally using the complete set (as far as known) of the forces of nature, which are revealing exciting and hitherto unknown facts about the cosmos and its denizens.
Compared with most EM emissions, these new non-photonic messengers are generally more challenging to detect and to trace back to their cosmic sources. When detected, they are usually associated with extremely high-mass or high-energy-density configurations (for example, the dense core of normal stars, stellar explosions at the end of the life of massive stars, the surface neighbourhood of extremely compact stellar remnants such as white dwarfs, neutron stars or black holes, the strong and fast-varying gravitational field near either stellar mass black holes or the more massive black holes in the core of galaxies, or in energetic shocks in high-velocity plasmas associated with compact astrophysical sources). This association with the most violent astrophysical phenomena known means that the interpretation of observations enabled by multiple messengers requires, and can have implications for, our theories of fundamental physics, including strong-field gravity, nuclear physics and particle interactions.
The study of such high-energy compact objects started in the 1950s, after decades of a slow build-up with increasingly larger ground-based optical telescopes. The first major breakthrough came from the deployment of large radio-telescopes, followed by the launch of satellites equipped with X-ray and later γ-ray detectors, which established the existence of active galactic nuclei (AGN), neutron stars and black holes, and revealed dramatic high-energy transient phenomena, including X-ray novae, X-ray bursts and gamma-ray bursts (GRBs).
In the late 1960s, large underground neutrino detectors were built, first measuring the neutrinos produced in the Sun and later those arising from a supernova explosion (see joint multi-messenger results section below). It was only in the current decade that extragalactic neutrinos in the TeV to PeV range were discovered1,2. Cosmic rays in the GeV energy range started to be measured in the 1910s, but it was only in the 1960s that large detectors started measuring higher energies, suggesting an extragalactic origin, and only in the past decade has it become practical to start investigating the spectrum and composition in the 1018–1020 eV energy range, for example, see ref.3. GW detectors were first built in the 1970s, but it was not until the 1990s that the sensitivity required for detection was achieved owing to new technologies and sufficiently large arrays. The first GW detections4 came in 2015.
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