The principal mechanism for generating high-energy neutrinos is through decay cascades induced by high-energy protons. Interactions of protons with matter or radiation produce mesons, essentially pions and kaons, whose leptonic decay modes will yield neutrinos.
The first detection of an excess of high-energy neutrinos of astrophysical origin was achieved in 2013 by the IceCube collaboration. This flux is, in the 100 TeV–PeV range, at the level of 10-8 GeV cm−2 s−1 sr-1. However the origin of these neutrinos is still unknown. A list of possible high-energy astrophysical neutrino sources, which involves astrophysical sites for proton acceleration, is shown below.
This class of binary stars, which are among the brightest cosmic X-ray sources, consists of compact objects, such as neutron stars or black holes, which accrete matter from their companion stars. The accretion process leads to plasma waves in the strong magnetic field of the compact object, which bring protons to high energies by stochastic acceleration. Interactions of the accelerated particles with the accreting matter or with the companion star would then produce a neutrino flux comparable to that in high-energy particles with a spectral index close to 2.
Supernova remnants and cosmic rays
Explosions of massive stars (supernovae) produce an expanding shell of material which is known from radio observations to accelerate high-energy particles. In some cases, the remnant of the supernova is a neutron star which is detectable as a pulsar.
Protons inside supernova shells can be accelerated by a first-order Fermi mechanism if (as seems likely) the shell is turbulent. If a pulsar is present there are additional acceleration mechanisms: in the magnetosphere of the pulsar, or at the front of the shock wave produced by the magneto-hydrodynamic wind in the shell.
The interaction of these protons with the matter of the shell gives rise to neutrinos and photons (from charged and neutral pion decays respectively). It is thought that supernova remnants are the principal galactic source of cosmic-ray protons. In that regard, the Fermi-LAT collaboration has been able to detect the characteristic pion-decay signature in Supernova Remnants.
However, electromagnetic radiation is an ambiguous signature, as it can come from either accelerated electrons or protons. In fact it is thought that at very-high energies the gamma-ray production should be dominated by hadronic processes. However, recent observation by HAWC and LHAASO have shown that leptonic acceleration is possible for sources emitting at sub-PeV energies.
An observation of neutrinos would provide a clear indication of proton acceleration with the direction identifying the source.
Active galactic nuclei
Active Galactic Nuclei (AGN) such as quasars are, averaged over time, the most powerful known objects in the Universe. Their observed total luminosities are in the range 1035 – 1041 W.
In the generic model of AGN, these high luminosities arise from accretion of matter, at a rate of at least a few solar masses per year, onto a supermassive black hole, ranging from 106 to 1010 solar masses. A minority of AGN also produces relativistic jets which transport synchrotron-emitting electrons, most easily observable in the radio, over distances up to 1 Mpc. Objects of this type whose jets are directed almost exactly towards us appear as intense, compact and variable sources because their emission is amplified by Doppler boosting. Such objects are known as “blazars”.
Electrons are thought to be accelerated by the first-order Fermi process, and this is also efficient for protons.
In generic AGN models, which do not necessarily involve jets, protons may be accelerated by shock waves associated with the accretion flow into the black hole. These protons may then interact with the matter of the accretion disk or with its ambient radiation field. Such models produce neutrinos with no associated high energy gammas, because the photons do not escape from the core of the AGN.
In active galactic nuclei with jets, protons may also be accelerated in the inner regions of jets. These will then produce neutrinos by interaction with ambient radiation either emitted by the accretion disk or generated as synchrotron radiation within the jet. Similarly, Fermi acceleration in the hot-spots of powerful (Fanaroff-Riley Class II) radio sources may produce a population of high-energy protons. Such protons contribute to the high-energy gamma-ray emission via a proton-induced cascades, which will also generate neutrinos. Both neutrino and photon flux will be increased by Doppler beaming in the case of blazars.
There are several recent works in which an excess of neutrinos coming from a catalog of selected blazars has been claimed, e.g. [Astrophys. J. 2021, 908, 157] [Sara Buson et al 2022 ApJL 933 L43]. This possible association is also supported by ANTARES observations. Moreover, an interesting association coming from J0242+1101 has been also observed. In fact, several neutrino coincidences with some particular blazars have been suggested, e.g, [ApJL 911 (2021) L18]. Until recently, the only compelling evidence of high-energy neutrino sources came from the multi-messenger observation of neutrinos coming from the blazar TX0506-056. (see the Multi-messenger astronomy sub-section). However, recent IceCube observations have strengthened the idea that AGNs may be sources of cosmic neutrinos, in particular with the detection of an excess of neutrinos coming from NGC1068.
Future observations with neutrino telescopes will play a crucial role to get a better understanding of the acceleration mechanisms in AGNs, and to figure out the extent to which AGNs contribute to the overall astrophysical neutrino flux.
Gamma ray bursts
Discovered by chance in 1967, Gamma-Ray Bursts (GRBs) are extremely energetic explosions that outshine any other event in the Universe, making them a perfect laboratory to study high energy astrophysics. GRBs are therefore promising candidate sources of astrophysical neutrinos.
We know that GRBs are of extragalactic origin, with a prompt emission that lasts from fractions of seconds to several hours, which can be followed by an afterglow emission of longer duration and lower energy (X-ray, optical, and radio). The light-curve, i.e. the time profile of the intensity of the emission of the GRBs is diverse and complex, with no two GRBs being identical. GRBs are usually classified into two main groups: short GRBs which have a duration shorter than 2 seconds, and long GRBs which have a longer duration.
The origin of the GRBs is not completely clear yet. Regarding long GRBs, one of the main candidates are hypernovas, i.e. the collapse of a supermassive star above 100 solar masses. Short GRBs have been long thought to be attributed to kilonovas, i.e. the merger of two compact stellar remnants (double neutron stars or neutron star-black hole binaries) [Astrophys. J., 395:L83–L86, 1992, Astrophys. J., 507:L59, 1998]. This scenario has been confirmed after the detection of a short GRB associated with the GW170817 event, showing that kilonovas should be connected with short GRBs [Nature, 551,71(2017), Nature, 551:64, 2017].
Concerning the GRB emission mechanisms, there is still a lot to learn. Presently, there is no theory that can explain all the GRBs. The prompt emission is usually fairly described by the Band function which consists of a power law with an exponential cut-off at low energies, smoothly connected to a steeper power law at high energies. The parameters of the spectral function vary from GRB to GRB. The Band function spectrum is usually attributed to the synchrotron radiation from shock-accelerated electrons. However this synchrotron emission cannot explain the very high-energy (VHE) emission (above 100 GeV) by itself and other components in the spectrum are required.
Recent observations of GRBs emitting at TeV energies [Nature 575, 464–467 (2019), Nature 575, 455–458 (2019)] have brought back the discussion on the nature of particle acceleration in GRBs, whether it occurs through hadronic or leptonic processes. Leptonic models have been traditionally favored for GRB modeling, but the detection of high-energy photons suggests the presence of hadronic acceleration.
Gravitational Waves (GWs) were predicted more than a century ago by A. Einstein. However, it took a century from their theoretical prediction to the first confirmation in 2015 by the LIGO and VIRGO collaborations. GWs are ripples in space-time typically caused by two massive objects orbiting each other which eventually merge into a single object. Some of these high-energy astrophysical processes, that give rise to the emission of gravitational waves, are also expected to produce high-energy neutrinos since they are associated with some of the sources mentioned before.
One of the most notable cases was the binary neutron star merger GW170817 event which was detected in spatial and temporal coincidence with gamma-ray emission GRB170817A.
Some estimates of neutrino production from GW170817 show that the expected flux should be already detectable with current neutrino telescopes under favorable circumstances, see Figure 1.
The way of studying the Universe at high-energies has substantially evolved in the last decade due to the discovery of a flux of astrophysical high-energy neutrinos and the first detection of gravitational waves, opening two new observational windows.
Now multi-messenger observations, detecting different types of particles (photons, CRs, neutrinos, and GWs) from the same source, is possible. Multi-messenger astronomy exploits the fact that combined observations provide a complementary view of the same astrophysical phenomena and, at the same time, enhance the discovery potential achieved by single messenger observations.
There are some notable examples of high-energy multi-messenger coincidences. One of them is the observation, in August 2017, of a high-energy neutrino from IceCube (IceCube-170922A) in spatial and temporal coincidence with a blazar flaring in gamma-rays (TXS 0506+056). An independent IceCube analysis on archival data revealed a flare of neutrinos between September 2014 and March 2015 with 3.5σ statistical significance coming from the same source. Moreover, TXS 0506+056 also appears as the second most significant source (2.8σ pre-trial) in the ANTARES telescope source search. Thanks to these multi-messenger observations, TXS0506+056 has been accepted by the community as the first compelling evidence (>3σ significance) of a cosmic neutrino source.
ANTARES has been participating actively on the follow-up of interesting alerts in the context of multi-messenger astronomy.