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Astronomy and astrophysics

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,  whose leptonic decay modes will yield neutrinos. A list of possible high energy astrophysical neutrino sources therefore involves a list of candidate astrophysical sites for proton acceleration.

X-ray binaries

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 normal 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 residue 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, but, as yet, there is no direct confirmation of this hypothesis. Recent observations above 108eV by the EGRET detector have found gamma-ray signals associated with at least 2 supernova remnants (IC 443 and $\gamma$ Cygni). However, electromagnetic radiation is an ambiguous signature, as it can come from either accelerated electrons or protons. An observation of neutrinos would provide a clear indication of proton acceleration with the direction identifying the source.

Active galactic nuclei

A view of the Crab supernova remnant (click to enlarge) 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 super-massive 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''.

An unexpected result, now well established by the EGRET satellite, is that many blazars emit gamma-rays with energies up to 1010 eV. Even more surprisingly, at least four members of this class have been detected by atmospheric Cherenkov telescopes as highly variable emitters of gamma-rays with energies exceeding 1012eV. The mechanism by which such high energy particles are generated is a matter of active debate, as is the jet composition.

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 cascade, which will also generate neutrinos. Both neutrino and photon flux will be increased by Doppler beaming in the case of blazars.

Theoretical models of these sources are subject to large uncertainties, so that observations are of paramount importance.

Gamma ray bursts

Gamma Ray Bursts (GRBs) are the most spectacularly violent phenomena in the universe known to this date, and up until recently they were declared to be one of the outstanding mysteries of modern astrophysics.

However, early 1997 brought a major break though when the BeppoSAX satellite located a burst precisely enough to permit the identification of its optical counterpart. Since then, further advances have been made, with the discovery of GRB afterglows, measurements of their redshift and recognition of the magnitude of the energy release occurring in these second-long flashes.

So far about a dozen GRB afterglows have been detected. The distances measured place the burst sources at cosmological distances with redshifts in the range z=0.8-3.4 and indicate an energy release of 10451 J in gamma-rays alone (assuming isotropic emission). 

The location of GRBs in their host galaxies somewhat correlates the population of GRB progenitors with the star formation rates, and supports models which involve a black hole formation through coalescence of a binary system of either a black hole-neutron star or a neutron star-neutron star.

Detailed studies of afterglows from X-rays to optical and radio wavelengths provided crucial constraints on physical parameters for theoretical models. In the current standard model for gamma-ray bursts and their afterglows, the fireball-plus-blastwave model, the initial event deposits a solar mass of energy into a region with a radius of about 100 km. The resulting fireball expands ultra-relativistically with Lorentz factor $\gamma\geq 300$ into the surrounding medium. While the forward shock sweeps material and heats it, the reverse shock collides with the ejecta. Therefore, the afterglow is produced by synchrotron radiation when external shocks decelerate. However, the multi-peaked light curve of the gamma-ray burst itself is produced by the collisions of several internal shocks which are catching up each other with different Lorentz factors within the inner engine. There is clear evidence of these three distinct regions of the fireball in the light curve of GRB990123.

A hadronic component would naturally be expected in such extreme phenomena. 

Nearby GRBs could therefore be the long sought after sources of the highest energy cosmic rays.

The rate predicted in some publications is very low for ANTARES, which has a detector area of 0.1 km2 and an angular acceptance of $2\pi$ sr. Nevertheless, a significantly higher event rate can be obtained by lowering the energy threshold to around 100 GeV. The background is greatly reduced by requiring a spatial and temporal coincidence with an observed GRB, offering a unique opportunity for high energy neutrino detectors to observe neutrinos associated with individual bursts.

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Author : Thierry Stolarczyk