Astroparticle Physics

6.2 Neutrino Astronomy 105muon spectra. However, they are also directly measured (seeFigs. 6.11 and 6.12). Their intensity is only known with anaccuracy of about 30%. The spectral shape and intensity ofneutrinos from extragalactic sources (AGN – Active GalacticNuclei) shown in Fig. 6.23 represents only a very roughestimate.It is generally assumed that binaries are good candidatesfor the production of energetic neutrinos. A binary consistingof a pulsar and a normal star could represent a strongneutrino source (Fig. 6.24).The pulsar and the star rotate around their common centerof mass. If the stellar mass is large compared to the pulsarmass, one can assume for illustration purposes of the neutrinoproduction mechanism that the pulsar orbits the companionstar on a circle. There are models which suggest thatthe pulsar can manage to accelerate protons to very highenergies. These accelerated protons collide with the gas ofthe atmosphere of the companion star and produce predominantlysecondary pions in the interactions. The neutral pionsdecay relatively fast (τ π 0 = 8.4 × 10 −17 s) into two energeticγ rays, which would allow to locate the astronomicalobject in the light of γ rays. The charged pions produce energeticneutrinos by their (π → µν) decay. Whether such asource radiates high-energy γ quanta or neutrinos dependscrucially on subtle parameters of the stellar atmosphere. Ifpions are produced in a proton interaction such asp + nucleus → π + + π − + π 0 + anything , (6.51)equal amounts of neutrinos and photons would be producedby the decays of charged and neutral pions (π + → µ + +ν µ ,π − → µ − +¯ν µ , π 0 → γ +γ). With increasing column densityof the stellar atmosphere, however, photons would bereabsorbed, and for densities of stellar atmospheres of ϱ ≤10 −8 g/cm 3 and column densities of more than 250 g/cm 2this source would be only visible in the light of neutrinos(Fig. 6.25).The source would shine predominantly in muon neutrinos(ν µ or ¯ν µ ). These neutrinos can be recorded in a detectorvia the weak charged current in which they produce muons(Fig. 6.26).Muons created in these interactions follow essentiallythe direction of the incident neutrinos. The energy of themuon is measured by its energy loss in the detector. For energiesexceeding the TeV range, muon bremsstrahlung andpulsar 0p + rest +starstellar atmospherelocal density (x)column density (x)dxFig. 6.24Production mechanism ofhigh-energy neutrinos in a binarysystemFig. 6.25Competition between productionand absorption of photons andneutrinos in a binary systemcompetition:neutrino, gamma emission –nW +Fig. 6.26Reaction for muon neutrinodetectionp