Astroparticle Physics

6.3 Gamma Astronomy 111The energy spectrum of synchrotron photons is continuous.The power P radiated by an electron of energy E ina magnetic field of strength B isatomic nucleusP ∼ E 2 B 2 . (6.60)b)Bremsstrahlung:A charged particle which is deflected in the Coulomb fieldof a charge (atomic nucleus or electron) emits bremsstrahlungphotons (Fig. 6.33). This mechanism is to a certainextent similar to synchrotron radiation, only that in thiscase the deflection of the particle occurs in the Coulombfield of a charge rather than in a magnetic field.The probability for bremsstrahlung φ varies with thesquare of the projectile charge z and also with the squareof the target charge Z, see also (4.7). φ is proportional tothe particle energy E and it is inversely proportional tothe mass squared of the deflected particle:φ ∼ z2 Z 2 Em 2 . (6.61)Because of the smallness of the electron mass bremsstrahlungis predominantly created by electrons. The energyspectrum of bremsstrahlung photons is continuous anddecreases like 1/E γ to high energies.c) Inverse Compton Scattering:In the twenties of the last century Compton discoveredthat energetic photons can transfer part of their energyto free electrons in a collision, thereby losing a certainamount of energy. In astrophysics the inverse Compton effectplays an important rôle. Electrons accelerated to highenergies in the source collide with the numerous photonsof the blackbody radiation (E γ ≈ 250 µeV, photon densityN γ ≈ 400/cm 3 ) or starlight photons (E γ ≈ 1eV,N γ ≈ 1/cm 3 ) and transfer part of their energy to the pho-tons which are ‘blueshifted’ (Fig. 6.34).d)π 0 Decay:Protons accelerated in the sources can produce chargedand neutral pions in proton–proton or proton–nucleus interactions(Fig. 6.35). A possible process iselectronphotonFig. 6.33Production of bremsstrahlung bydeflection of charged particles inthe Coulomb field of a nucleusbremsstrahlung probabilityspectrumof bremsstrahlung photonsinverse Compton scatteringFig. 6.34Collision of an energetic electronwith a low-energy photon. Theelectron transfers part of its energyto the photon and is consequentlyslowed downp+nucleus → p ′ +nucleus ′ +π + +π − +π 0 . (6.62)Charged pions decay with a lifetime of 26 ns into muonsand neutrinos, while neutral pions decay rapidly (τ =8.4 × 10 −17 s) into two γ quanta,