Gamma Ray Emission Mechanisms 1) Gamma-Ray Lines A) Spontaneous decay of excited states of atomic nuclei typically resulting from radioactive decays of other nuclei (produced in supernovae) or cosmic-ray interactions -> mostly concentrated in the plane of the Milky Way E2 = E1 + hn0 E1 Some commonly observed g-ray lines from space: 26Al (1.809 MeV); 12C (4.44 MeV), 16O (6.13 MeV), 2H (2.223 MeV)

Gamma-Gay Lines (cont.) B) Electron-positron pair annihilation Prominent peak at 511 keV, plus weak continuum at lower energies due to annihilation via formation of Positronium: Bound state of e+ and e- (similar to H atom); Has a singlet (S = 0) and triplet (S = 1) state; Triplet state (3/4 of all positronium “atoms”) must decay into 3 photons (angular momentum conservation!) => Continuum of photons with E < 511 keV Most pair annihilation radiation from space is concentrated in a small region around the Galactic center. FE E

Gamma-gamma pair production The inverse process of pair annihilation can absorb g-rays with energies E > 511 keV. Threshold energy Ethr for a g-ray interacting with a background photon field of photons with characteristic photon energy E1: e+ e- 2 (mec2)2 Ethr = q E1 (1 – cosq) Eg E1 sgg GeV – TeV photons are absorbed in intergalactic space by interacting with the cosmic microwave background! E Epeak = 2 Ethr

Nonrelativistic electrons 2) Cyclotron/Synchrotron Emission Cyclotron frequency: ncy = eB/(2pmec) ~ 2.8*106 (B/G) Hz Magnetic field B Nonrelativistic electrons Cyclotron radiation In Harmonics: In ~ (v/c)n ncy n

Relativistic electrons: Synchrotron Radiation Relativistic electrons: nsy ~ 3.4*106 (B/G) g2 Hz e-n/nsy In n1/3 n nsy

Power-law spectra of astronomical objects X-Ray Binaries Galactic Jet Sources (Microquasars)

For a power-law distribution of relativistic electrons, Synchrotron Radiation (cont.) For a power-law distribution of relativistic electrons, Ne(g) ~ g-p Fn ~ n-a a = (p-1)/2 Total synchrotron energy loss rate for an individual electron with energy E = gmec2: dE/dt = - (4/3) c sT uB (g2 – 1) uB = B2 / (8p)

3) Compton scattering Most important leptonic mechanism to produce very-high-energy g-rays Determined by Thomson cross section: sT = 6.65*10-25 cm2

Compton scattering (cont.) Compton scattering (cont.) Power-law distribution of relativistic electrons: Ne(g) ~ g-p jn ~ n-a a = (p-1)/2 Total Compton energy loss rate for an individual electron with energy E = gmec2: dE/dt = - (4/3) c sT uph (g2 – 1) uph = energy density of the photon field

4) Nonthermal Bremsstrahlung 4) Nonthermal Bremsstrahlung For a non-thermal distribution of electrons, N(g) ~ g-p, the photon spectrum essentially reproduces the electron spectrum: FE ~ E-p Total bremsstrahlung energy loss rate for an individual electron with energy E = gmec2: 3 a c sT dE/dt = - g S nZ Z (Z + 1) (lng + ln2 – 1/3) 2 p nZ = number density of nuclei with nuclear charge Z

Total Energy Loss Rate of Relativistic Electrons -dg/dt Compton Scattering Synchrotron Nonthermal Bremsstrahlung g

5) Hadronic processes Cross sections for most electromagnetic radiation mechanisms for electrons scale with the Thomson cross section, which is ~ me-2. => Same radiation mechanisms are suppressed for protons by a factor (me/mp)2. But ultrarelativistic protons can effectively produce radiation through the production of secondary particles.

Hadronic processes (cont.) A) Radiation mechanisms based on photopion production: pg → pp0 p0 → 2g FE E Epeak ≈ gp * 70 MeV pg → np+ p+ → m+nm m+ → e+nenm FE Sy. + Compton emission from first-generation leptons Sy. + Compton emission from leptons in electromagnetic cascades E

Hadronic processes (cont.) B) Radiation mechanisms based on proton-proton collisions: pp → ppp0 p0 → 2g FE E Epeak ≈ gp * 70 MeV pp → pnp+ pp → ppp+p- p+ → m+nm m+ → e+nenm p- → m-nm m- → e-nenm FE Sy. + Compton emission from first-generation leptons Sy. + Compton emission from leptons in electromagnetic cascades E