1. 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)

2. 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

3. 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

4. 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

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

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

7. 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)

8. 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

9. 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

10. 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

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

12. 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.

13. 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

14. 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