Introduction

Gamma-ray astronomy provides a fundamental tool to observe the Universe using neutral deep penetrating gamma-ray particles.

The Electromagnetic Spectrum

Originally the distinction between gamma-rays and X-rays was due to the nuclear production mechanism. X-rays are the product of transition of electrons in the atomic shell, while gamma-rays are produced in the atomic nucleus. This distinction also leads to a classification in energy. X-rays are typically below $100 {\rm \;keV}$. Photons with $E > 100 {\rm \; keV}$ are called gamma-rays.

Spectral Energy Distribution

In gamma-ray astronomy is usual to study an object emission by its spectral energy distribution (SED). The SED on an object represents its energy emitted as function of frequency or wavelength. The reason astronomers do this is to see how much energy is produced by the object at each frequency or wavelength. The SED is typically characterized by $\nu F_\nu$ and it is measured in units of ${\rm ergs \; cm^{-2}\; s^{-1}}$, ie. it indicates the rate of energy emitted per surface. The function $F_\nu$ is the flux density which indicates the rate of energy emitted per surface and also per frequency, so it is expressed as ${\rm ergs\; cm^{-2}\; s^{-1}\; Hz^{-1}}$.

Don't confuse SED with the spectrum. In cosmic-ray physics spectrum is the rate of particles per unite energy and surface and it is measured ${\rm GeV^{-1}\; cm^{-2}\; s^{-1}}$.

If we multiply the spectrum by energy, we obtain the rate of particles per unit surface. Multiplying again by energy give us the energy rate per unit surface. Therefore we have the relation:

$$\nu F_\nu = E^2 \frac{{\rm d}N}{{\rm d}E}$$

Gamma-ray production mechanism

Sources of CR can also produce gamma-rays by different mechanisms roughly divided in two main categories:

• Leptonic models. In this models only leptons (mostly electrons) will produce gamma-ray emission. The mechanism are mostly:

  • Synchrotron radiation.

  • Inverse Compton Scattering.

• Hadronic model. In hadronic models, gamma-rays are produced as a result of acceleration to high energies of protons or other hadrons. The mechanism of gamma-ray production in hadronic models are mainly:

  • $\pi^0$ decay.

  • Proton synchrotron radiation.

Another categories for gamma-ray production can some exotic models like dark matter annihilation, or matter-antimatter annihilation and nuclear transformation.

We are going to review the different process of gamma-ray production but before we get there we need first to see about radiation.