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Processes Which Create Cosmic Gamma-Rays

There are several physical processes by which cosmic gamma-rays are generated. These include:

  1. a high-energy particle can collide with another particle,
  2. a particle can collide and annihilate with its anti-particle,
  3. an element can undergo radioactive decay, or
  4. a charged particle can be accelerated.
Particle-Particle collisions

Particle-Particle Collisions

In gamma-ray astronomy, this usually means a high-energy proton, or cosmic ray, strikes another proton or atomic nucleus. This collision produces, among other things, one or more neutral pi mesons (or pions). These are unstable particles that decay into a pair of gamma-rays. Since the pion is usually moving at a high velocity as a result of its violent birth, the gamma-rays are projected forward... in a slight 'V' formation. This process gives rise to gamma-rays with a broad spectrum of energies (all greater than 72 MeV), which reflects the energies of the incident particles.

Matter-Antimatter Annihilation

A particle and its anti-particle, such as an electron and a positron, will undergo what in physics is called an annihilation process. This process produces neutral pions which quickly decay into gamma-rays.

Matter-Antimatter Annihilation

OSSE 511 keV image of galactic center


The results from electron-positron annihilations have been seen by the OSSE experiment aboard the CGRO satellite. The colors in this map represent the intensity of gamma-ray emission from positron-electron annihilation in the plane of our Galaxy near the galactic center. The emission is at 511 keV, which is the rest-mass energy of the positron. The map is of a model that fits the OSSE 511 keV observations. OSSE has discovered that the radiation is mostly contained in a region of about 10 degrees diameter centered on the center of the Galaxy . The line plot superimposed on the map represents an OSSE observation of the 511 keV emission line.

Radioactive decay

Radioactive Decay

Radioactive decay, or the electromagnetic deexcitation of nuclei, is a source of gamma-ray line emission. Observation of such gamma-rays confirms that the excited states of nuclei are being produced, while the measured fluxes and spectra identify the specific nuclei and the rate of their excitation. Extreme physical conditions are required to produce excited nuclei, thus allowing us to probe unique physical environments with these observations. Radioactive gamma-ray sources in space are associated with events of nucleosynthesis, such as supernovae.

Charged particle accelerated by a rotating magnetic field

Acceleration of Charged Particles

A magnetic field exerts a force on a charged particle which is moving in it. This causes the particle to radiate, with the emitted power being proportional to the square of the force divided by the square of the mass of the particle. For electrons, this radiation is often in the gamma-ray region of the electromagnetic spectrum. The character of the radiation (and the name given to it) depends on the nature of the accelerating force. If the electron is accelerated in the electrostatic field around a nucleus, the resulting radiation is called bremsstrahlung; it is cyclotron or synchrotron radiation when the acceleration takes place in a static magnetic field; and the process is called Thomson scattering or Compton scattering when the acceleration occurs in the electromagnetic field of a photon.

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Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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This page last updated: Monday, 27-Sep-2004 11:26:11 EDT