Processes Which Create Cosmic Gamma-Rays
There are several physical processes by which cosmic
gamma-rays are
generated. These include:
- a high-energy particle can collide with another particle,
- a particle can collide and annihilate with its anti-particle,
- an element can undergo radioactive decay, or
- a charged particle can be accelerated.
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Particle-Particle Collisions
In gamma-ray astronomy, "particle-particle collision" 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. |
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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.
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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. |
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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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