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Gamma Rays Part 1

I knew nothing about gamma rays but now they are interesting because they relate to cosmic rays. Perhaps others are interested.
There are 2 parts in this series rather than  one very long post.
The part 2 is more interesting but part 1 provides the background for part 2.

quotes from NASA ' ':
What can generate gamma rays?
a) from the radioactive decay of atomic nuclei.
I will assume this source is not relevant to cosmology.

b) 'A particle and its anti-particle, such as an electron and a positron, will undergo something called an annihilation process. In physics, this process produces neutral pions that quickly decay into gamma rays.'

I will assume this source is not relevant.

c) high energy particle collisions. This source is relevant.
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 mega-electron-volts, which is a measurement of the kinetic energy in the incident particles).
This means just cosmic rays can generate gamma rays, even here on Earth.

'Most cosmic rays are atomic nuclei: most are hydrogen nuclei, some are helium nuclei, and the rest heavier elements.'

Where are these sources in the universe?

The Fermi Gamma-ray Space Telescope has detected them from its low orbit around the Earth.

from online:
 NASA's Fermi telescope detects some of the universe's most powerful explosions, and these include terrestrial gamma-ray flashes triggered in the intense electrical fields of storms.

Wikipedia has a topic for this satellite. That page includes an image of the universe in gamma rays of > 1 Gev. There is an intense band in the Milky Way disk but just diffused emissions from the rest of the universe.

This image does not reveal any of the 'universe's most powerful explosions.'

The NASA Goddard site has this image but it also has positions of 'the five most distant gamma-ray blazars yet known.'

Blazars are the subject of Gamma Rays part 2.

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