Finding evidence for the acceleration of protons has long been a key issue in the efforts to explain the origin of cosmic rays. The pair of spectra, from two supernova remnants, are the “smoking gun” that researchers have been looking for. The Fermi Large Area Telescope’s observations fit very neatly with predictions of neutral pion decay. (Courtesy: NASA/DOE/Fermi LAT Collaboration, Chandra X-ray Observatory, ESA Herschel/XMM-Newton)

Supernova origin of galactic cosmic rays confirmed

Feb 14, 2013

The first direct evidence that galactic cosmic rays are accelerated within supernova remnants has been provided by observations by the Fermi Large Area Telescope collaboration. The results make use of four years of data collected by the telescope observing two supernova remnants – IC 443 and W44 – within our galaxy. The observations fit very neatly with predictions of neutral pion decay.

Galactic cosmic rays – the hypothesis

Cosmic rays are highly energetic particles, mainly protons, whizzing through space, most of which have their origins outside the solar system. A popular explanation of the origins of galactic cosmic rays – those produced within the Milky Way – is that they come from supernova remnants (SNRs), but until now there has been no unambiguous observational evidence linking the two.

When a star goes supernova, its remnants – including strong magnetic fields – can linger for thousands of years. According to the SNR cosmic-ray hypothesis, protons are accelerated by the shock front created in a supernova and then further accelerated by the magnetic fields until they gain enough energy to escape this process and become a newly formed cosmic ray. These energetic protons, the hypothesis claims, sometimes collide with other protons – in interstellar clouds, for example – to produce a neutral pion, which decays almost instantly into two gamma-ray photons.

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