In the Introductory Astronomy book I'm studying (Astronomy 2e, OpenStax.org, Fraknoi et al.), cosmic rays are said to include protons and electrons, among other components. It also says that the best candidates for the origin of cosmic rays are "supernova explosions". Since electrons are much lighter than protons, can we expect a supernova to accelerate electrons to higher velocities than protons? If so, is this expectation borne out by observations of cosmic rays, e.g., (a) in higher average velocity of electrons among observed cosmic rays, or (b) in an observed time delay between arrival of electrons and protons in a stream of cosmic rays?


1 Answer 1


We cannot directly observe cosmic rays from supernovae so measurement of a time delay between energetic electrons and nuclei isn't possible. In any case, the acceleration mechanism isn't immediately associated with the supernova explosion, so there isn't a "pulse" of cosmic rays on which the timing could be based.

Electrons as cosmic rays are comparatively rare. They have not been observed to the same high energies as the highest energy cosmic ray protons and the origins of the highest energy electrons is not completely understood and may be from multiple mechanisms and from sources closer to Earth. i.e. The high energy protons/nuclei and electrons we receive at Earth aren't even necessarily from the same source(s).

Some details:

The high energy charged particles originating from supernovae are actually thought to be accelerated by turbulence caused by shocks in expanding supernova remnants. They are not a "prompt" outcome of a supernova and the accelerating processes can continue to occur for thousands of years after the explosion.

The cosmic rays from supernova remnants cannot be observed directly. Cosmic rays arrive at the Earth from all over the sky. As charged particles they are easily deflected by magnetic fields and their directions are quickly scrambled by interstellar magnetic fields such that their points of origin cannot be unambiguously identified.

That supernova remnants are the production sites for cosmic rays of up to $\sim 10^{15}$ eV is indirectly inferred by observing gamma ray photons that are produced either by accelerated electrons (synchrotron emission, bremsstrahlung) or by various particle collisions in the case of high energy protons or heavier nuclei.

The process by which charged particles are accelerated in the supernova shocks is the first-order Fermi acceleration mechanism. There is no obvious reason why electrons and heavier nuclei should be accelerated to exactly the same energies and I have read (e.g. Bykov et al. 2018) that electrons do not gain energies as large as protons. It is also far easier for electrons to lose their energy in radiative processes before they arrive and are detected at Earth. Indeed, these loss mechanisms (synchrotron, inverse Compton scattering) are such that the sources of high energy electrons as cosmic rays at Earth are likely to be relatively local in the galaxy, compared to protons which could have even originated in other galaxies.


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