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AGN (Active Galactic Nuclei) produce protons in their jets and they are relativistic. I was reading about photo-pion production, where a proton and photon annihilate to produce a pion. Could this happen with a proton and a photon from the CMB (Cosmic Microwave Background)? I know the CMB will be of low energy so I am not sure if it is possible for the proton to have sufficient energy to reach the rest mass threshold of the pion.

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    $\begingroup$ Hi Sean, I'm guessing you mean "Active Galactic Nucleusses" by "AGN"? I've spelt out a couple of your acronyms to make your question a little more widely readable. $\endgroup$ – WetSavannaAnimal May 27 '16 at 10:00
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You need to look in to the GZK cutoff. In 1966, Greisen, Kuzmin and Zatsepin calculated that above a threshold of $5\times10^{19} \,\mathrm{eV}$ cosmic ray protons would lose energy to photo-pion production on the cosmic microwave background fairly rapidly. The consequence of this is that cosmic rays above that energy can't travel more than about $50 \,\mathrm{Mpc}$ without losing energy to pion production (the GZK horizon), and so cosmic rays detected with energy above that threshold presumably come from sources closer to us than that.

There are a few Active Galactic Nuclei within that distance of us, but not very many. It is not known if they are the source of these ultra-high energy cosmic rays (UHECR), but cosmics rays up to energies of $\sim3\times10^{20}\,\mathrm{eV}$ have been observed, so cosmic rays above the GZK cutoff do exist. The Pierre Auger cosmic ray observatory in Argentina detects roughly one trans-GZK particle per month.

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  • $\begingroup$ "above a threshold of 5$\times10^{19}$ eV cosmic ray protons would lose energy to photo-pion" This is only the case for protons, the GZK cut-off for iron is 56 times higher (due to its $A=56$). It is true that there are not many AGN within 50 Mpc and the origin of the UHECR still remains a mystery nowadays. $\endgroup$ – Juanjo May 18 '18 at 14:50

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