Seminar of Theoretical Physics - Anabella Araudo (24.10.2017)

Tuesday 24.10.2017 at 14:00, Lecture room F2/125

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Anabella Araudo (Astronomical Institute of the Czech Academy of Sciences):
On the maximum energy of particles accelerated in the termination shocks of AGN jets

Abstract:
It has been suggested that relativistic shocks in extragalactic sources may accelerate the highest energy cosmic rays, but recent theoretical advances indicate that relativistic shocks are probably unable to accelerate particles to energies much larger than a $\mathrm{PeV}$. We study the hotspots of radiogalaxies. The observed turnover of the synchrotron spectrum indicates that the maximum energy of electrons accelerated at the jet termination shock is less than $1~\mathrm{TeV}$ in a $100~\mathrm{\mu G}$ magnetic field. We show that this maximum energy cannot be constrained by synchrotron losses as usually assumed. We propose that the maximum energy is determined by ceasing the cross-field diffusion in a perpendicular $1~\mathrm{\mu G}$ magnetic field. We demonstrate that Bell instabilities generated by the streaming of cosmic rays with the same energy as the most energetic electrons in the hotspot can amplify the turbulent field up to $100~\mathrm{\mu G}$. If the maximum energy of electrons is determined by the diffusion condition, the same limit applies to protons and therefore the maximum energy of ions is also less than $1~\mathrm{TeV}$. As a consequence, relativistic jet termination shocks are poor cosmic ray accelerators. We test this result by considering the radiogalaxy Cygnus A as a case study.

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