Evidence that the maximum electron energy in hotspots of FR II galaxies is not determined by synchrotron cooling

Abstract

It has been suggested that relativistic shocks in extragalactic sources may accelerate the highest energy cosmic rays. The maximum energy to which cosmic rays can be accelerated depends on the structure of magnetic turbulence near the shock but recent theoretical advances indicate that relativistic shocks are probably unable to accelerate particles to energies much larger than a PeV. We study the hotspots of powerful radiogalaxies, where electrons accelerated at the termination shock emit synchrotron radiation. The turnover of the synchrotron spectrum is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the shock is TeV for a canonical magnetic field of ∼100 µG. Based on theoretical considerations we show that this maximum energy cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonably large and most of the jet upstream energy goes to non-thermal particles. We test this result by considering a sample of hotspots observed with high spatial resolution at radio, infrared and optical wavelengths.

Ultrahigh energy cosmic rays from shocks in the lobes of powerful radio galaxies

“…In a recent paper, we showed that the maximum energy in an ultra-relativistic shock is well below the EeV range . We also applied similar arguments to the observed radio spectra in the hotspots of Cygnus A ) and other FRII sources (Araudo et al 2015(Araudo et al , 2016. These studies show that while magnetic field amplification to above 100µG can occur, the maximum energy of the non-thermal electrons is rather low, on the order of a TeV.…”

“…usually assumed (Araudo et al 2016. The same constraint applies to protons and therefore the CR maximum energy is about 1 TeV, the same as the maximum electron energy inferred by the optical-IR synchrotron cutoff.…”

Cosmic-ray acceleration by relativistic shocks: limits and estimates

“…We now show that the scattering mean free path λ is in fact much larger than the CR Larmor radius rg as deduced from observations of termination shocks in radio galaxies (Araudo et al 2015(Araudo et al , 2016. This limits further the energy to which CR can be accelerated by relativistic shocks.…”

“…There are other means by which the magnetic field might be amplified (Giacalone & Jokipii 2007) but these amplify the field over a large distance downstream of the shock and we do not consider them here since field amplification needs to take place close to a relativistic shock. Similar, presumably CR-driven, magnetic field amplification appears to happen at the termination shocks of the jets of radio galaxies where the hot-spot field is again typically 100s µG (eg Araudo et al 2016). Magnetic field amplification is a general feature of optimal CR acceleration to high energy since the magnetic energy density upstream of the shock can be expected to be much smaller than the kinetic energy density (Γ sh − 1)ρc 2 produced by any event releasing sufficient energy to be capable of particle acceleration to high energy (where ρ is the density and Γ sh the Lorentz factor of the bulk flow).…”

ALMA polarization observations of the particle accelerators in the hotspot of the radio galaxy 3C 445

“…Perucho et al 2014;Tchekhovskoy & Bromberg 2016). In principle, shocks would accelerate electrons to very high energies and this has been used to highlight some tension between shock acceleration models and the optical synchrotron cut-off observed in some hot spots (Araudo et al 2016). Given our results this tension may be circumvent by assuming that the shock acceleration occurs in an intermittent way.…”