# Evidence that particle acceleration in hotspots of FR II galaxies is not constrained by synchrotron cooling

## TL;DR

This study demonstrates that the maximum energy of electrons in FR II galaxy hotspots is not limited by synchrotron cooling, challenging previous assumptions and suggesting alternative constraints on particle acceleration.

## Contribution

The paper provides evidence that synchrotron cooling does not constrain electron energies in hotspots, contrary to common belief, based on observational data and analysis.

## Key findings

- Maximum electron energy is ~TeV for typical magnetic fields.
- Synchrotron losses do not limit electron acceleration unless jet density is unrealistically high.
- Observational data from radio, infrared, and optical wavelengths support the conclusions.

## Abstract

We study the hotspots of powerful radiogalaxies, where electrons accelerated at the jet 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 micro Gauss. 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 at radio, infrared and optical wavelengths.