Electron Energy Distributions at Relativistic Shock Sites: Observational Constraints from the Cygnus A Hotspots

TL;DR

This study combines new infrared observations with existing radio and X-ray data to analyze electron energy spectra at Cygnus A hotspots, revealing non-standard acceleration features and challenging canonical shock models.

Contribution

It provides the first detailed reconstruction of electron energy spectra at Cygnus A hotspots across multiple wavelengths, highlighting deviations from expected shock acceleration slopes.

Findings

Electron spectra have flat slopes (~1.5) up to a break energy near the proton-electron mass ratio.

Beyond the break, electron spectra steepen to slopes greater than 3.

No evidence of the canonical slope s=2 expected from first-order Fermi acceleration.

Abstract

We report new detections of the hotspots in Cygnus A at 4.5 and 8.0 microns with the Spitzer Space Telescope. Together with detailed published radio observations and synchrotron self-Compton modeling of previous X-ray detections, we reconstruct the underlying electron energy spectra of the two brightest hotspots (A and D). The low-energy portion of the electron distributions have flat power-law slopes (s~1.5) up to the break energy which corresponds almost exactly to the mass ratio between protons and electrons; we argue that these features are most likely intrinsic rather than due to absorption effects. Beyond the break, the electron spectra continue to higher energies with very steep slopes s>3. Thus, there is no evidence for the `canonical' s=2 slope expected in 1st order Fermi-type shocks within the whole observable electron energy range. We discuss the significance of these observations and the insight offered into high-energy particle acceleration processes in mildly relativistic shocks.