The interstellar cosmic-ray electron spectrum from synchrotron radiation and direct measurements

TL;DR

This study uses synchrotron radiation and direct measurements to constrain the low-energy interstellar cosmic-ray electron spectrum, revealing a need for a spectral break and implications for cosmic-ray propagation models.

Contribution

It demonstrates how synchrotron data constrains the low-energy cosmic-ray electron spectrum and challenges standard models, highlighting the importance of secondary leptons and propagation effects.

Findings

Synchrotron data confirms a low-energy break in the electron spectrum.

The interstellar spectrum below a few GeV is lower than standard models predict.

Reacceleration models are less compatible with synchrotron constraints.

Abstract

Context: The relation between Galactic cosmic-ray electrons, magnetic fields and synchrotron radiation. Aims: We exploit synchrotron radiation to constrain the low-energy interstellar electron spectrum, using various radio surveys and connecting with electron data from Fermi-LAT and other experiments. Methods: The GALPROP programme for cosmic-ray propagation, gamma-ray and synchrotron radiation is used. Secondary electrons and positrons are included. Propagation models based on cosmic-ray and gamma-ray data are tested against synchrotron data from 22 MHz to 94 GHz. Results: The synchrotron data confirm the need for a low-energy break in the cosmic-ray electron injection spectrum. The interstellar spectrum below a few GeV has to be lower than standard models predict, and this suggests less solar modulation than usually assumed. Reacceleration models are more difficult to reconcile with the synchrotron constraints. We show that secondary leptons are important for the interpretation of synchrotron emission. We also consider a cosmic-ray propagation origin for the low-energy break. Conclusions: Exploiting the complementary information on cosmic rays and synchrotron gives unique and essential constraints on electrons, and has implications for gamma rays. This connection is especially relevant now in view of the ongoing PLANCK and Fermi missions.