Synchrotron radiation from electrons with a pitch-angle distribution

TL;DR

This paper investigates how anisotropic electron pitch-angle distributions affect synchrotron radiation spectra, revealing conditions under which the spectrum follows different power laws, with implications for astrophysical sources like blazars and GRBs.

Contribution

It introduces a model for synchrotron radiation from anisotropic electron distributions, deriving spectral behaviors based on the pitch-angle scatter and applying it to astrophysical phenomena.

Findings

For small pitch-angle scatter, the spectrum follows F_ν ∝ ν^{2/3}.

For large scatter, a broken power-law spectrum appears with a specific break frequency.

The theory constrains the electron pitch-angle distribution effects on observed synchrotron spectra.

Abstract

In most astrophysical processes involving synchrotron radiation, the pitch-angle distribution of the electrons is assumed to be isotropic. However, if electrons are accelerated anisotropically, e.g, in a relativistic shock wave with an ordered magnetic field or in magnetic reconnection regions, the electron pitch angles might be anisotropic. In this work, we study synchrotron radiation from electrons with a pitch-angle distribution with respect to a large-scale uniform magnetic field. Assuming that the pitch-angle distribution is normal with a scatter of $\sigma_p$ and that the viewing direction is where the pitch-angle direction peaks, we find that for electrons with a Lorentz factor $\gamma$, the observed flux satisfies $F_\nu\propto\nu^{2/3}$ for $\nu\ll\nu_{\rm cr}$ ($\nu_{\rm cr}$ is the critical frequency of synchrotron), if $\sigma_p\lesssim1/\gamma$ is satisfied. On the other hand, if $\sigma_p\gg1/\gamma$, the spectrum below $\nu_{\rm cr}$ is a broken power law with a break frequency $\nu_{\rm br}\sim2\nu_{\rm cr}/\sigma_p^3\gamma^3$, e.g., $F_\nu\propto\nu^{2/3}$ for $\nu\ll\nu_{\rm br}$ and $F_\nu\propto\nu^{1/3}$ for $\nu_{\rm br}\ll\nu\ll\nu_{\rm cr}$. Thus the ultimate synchrotron line of death is $F_\nu\propto\nu^{2/3}$. We discuss the application of this theory to blazars and gamma-ray bursts (GRBs).