Impact of electron spectra on morphology of pulsar halos at ultra-high energies

TL;DR

This paper explores how the spectrum of high-energy electrons influences the shape of pulsar gamma-ray halos at ultra-high energies, revealing a degeneracy that complicates understanding electron diffusion.

Contribution

It highlights the importance of considering electron spectral shape when analyzing gamma-ray halo morphologies to accurately determine diffusion properties.

Findings

Electron spectra significantly affect gamma-ray halo morphology.

Degeneracy exists between electron spectrum and diffusion coefficient.

Spectral effects must be accounted for in diffusion studies.

Abstract

The extended $\gamma$-ray halos around pulsars are unique probe of transportation of high-energy electrons (and positrons) in vicinities of such pulsars. Observations of morphologies of several such halos indicate that particles diffuse very slowly around pulsars, compared with that in the Milky Way halo. The energy-dependent morphologies are expected to be very important in studying the energy-dependence of the diffusion coefficient. In this work we point out that the spectrum of high-energy electrons takes effect in shaping the $\gamma$-ray morphologies at the ultra-high-energy bands, and thus results in a degeneracy between the electron spectrum and the energy-dependence of the diffusion coefficient. The reasons for such a degeneracy include both the Klein-Nishina effect of the inverse Compton scattering and the curvature (if any) of the electron spectrum. It it thus necessary to take into account the spectral shape of electrons when deriving the energy-dependence of diffusion coefficient using ultra-high-energy $\gamma$-ray measurements of extended pulsar halos.