The Highest Energy HAWC Sources are Likely Leptonic and Powered by Pulsars

TL;DR

This paper argues that the highest energy gamma-ray sources observed by HAWC are likely leptonic pulsar wind nebulae, not hadronic sources, based on spectral analysis and energy loss considerations.

Contribution

The study provides evidence favoring leptonic models over hadronic ones for the highest energy HAWC sources, emphasizing the role of pulsars in powering these emissions.

Findings

Spectral features match inverse Compton models.

Spectral break aligns with pulsar age and energy losses.

Hadronic models cannot easily explain observed spectra.

Abstract

The HAWC Collaboration has observed gamma rays at energies above 56 TeV from a collection of nine sources. It has been suggested that this emission could be hadronic in nature, requiring that these systems accelerate cosmic-ray protons or nuclei up to PeV-scale energies. In this paper, we instead show that the spectra of these objects favor a leptonic (inverse Compton) origin for their emission. More specifically, the gamma-ray emission from these objects can be straightforwardly accommodated within a model in which $\sim \mathcal{O}(10\%)$ of the host pulsar's spindown power is transferred into the acceleration of electrons and positrons with a power-law spectrum that extends to several hundred TeV or higher. The spectral break that is observed among these sources is naturally explained within the context of this simple model, and occurs at the energy where the timescale for energy losses matches the age of the pulsar. In contrast, this spectral feature cannot be straightforwardly accommodated in hadronic scenarios. Furthermore, hadronic models predict that these sources should produce more emission at GeV-scale energies than is observed. In light of these considerations, we conclude that HAWC's highest energy sources should be interpreted as TeV halos or pulsar wind nebulae, which produce their emission through inverse Compton scattering, and are powered by the rotational kinetic energy of their host pulsar.