Ultrahigh-Energy Gamma-Ray Sources Need Not Be Hadronic PeVatrons

TL;DR

This paper argues that ultrahigh-energy gamma-ray sources do not necessarily have to be hadronic PeVatrons, as they can be explained by leptonic models, challenging previous assumptions about their origin.

Contribution

The study demonstrates that high-energy gamma rays can originate from leptonic processes, providing alternative explanations for sources previously thought to be hadronic PeVatrons.

Findings

Leptonic models can explain PeV gamma-ray observations.

The angular extension of SS 433 matches leptonic model predictions.

Identifying a PeVatron requires additional observational evidence.

Abstract

Ultrahigh-energy gamma rays ($E_{\gamma}>100 \, {\rm TeV}$) have been detected from a handful of astrophysical sources. Due to the Klein-Nishina suppression of inverse Compton scattering at such high energies, it has sometimes been argued that these sources must be accelerators of PeV-scale protons, making them the long-sought-after Galactic ''PeVatrons.'' Here, we challenge this conclusion, demonstrating that these sources can be straightforwardly explained by simple leptonic models. In this context, we consider the microquasar SS 433, the Galactic Center, and TeV halos, showing in each case that the observation of PeV-scale gamma rays from these sources does not indicate that they are accelerators of hadronic cosmic rays. We also note that the measured angular extension of SS 433 is in good agreement with the predictions of our model, favoring a leptonic origin for the gamma-ray emission from this source. A definitive identification of a PeVatron would require additional information, such as the combined observation of the pion bump and synchrotron peak, the spatial correlation of gamma-ray emission with gas, or the detection of neutrinos with $E_{\nu} \gtrsim 100 \, {\rm TeV}$.