On the Contribution of Unresolved Pulsars to the Ultra-high-energy Galactic Diffuse Gamma-Ray Emission

TL;DR

This study estimates the contribution of unresolved pulsars to the ultra-high-energy Galactic diffuse gamma-ray emission, finding it to be limited and not dominant above tens of TeV, thus refining our understanding of cosmic ray propagation.

Contribution

It introduces a novel data-driven method to quantify unresolved pulsar contributions using ATNF and LHAASO catalogs, providing new constraints on their impact.

Findings

Unresolved pulsars contribute up to 38% of the diffuse flux at 20 TeV in the inner Galaxy.

The contribution decreases with energy, dropping below 21% above 100 TeV.

In the outer Galaxy, the contribution remains subdominant across energies.

Abstract

The ultra high-energy (UHE) Galactic diffuse gamma-ray emission holds important information on the propagation of cosmic rays in the Galaxy. However, its measurements suffer from a contamination from unresolved sources whose contribution remains unclear. In this Letter, we propose a novel data-driven estimate of the contribution of unresolved pulsar wind nebulae and TeV halos based on the information present in the ATNF and the LHAASO catalogs. We find that in the inner Galaxy, this contribution is limited to $\sim38\%\pm10\%$ of the diffuse flux measured by LHAASO at $\sim20\,\rm{TeV}$ in the case where all sources associated to pulsars contribute as unresolved sources, and this fraction drops with energy to less than $21\%\pm6\%$ above $100\,\rm{TeV}$. In the outer Galaxy, this contribution is always subdominant. In particular, it reaches at most $\sim18\%\pm2\%$ at $10\,\rm{TeV}$ and is less than $\sim7\%\pm1\%$ above $\sim25\,\rm{TeV}$. We conclude that the UHE Galactic diffuse gamma-ray emission cannot be dominated by unresolved pulsar sources above a few tens of $\rm{TeV}$.