Hadronic Scenario for Galactic PeVatron LHAASO J1912+1014u Supported by Fermi-LAT $\gamma$-ray Data and FUGIN CO Data

TL;DR

This study analyzes multi-wavelength data around LHAASO J1912+1014u, providing evidence that supports a hadronic origin of the PeV gamma-ray emission, indicating a proton PeVatron in our galaxy.

Contribution

It offers a detailed multi-wavelength analysis favoring a hadronic scenario for the PeV gamma-ray source, with improved diffuse emission modeling and spectral fitting.

Findings

Detected a significant GeV excess with a hard spectrum associated with the source.

Favored a hadronic origin over leptonic due to spectral and energy considerations.

Estimated the CR proton energy content to be (1--5) × 10^{49} erg.

Abstract

LHAASO has reported 43 sub-PeV $\gamma$-ray sources, which are promising candidates for cosmic-ray (CR) accelerators above the PeV energy, commonly called as PeVatrons. Multi-wavelength observations are crucial for identifying the underlying particle species and estimating the CR energy content of these sources. In this work we investigate the region around LHAASO J1912+1014u (and HESS J1912+101) using Fermi-LAT $\gamma$-ray data and FUGIN CO data. We analyzed 15 years of Fermi-LAT data in the 0.4--409.6 GeV energy range. By improving the standard Fermi-LAT diffuse emission model, we significantly reduced the large residuals around the source in the 1.6-12.8 GeV band. We detected a statistically significant excess above the diffuse background, which likely represents $\ge$10 GeV emission associated with the LHAASO/H.E.S.S. source. The GeV excess exhibits a hard spectrum (photon index of about 2.1) and is well reproduced by interstellar gas templates with systemic velocities of about 25 $\mathrm{km~s^{-1}}$ or 60 $\mathrm{km~s^{-1}}$. We performed a comprehensive fit to the GeV--TeV spectral energy distribution. Although a leptonic scenario can reproduce the observed spectrum, a hadronic scenario is favored once electron cooling is considered. The inferred CR proton spectrum has an index of $\sim$2.2, and the total CR proton energy above 1 GeV is (1--5) $\times 10^{49}~\mathrm{erg}$, depending on the assumed velocity range of the associated interstellar gas. A stringent upper limit on diffuse X-ray emission further supports the proton PeVatron scenario.