Galactic PeVatrons and helping to find them: Effects of Galactic absorption on the observed spectra of very high energy $\gamma$-ray sources

Abstract

Identification of the cosmic-ray (CR) `PeVatrons', which are sources capable of accelerating particles to $\sim 10^{15}$ eV energies and higher, may lead to resolving the long-standing question of the origin of the spectral feature in the all-particle CR spectrum known as the `knee'. Because CRs with these energies are deflected by interstellar magnetic fields identification of individual sources and determination of their spectral characteristics is more likely via very high energy $\gamma$-ray emissions, which provide the necessary directional information. However, pair production on the interstellar radiation field (ISRF) and cosmic microwave background leads to steepening of the high-energy tails of $\gamma$-ray spectra, and should be corrected for to enable true properties of the spectrum at source to be recovered. Employing recently developed three-dimensional ISRF models this paper quantifies the pair-absorption effect on spectra for sources in the Galactic centre direction at 8.5 kpc and 23.5 kpc distance, with the latter corresponding to the far side of the Galactic stellar disc where it is expected that discrimination of spectral features $>10$ TeV will be possible by the forthcoming Cherenkov Telescope Array. The estimates made suggest spectral cutoffs could be underestimated by factors of a few in the energy range so far sampled by TeV $\gamma$-ray telescopes. As an example to illustrate this, the recent HESS measurements of diffuse $\gamma$-ray emissions possibly associated with injection of CRs nearby Sgr A${}^{\ast }$ are ISRF-corrected, and estimates of the spectral cutoff are re-evaluated. It is found that it could be higher by up to a factor $\sim 2$, indicating that these emissions may be consistent with a CR accelerator with a spectral cutoff of at least 1 PeV at the 95% confidence level.

6 pages, 3 figures. Accepted by Physical Review D, Rapid Communications

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• Deciphering Residual Emissions: Time-Dependent Models for the Non-Thermal Interstellar Radiation from the Milky Way
• Using Interstellar Clouds to Search for Galactic PeVatrons: Gamma-ray Signatures from Supernova Remnants
• The Steady-State Multi-TeV Diffuse Gamma-Ray Emission Predicted with GALPROP and Prospects for the Cherenkov Telescope Array
• Mind the gap: The discrepancy between simulation and reality drives interpretations of the Galactic Center Excess
• Signatures of Recent Cosmic-Ray Acceleration in the High-Latitude $\gamma$-Ray Sky
• Exploring the high energy frontiers of the Milky Way with ground-based gamma-ray astronomy: PeVatrons and the quest for the origin of Galactic cosmic-rays
• Modeling the Galactic center gamma-ray emission with more realistic cosmic-ray dynamics
• Constraints on Axion-like Particles from the gamma-ray observation of the Galactic Center
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• Search for the Galactic accelerators of Cosmic-Rays up to the Knee with the Pevatron Test Statistic
• The GALPROP Cosmic-ray Propagation and Non-thermal Emissions Framework: Release v57
• Stochastic modelling of cosmic-ray sources for Galactic diffuse emissions
• The GPSP Atlas: High-resolution Galactic Photon Survival Probability Atlas for Very-High- and Ultra-High-Energy Gamma-Ray Astronomy