The Galactic Center: A PeV Cosmic Ray Acceleration Factory

TL;DR

This paper proposes a model where PeV cosmic rays are accelerated in the Galactic Center, explaining gamma-ray spectra and predicting observable neutrino signals for future detectors.

Contribution

It introduces a self-consistent model of PeV cosmic-ray acceleration and gamma-ray production in the Galactic Center, accounting for spectral cutoffs and predicting neutrino signals.

Findings

PeV cosmic-ray acceleration occurs in the Galactic Center.

Gamma-ray spectra are explained by cosmic-ray interactions with molecular clouds.

Future observations by CTA, LHAASO, and KM3Net can test the model.

Abstract

The multi-TeV $\gamma$-rays from the Galactic Center (GC) have a cutoff at tens of TeV, whereas the diffuse emission has no such cutoff, which is regarded as an indication of PeV proton acceleration by the HESS experiment. It is important to understand the inconsistency and study the possibility that PeV cosmic-ray acceleration could account for the apparently contradictory point and diffuse $\gamma$-ray spectra. In this work, we propose that the cosmic rays are accelerated up to $>$PeV in GC. The interaction between cosmic rays and molecular clouds is responsible for the multi-TeV $\gamma$-ray emissions from both the point source and diffuse sources today. Enhanced by the small volume filling factor (VFF) of the clumpy structure, the absorption of the $\gamma$-rays leads to a sharp cutoff spectrum at tens of TeV produced in the GC. Away from galactic center, the VFF grows and the absorption enhancement becomes negligible. As a result, the spectra of $\gamma$-ray emissions for both point source and diffuse sources can be successfully reproduced under such self-consistent picture. In addition, a "surviving-tail" at $\sim$100 TeV is expected from the point source, which can be observed by future projects CTA and LHAASO. Neutrinos are simultaneously produced during proton-proton (PP) collision. With 5-10 years observations, the KM3Net experiment will be able to detect the PeV source according to our calculation.