The Galactic Center: A Petaelectronvolt Cosmic-ray Acceleration Factory

Abstract

The multi-TeV γ-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 γ-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 γ-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 γ-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 γ-ray emissions for both point source and diffuse sources can be successfully reproduced under such self-consistent picture. In addition, a "surviving-tail" at ∼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.

Science Case for a Wide Field-of-View Very-High-Energy Gamma-Ray Observatory in the Southern Hemisphere

“…Observations helped identifying the presence of PeV protons in the central ∼10 parsec of the Galaxy, but there is still no unequivocal identification of the source of these PeV protons. The most plausible explanation is that the supermassive black hole Sagittarius A * is somewhat involved in the acceleration process [48][49][50]. Other hypotheses have been, and are still being investigated, such as for example the increased SN rate in the Galactic center [51], the role of massive stars [52], or the contribution from millisecond pulsars [53], among others.…”

High-Energy Particles and Radiation in Star-Forming Regions

“…data likely indicates the acceleration of the PeV protons within the central 10 pc of the Milky Way. A scenario of PeV CRs acceleration in the vicinity of massive black hole in the GC was discussed by Aharonian and Neronov (2005) (see also Guo et al 2017).…”

Science Case for a Wide Field-of-View Very-High-Energy Gamma-ray Observatory in the Southern Hemisphere

“…Observations helped identifying the presence of PeV protons in the central ∼10 parsec of the Galaxy, but there is still no unequivocal identification of the source of these PeV protons. The most plausible explanation is that the supermassive black hole Sagittarius A * is somewhat involved in the acceleration process [48][49][50]. Other hypotheses have been, and are still being investigated, such as for example the increased SN rate in the Galactic center [51], the role of massive stars [52], or the contribution from millisecond pulsars [53], among others.…”