Diffusive Cosmic-ray Acceleration in Sagittarius A*

TL;DR

This paper demonstrates that stochastic acceleration in a complex environment around Sgr A* can explain its high-energy spectrum, challenging the idea that observed variability is necessary to account for the data.

Contribution

It introduces a model of stochastic acceleration in a two-phase environment around Sgr A* that fits the observed high-energy emission without requiring strong variability.

Findings

Stochastic acceleration can reproduce Sgr A*'s spectrum.

Fermi-LAT and HESS data do not necessarily imply rapid variability.

Two-phase environment models are consistent with observations.

Abstract

Together, the Fermi-LAT and HESS have revealed the presence of an unusual GeV-TeV source coincident with Sgr A* at the Galactic center. Its high-energy emission appears to be bimodal, hinting at an energizing process more sophisticated than mere shock acceleration. It has been suggested that this may be evidence of strong, rapid variability, as required if Sgr A*'s emission were responsible for the fluorescent X-ray echos detected in nearby molecular clouds. In this {\it Letter}, however, we show that stochastic acceleration in a more realistic two-phase environment surrounding the central black hole can accommodate Sgr A*'s high-energy spectrum quite well. We therefore suggest that the Fermi-HESS data alone do not necessarily provide evidence for strong variability in Sgr A*.