Past activity of Sgr A* is unlikely to affect the local cosmic-ray spectrum up to the TeV regime (2405.18447v1)

Abstract: The presence of kiloparsec-sized bubble structures in both sides of the Galactic plan suggests active phases of Sgr A$^\star$, the central supermassive black hole of the Milky-Way in the last 1-6 Myr. The contribution of such event on the cosmic-ray flux measured in the solar neighborhood is investigated with numerical simulations. We evaluate whether the population of high-energy charged particles emitted by the Galactic Center could be sufficient to significantly impact the CR flux measured in the solar neighborhood. We present a set of 3D magnetohydrodynamical simulations, following the anisotropic propagation of CR in a Milky - Way like Galaxy. Independent populations of cosmic-ray are followed through time, originating from two different sources types, namely Supernovae and the Galactic Center. To assess the evolution of the CR flux spectrum properties, we split these populations into two independent energy groups of 100 GeV and 10 TeV. We find that the anisotropic nature of cosmic-ray diffusion dramatically affects the amount of cosmic-ray energy received in the solar neighborhood. Typical timescale to observe measurable changes in the CR spectrum slope is of the order 10 Myr, largely surpassing estimated ages of the Fermi bubbles in the AGN jet-driven scenario. We conclude that a cosmic-ray outburst from the Galactic center in the last few Myr is unlikely to produce any observable feature in the local CR spectrum in the TeV regime within times consistent with current estimates of the Fermi bubbles ages.

• The paper demonstrates through detailed 3D simulations that cosmic rays from past Sgr A* outbursts have negligible impact on the local TeV spectrum.
• The research employs anisotropic diffusion modeling, accurately representing the magnetic constraints on cosmic-ray propagation.
• The findings indicate that significant cosmic-ray energy escapes the Galactic disk, supporting the formation of structures like the Fermi bubbles.

Past Activity of Sgr A* and Its Influence on Local Cosmic-Ray Spectra

The paper "Past activity of Sgr A$^\star$ is unlikely to affect the local cosmic-ray spectrum up to the TeV regime," written by M. Fournier, J. Fensch, and B. Commerçon, addresses the potential impact of cosmic-ray (CR) emissions from significant historical activity of the central supermassive black hole in the Milky Way, Sgr A$^\star$, on the CR flux in the solar neighborhood. Through a series of 3D magnetohydrodynamical simulations accounting for anisotropic CR diffusion in a Milky Way-like galaxy, the authors contribute to the ongoing debate regarding the origin and influence of cosmic rays arising from the Galactic Center, particularly in connection with features such as the Fermi bubbles.

Numerical Simulations and CR Propagation

The paper utilizes a comprehensive numerical approach involving magnetohydrodynamical simulations to model the propagation of CRs emitted from two sources: supernovae remnants (SNRs) and the Galactic Center. Specifically, the simulations consider two distinct energy groups of CRs — 100 GeV and 10 TeV — representing different segments of the CR energy spectrum. A critical aspect of the research is the modeling of anisotropic CR diffusion, which dramatically influences how CRs are transported through the galaxy due to magnetic field lines.

One of the central findings is that the anisotropic diffusion significantly affects the CR energy flux observed in the solar neighborhood. The paper found the diffusion timescale to significantly alter the CR spectrum slope is on the order of 10 Myr, much longer than the estimated ages of the Fermi bubbles in scenarios driven by active galactic nucleus (AGN) jets.

Key Findings and Implications

1. Contribution to the Local CR Spectrum: The simulations reveal that recent CR outbursts from the Galactic Center are unlikely to significantly impact the CR spectrum observed locally in the TeV energy regime. The observations suggest that within the time frames accounted for the Fermi bubbles ages, the observed effects on CR flux in the solar neighborhood remain negligible.
2. Anisotropic Versus Isotropic Propagation: The research emphasizes that anisotropic propagation, as considered in the model, better reflects the Galactic environment’s magnetic constraints compared to isotropic models. In the isotropic diffusion model, the contribution from Sgr A$^\star$ to local CR spectra would be overestimated.
3. Escape of CR Energy: A noteworthy aspect is the significant fraction of CR energy injected from the Galactic Center dissipates out of the Galactic disk, contributing to the formation of bubble-like structures, thereby supporting the hypothesis of outflow dynamics in the observed Fermi bubbles.

Theoretical and Practical Significance

The paper’s findings have substantial implications for the theoretical understanding of CR propagation in the galaxy. They challenge previous assumptions regarding the extent of influence that Sgr A$^\star$ could have on locally measurable cosmic rays, especially given the energy confinement and dispersal mechanisms illuminated by the anisotropic diffusion framework. Practically, this work may guide the future observational strategies aimed at discerning the origins of cosmic-ray signatures and inform models predicting the interactions of CR with the interstellar medium.

Future Research Directions

Further investigation into the roles of various astrophysical processes associated with CR acceleration and propagation may be warranted. Improved observational data regarding magnetic field topology and CR interactions will enhance the fidelity of models like this one. Additionally, a more granular spectrum model with energy-dependent diffusion could elucidate finer details in the composition of local and extra-galactic CR sources.

Overall, the research provides valuable insights into cosmic-ray propagation from galactic phenomena, enhancing both the theoretical and applied understanding of cosmic ray astrophysics.