Radio surface fluctuations in radio relics (2402.12375v1)

Abstract: Recent observations have revealed detailed structures of radio relics in a wide range of frequencies. In this work, we perform three-dimensional magnetohydrodynamical simulations of merger shocks propagating through a turbulent magnetized intracluster medium, and employ on-the-fly Lagrangian particles to explore the physical processes originating radio substructures and their appearances in high and low-frequency observations. We employ two cosmic-ray (CR) electron acceleration models: the fresh injection of electrons from the thermal pool and the re-acceleration of mildly relativistic electrons. We use the relative surface brightness fluctuations, $\delta S_{\nu}$, to define a "degree of patchiness''. We find that: 1) Patchiness is produced if the shock's surface has a distribution of Mach numbers, rather than a single Mach number; 2) Radio relics appear patchier if the Mach number distribution consists of a large percentage of low Mach numbers ($\mathcal{M}\lesssim2.5$); 3) As the frequency increases, the patchiness also becomes larger. Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz), they are necessarily also at low frequencies (e.g., 150 MHz); 4) To produce noticeable differences in the patchiness at low and high frequencies, the shock front should have a Mach number spread of $\sigma_{\mathcal{M}}\gtrsim0.3$-0.4; 5) The amount of the patchiness depends on the Mach number distribution as well as the CR acceleration model. We propose $\delta S_{\nu}$ as a potential tool for extracting merger shock properties and information about particle acceleration processes at shocks in radio observations.

• The paper demonstrates that radio relic surface patchiness is directly linked to the distribution of shock Mach numbers.
• It employs high-resolution 3D MHD simulations with on-the-fly Lagrangian particles to analyze electron acceleration models.
• The findings highlight that frequency-dependent observations offer new insights into cosmic-ray acceleration and ICM turbulence.

Overview of "Radio Surface Fluctuations in Radio Relics"

The research paper titled "Radio Surface Fluctuations in Radio Relics" by Dominguez-Fernandez et al. addresses the detailed structures and dynamics of radio relics, which are diffuse radio sources found in the outskirts of galaxy clusters. These structures are associated with merger shocks in the intracluster medium (ICM), where they accelerate cosmic-ray electrons to emit synchrotron radiation detectable across multiple frequencies. The paper utilizes 3D magnetohydrodynamical (MHD) simulations to explore the emergence of substructures within radio relics and their observational implications across a spectrum of radio frequencies.

Methodology

The authors employ high-resolution MHD simulations to model merger shocks propagating through turbulent, magnetized ICM. This setup involves the use of on-the-fly Lagrangian particles to simulate the physics of cosmic-ray electron acceleration and re-acceleration. Two primary electron acceleration models are investigated: the fresh injection from the thermal pool and the re-acceleration of pre-existing mildly relativistic electrons. The simulation tracks the resulting radio substructures, which are synthesized and analyzed across a wide frequency range to assess their impact on radio surface brightness.

Key Findings

1. Patchiness and Mach Number Distribution: The findings highlight that surface patchiness in radio relics can be linked directly to the distribution of Mach numbers at the shock surface. A variety of Mach numbers, especially those skewed towards lower values (𝒪(2.5)), leads to greater patchiness in the observed radio emission.
2. Frequency Dependence: The authors demonstrate that the degree of patchiness is frequency-dependent, increasing with higher observing frequencies. Specifically, they note that relics that appear patchy at high frequencies retain this property at lower frequencies, although the effect is more pronounced at higher spectral resolutions.
3. Impact of Critical Mach Number: The results indicate that shock properties such as the spread of Mach numbers play a crucial role in defining the substructures of radio relics. Specifically, a broader Mach number distribution enhances differences in the patchiness observed at low versus high frequencies.
4. Model Implications: The paper concludes that the different electron acceleration models affect the degree and pattern of patchiness. Moreover, the re-acceleration model generally results in smoother radio structures compared to the fresh injection model.

Implications and Future Directions

The insights gained from this research enhance the theoretical understanding of radio relic emissions, potentially refining methodologies for using radio observations to infer physical conditions in galaxy clusters. The proposed metric, $\delta S_{\nu}$, offers a promising, quantifiable measure for extracting information about shock properties and cosmic-ray acceleration processes from observational data. Additionally, the research has implications for understanding the characteristic turbulence and magnetic field distributions in the ICM.

Looking forward, continued development of radio telescope capabilities, including those in the millimeter and sub-millimeter regimes, will facilitate even more refined observations. This will likely further validate and refine theoretical models of radio relics discussed in this paper. Moreover, future work should focus on integrating observational constraints with simulations, potentially using high-resolution spectral indices to provide more precise insights into the dynamics and evolution of these fascinating astrophysical phenomena.