New mysteries and challenges from the Toothbrush relic: wideband observations from 550 MHz to 8 GHz (1911.08904v2)

Abstract: Radio relics are diffuse extended synchrotron sources that originate from shock fronts induced by galaxy cluster mergers. The actual particle acceleration mechanism at the shock fronts is still under debate. The galaxy cluster 1RXS J0603.3+4214 hosts one of the most intriguing examples of radio relics, known as the Toothbrush. We present new wideband radio continuum observations made with uGMRT and VLA. Our new observations, in combination with previously published data, allowed us to carry out a detailed high spatial resolution spectral and curvature analysis of the known diffuse radio emission sources, over a broad range of frequencies. The integrated spectrum of the Toothbrush follows closely a power law over close to 2 decades in frequency, with a spectral index of $-1.16\pm0.02$. We do not find any evidence of spectral steepening below 8 GHz. The subregions of the Toothbrush show an identical spectral slopes, suggesting that observed spectral index is rather set by the distribution of Mach numbers which may have a similar shape at different parts of the shock front. Indeed, numerical simulations show an intriguing similar spectral index, indicating that the radio spectrum is dominated by the average over the inhomogeneities within the shock, with most of the emission coming from the tail of the Mach number distribution. In contrast to the Toothbrush, the spectrum of the fainter relics show a high frequency steepening. The integrated spectrum of the halo follows a power law from 150 MHz to 3 GHz with a spectral index of $-1.16\pm0.04$. We do not find any evidence for spectral curvature, not even in subareas of the halo. This suggest a homogeneous acceleration throughout the cluster volume. Between the brush region of the Toothbrush and the halo, the color-color analysis revealed emission that was consistent with an overlap between the two different spectral regions.

• The paper demonstrates that the integrated emission from the Toothbrush relic follows a consistent power law with a spectral index of -1.16±0.02 over nearly two orders of magnitude.
• The paper reveals detailed spatial spectral maps that uncover intricate filamentary structures and suggest a uniform shock Mach number distribution along the relic.
• The paper shows that numerical simulations support standard DSA processes with homogeneous post-shock magnetic fields to explain the relic-halo overlap and uniform spectral properties.

Analysis of the Toothbrush Relic in Galaxy Cluster 1RXS J0603.3+4214: Key Findings and Implications

The paper in question presents a comprehensive paper of the radio relic known as the "Toothbrush" within the merging galaxy cluster 1RXS J0603.3+4214. The paper utilizes deep, wideband radio observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Very Large Array (VLA), covering frequencies from 550 MHz to 8 GHz. The research aims to elucidate the underlying mechanisms that accelerate relativistic particles in the intracluster medium (ICM), focusing on the Toothbrush relic, other associated relics, and a massive radio halo present in the cluster.

Main Findings

1. Integrated Relic Spectrum: The paper finds that the integrated emission from the Toothbrush relic adheres closely to a power law across nearly two orders of magnitude in frequency. The spectral index is reported as $-1.16\pm0.02$, with no evident spectral bending below 8 GHz. This observation is consistent across separate regions of the relic and contrasts with some previous studies suggesting spectral steepening at higher frequencies. The uniform spectral index suggests that similar conditions underlie different parts of the relic.
2. Spatial and Spectral Analysis: The paper provides high spatial resolution spectral index maps, revealing intricate filamentary structures within the Toothbrush. The consistent power-law spectrum indicates a robust convergence of the spectral shape despite potential minor variations in local conditions. The researchers conclude that the Mach number distribution along the shock front might contribute to the uniform spectral features observed.
3. Comparison with Simulations: Numerical simulations support the observed homogeneity of the spectral index by considering the effects of shock-induced particle acceleration and downstream cooling. The simulations suggest that localized variations in Mach number, injected electron distributions, and magnetic fields average out to form the observed spectral slopes, consistent with the simplest DSA model assumptions.
4. Halo and Relic Connection: A consequential finding is the apparent overlap between the Toothbrush relic and the central radio halo. The observational data suggest a superposition of emission from the relic and the halo, with high-frequency observations indicating a more uniform spectral index, typical of halo emissions. This overlap is crucial for understanding the transition from shock-induced to turbulent-acceleration dominated regions within the cluster.
5. Implications on Particle Acceleration: The morphology and spectral properties revealed in this paper imply that some level of uniformity exists in the shock characteristics and magnetic field strengths across the Toothbrush relic. These findings challenge purely stochastic acceleration models and indicate that standard DSA processes with a homogeneous post-shock magnetic field may sufficiently describe the observed spectra.

Implications and Future Directions

The findings from this paper have significant implications for our understanding of particle acceleration mechanisms in galaxy clusters. The consistency of the spectral index across the Toothbrush relic suggests that variations in acceleration efficiency, shock strength, and magnetic field properties might be minimal compared to previously thought. The overlap of relic and halo emissions highlights the need for integrated models that consider both shock acceleration and turbulent re-acceleration processes.

Future studies could benefit from extending these observations to even higher frequencies to definitively rule out spectral steepening and to corroborate the findings with more detailed magnetohydrodynamical simulations. Exploration into the impact of environmental conditions on relic morphology and spectral properties could further refine models of ICM physics. Additionally, polarization studies could offer more nuanced insights into magnetic field structures and their role in the observed emission characteristics.

Overall, the paper sets a robust benchmark for analyzing relic and halo phenomena in galaxy clusters, offering insights into the physics of cosmic rays and magnetic fields within large-scale cosmic structures.