Search

We report the discovery of a radio relic in the north-eastern periphery of the galaxy cluster Abell 2108 (A2108). A2108 is part of the uGMRT LOw-MAss Galaxy Cluster Survey (GLOMACS), where our main aim is to search for diffuse radio emission signatures in very sparsely explored low-mass galaxy clusters using uGMRT band-3 (central frequency 400 MHz). We used our uGMRT band-3 observations along with the existing archival band-3 uGMRT data to improve image sensitivity. Along with the previously reported south-western relic, the discovery of the new relic makes A2108 one of the few low-mass clusters hosting a double relic. The new relic spans over a region of 610 kpc × 310 kpc and, interestingly, differs considerably in size and morphology from the other relic. Using XMM–Newton science archive data, we also report the tentative detection of a mildly supersonic shock of Mach number |$\mathcal {M}_\mathrm{SB}=1.42$| and |$\mathcal {M}_\mathrm{T} = 1.43$| from the surface brightness and temperature discontinuities, respectively, near this newly found relic. Both the relics in A2108 are found to be significantly under-luminous compared with other double-relic systems in the mass–luminosity plane. Moreover, the north-eastern relic is remarkably under-luminous in the size–luminosity plane. Although mild supersonic shocks resulting from an off-axis merger could have influenced the origin of both relics, we hypothesize that local environments have played a crucial role in shaping their morphologies. [ABSTRACT FROM AUTHOR]

A joint hadronic model is shown to quantitatively explain the observations of diffuse radio emission from galaxy clusters in the form of minihaloes, giant haloes, relics, and their hybrid, transitional stages. Cosmic-ray diffusion of order D ∼ 1031–32 cm2 s−1, inferred independently from relic energies, the spatial variability of giant-halo spectra, and the spectral evolution of relics, reproduces the observed spatio-spectral distributions, explains the recently discovered mega-haloes as enhanced peripheral magnetization, and quenches electron (re)acceleration by weak shocks or turbulence. For instance, the hard-to-soft evolution along secondary-electron diffusion explains both the soft spectra in most halo peripheries and relic downstreams, and the hard spectra in most halo centres and relic edges, where the photon index can reach α ≃ −0.5 regardless of the Mach number |$\mathcal {M}$| of the coincident shock. Such spatio-spectral modelling, recent γ-ray observations, and additional accumulated evidence are thus shown to support a 2010 claim that the seamless transitions among minihaloes, giant haloes, and relics, their similar energetics, integrated spectra, and delineating discontinuities, the inconsistent |$\mathcal {M}$| inferred from radio versus X-rays in leptonic models, and additional observations, all indicate that these diffuse radio phenomena are manifestations of the same cosmic-ray ion population, with no need to invoke less natural alternatives. [ABSTRACT FROM AUTHOR]

Radio relics are the manifestation of electrons presumably being shock (re-)accelerated to high energies in the outskirts of galaxy clusters. However, estimates of the shocks' strength yield different results when measured with radio or X-ray observations. In general, Mach numbers obtained from radio observations are larger than the corresponding X-ray measurements. In this work, we investigate this Mach number discrepancy. For this purpose, we used the cosmological code enzo to simulate a sample of galaxy clusters that host bright radio relics. For each relic, we computed the radio Mach number from the integrated radio spectrum and the X-ray Mach number from the X-ray surface brightness and temperature jumps. Our analysis suggests that the differences in the Mach number estimates follow from the way in which different observables are related to different parts of the underlying Mach number distribution: radio observations are more sensistive to the high Mach numbers present only in a small fraction of a shock's surface, while X-ray measurements reflect the average of the Mach number distribution. Moreover, X-ray measurements are very sensitive to the relic's orientation. If the same relic is observed from different sides, the measured X-ray Mach number varies significantly. On the other hand, the radio measurements are more robust, as they are unaffected by the relic's orientation. [ABSTRACT FROM AUTHOR]