Abstract
Radio relics are extended radio emission features which trace shock waves in the periphery of galaxy clusters originating from cluster mergers. Some radio relics show a highly polarised emission, which make relics an excellent probe for the magnetisation of the intra-cluster medium. The origin of the relic polarisation is still debated. It could be a result of tangentially stretching the magnetic field at the shock surface. This scenario would naturally explain the alignment of the polarisation (E-vectors) with the shock normal. We have implemented a toy model for the relic polarisation according to this scenario. We find that the magnetic field strength itself crucially affects the fractional polarisation. Moreover, we find that the shock strength has surprisingly little effect on the overall polarisation fraction. Finally, we find that the fractional polarisation may decrease downstream depending on the magnetic field strength. Our results demonstrates that the shock compression scenario provides a very plausible explanation for the radio relic polarisation which specific features permitting to test the origin of radio relic polarisation.
Highlights
When galaxy clusters merge, large shock waves form in the intra-cluster medium.These shock fronts have been observed as discontinuities in the X-ray surface brightness.large-scale radio emission features have been found in the periphery of galaxy clusters and are classified as radio relics [1]
We investigate how the fractional polarisation depends on the magnetic field strength for a given shock strength
Using our toy model MoCaRePo for relic polarisation, we demonstrated that shock compression can lead to polarisation fractions of 50% and above
Summary
Large shock waves form in the intra-cluster medium. These shock fronts have been observed as discontinuities in the X-ray surface brightness. Large-scale radio emission features have been found in the periphery of galaxy clusters and are classified as radio relics [1] Their existence is attributed to the acceleration of thermal or mildly relativistic electrons to higher relativistic energies at the shock fronts [2,3]. The radio relic emission manifests that synchrotron radiation is emitted at the shock fronts, magnetic fields and electrons with relativistic energies (cosmic ray electrons, CRe) are present in the outskirts of galaxy clusters. In the observed relics, there might be projection effects –which affect the spectral index profile–, causing a contribution of emission inclined to the line-of-sight and, reducing the fractional polarisation downstream [14]. We describe in brief our model calculations and discuss in particular how the polarisation properties depend on the magnetic field strength
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