Spectral age modelling of the `Sausage' cluster radio relic

Abstract

CIZA J2242.8+5301 is a post-core passage, binary merging cluster that hosts a large, thin, arc-like radio relic, nicknamed the `Sausage', tracing a relatively strong shock front. We perform spatially-resolved spectral fitting to the available radio data for this radio relic, using a variety of spectral ageing models, with the aim of finding a consistent set of parameters for the shock and radio plasma. We determine an injection index of $0.77_{-0.02}^{+0.03}$ for the relic plasma, significantly steeper than was found before. Standard particle acceleration at the shock front implies a Mach number $M=2.90_{-0.13}^{+0.10}$, which now matches X-ray measurements. The shock advance speed is $v_{\mathrm{shock}}\approx 2500$ km s${}^{-1}$, which places the core passage of the two subclusters $0.6-0.8$ Gyr ago. We find a systematic spectral age increase from $0$ at the northern side of the relic up to $\sim 60$ Myr at $\sim 145$ kpc into the downstream area, assuming a $0.6$ nT magnetic field. Under the assumption of freely-ageing electrons after acceleration by the `Sausage' shock, the spectral ages are hard to reconcile with the shock speed derived from X-ray and radio observations. Re-acceleration or unusually efficient transport of particle in the downstream area and line-of-sight mixing could help explain the systematically low spectral ages.

Accepted for publication in MNRAS. 10 pages, 6 figures, 2 tables

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