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(shock) approximate to 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 similar to 60 Myr at similar to 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