Previous studies have shown that CIZA J2242.8+5301 ( the ’ Sausage ’ cluster , z = 0.192 ) is a massive merging galaxy cluster that hosts a radio halo and multiple relics .
In this paper we present deep , high fidelity , low-frequency images made with the LOw-Frequency Array ( LOFAR ) between 115.5 and 179 MHz .
These images , with a noise of 140 \mbox { $ \mu$Jy / beam } and a resolution of \theta _ { \text { beam } } = 7.3 \arcsec \times 5.3 \arcsec , are an order of magnitude more sensitive and five times higher resolution than previous low-frequency images of this cluster .
We combined the LOFAR data with the existing GMRT ( 153 , 323 , 608 MHz ) and WSRT ( 1.2 , 1.4 , 1.7 , 2.3 GHz ) data to study the spectral properties of the radio emission from the cluster .
Assuming diffusive shock acceleration ( DSA ) , we found Mach numbers of \mathcal { M } _ { n } = 2.7 { } _ { -0.3 } ^ { +0.6 } and \mathcal { M } _ { s } = 1.9 _ { -0.2 } ^ { +0.3 } for the northern and southern shocks .
The derived Mach number for the northern shock requires an acceleration efficiency of several percent to accelerate electrons from the thermal pool , which is challenging for DSA .
Using the radio data , we characterised the eastern relic as a shock wave propagating outwards with a Mach number of \mathcal { M } _ { e } = 2.4 _ { -0.3 } ^ { +0.5 } , which is in agreement with \mathcal { M } _ { e } ^ { X } = 2.5 { } _ { -0.2 } ^ { +0.6 } that we derived from Suzaku data .
The eastern shock is likely to be associated with the major cluster merger .
The radio halo was measured with a flux of 346 \pm 64 \text { mJy } at 145 \text { MHz } .
Across the halo , we observed a spectral index that remains approximately constant ( \alpha ^ { \text { 145 MHz - 2.3 GHz } } _ { \text { across \ ( \sim \ ) 1 Mpc } ^ { 2 } } = -1.01 \pm 0.10 ) after the steepening in the post-shock region of the northern relic .
This suggests a generation of post-shock turbulence that re-energies aged electrons .