We report the discovery of extended radio emission in the Phoenix cluster ( SPT-CL J2344–4243 , z=0.596 ) with the GMRT at 610 MHz .
The diffuse emission extends over a region of at least 400–500 kpc and surrounds the central radio source of the Brightest Cluster Galaxy , but does not appear to be directly associated with it .
We classify the diffuse emission as a radio mini-halo , making it the currently most distant mini-halo known .
Radio mini-halos have been explained by synchrotron emitting particles re-accelerated via turbulence , possibly induced by gas sloshing generated from a minor merger event .
Chandra observations show a non-concentric X-ray surface brightness distribution , which is consistent with this sloshing interpretation .
The mini-halo has a flux density of 17 \pm 5 mJy , resulting in a 1.4 GHz radio power of ( 10.4 \pm 3.5 ) ~ { } \times 10 ^ { 24 } W Hz ^ { -1 } .
The combined cluster emission , which includes the central compact radio source , is also detected in a shallow GMRT 156 MHz observation and together with the 610 MHz data we compute a spectral index of -0.84 \pm 0.12 for the overall cluster radio emission .
Given that mini-halos typically have steeper radio spectra than cluster radio galaxies , this spectral index should be taken as an upper limit for the mini-halo .