Mapping the particle acceleration in the cool core of the galaxy cluster
  RX J1720.1+2638
  We present new deep, high-resolution radio images of the diffuse minihalo in
the cool core of the galaxy cluster RX J1720.1+2638. The images have been
obtained with the Giant Metrewave Radio Telescope at 317, 617 and 1280 MHz and
with the Very Large Array at 1.5, 4.9 and 8.4 GHz, with angular resolutions
ranging from 1" to 10". This represents the best radio spectral and imaging
dataset for any minihalo. Most of the radio flux of the minihalo arises from a
bright central component with a maximum radius of ~80 kpc. A fainter tail of
emission extends out from the central component to form a spiral-shaped
structure with a length of ~230 kpc, seen at frequencies 1.5 GHz and below. We
find indication of a possible steepening of the total radio spectrum of the
minihalo at high frequencies. Furthermore, a spectral index image shows that
the spectrum of the diffuse emission steepens with the increasing distance
along the tail. A striking spatial correlation is observed between the minihalo
emission and two cold fronts visible in the Chandra X-ray image of this cool
core. These cold fronts confine the minihalo, as also seen in numerical
simulations of minihalo formation by sloshing-induced turbulence. All these
observations favor the hypothesis that the radio emitting electrons in cluster
cool cores are produced by turbulent reacceleration.
