Particle acceleration in a nearby galaxy cluster pair: the role of
  cluster dynamics
  Diffuse radio emission associated with the intra-cluster medium (ICM) is
observed in a number of merging galaxy clusters. It is currently believed that
in mergers a fraction of the kinetic energy is channeled into non-thermal
components, such as turbulence, cosmic rays and magnetic fields, that may lead
to the formation of giant synchrotron sources in the ICM. Studying merging
galaxy clusters in different evolutionary phases is fundamental to
understanding the origin of radio emission in the ICM. We observed the nearby
galaxy cluster pair RXC J1825.3+3026 ($z\sim0.065$) and CIZA J1824.1+3029
($z\sim0.071$) at 120-168 MHz with the LOw Frequency ARray (LOFAR) and made use
of a deep (240 ks) XMM-Newton dataset to study the non-thermal and thermal
properties of the system. RXC J1825.3+3026 is in a complex dynamical state,
with a primary on-going merger in the E-W direction and a secondary later stage
merger with a group of galaxies in the SW, while CIZA J1824.1+3029 is
dynamically relaxed. These two clusters are in a pre-merger phase. We report
the discovery of a Mpc-scale radio halo with a low surface brightness extension
in RXC J1825.3+3026 that follows the X-ray emission from the cluster center to
the remnant of a galaxy group in the SW. This is among the least massive
systems and the faintest giant radio halo known to date. Contrary to this, no
diffuse radio emission is observed in CIZA J1824.1+3029 nor in the region
between the pre-merger cluster pair. The power spectra of the X-ray surface
brightness fluctuations of RXC J1825.3+3026 and CIZA J1824.1+3029 are in
agreement with the findings for clusters exhibiting a radio halo and the ones
where no radio emission has been detected, respectively. We provide
quantitative support to the idea that cluster mergers play a crucial role in
the generation of non-thermal components in the ICM.
