LOFAR discovery of radio emission in MACS$\,$J0717.5$+$3745
  We present results from LOFAR and GMRT observations of the galaxy cluster
MACS$\,$J0717.5$+$3745. The cluster is undergoing a violent merger involving at
least four sub-clusters, and it is known to host a radio halo. LOFAR
observations reveal new sources of radio emission in the Intra-Cluster Medium:
(i) a radio bridge that connects the cluster to a head-tail radio galaxy
located along a filament of galaxies falling into the main cluster, (ii) a 1.9
Mpc radio arc, that is located North West of the main mass component, (iii)
radio emission along the X-ray bar, that traces the gas in the X-rays South
West of the cluster centre. We use deep GMRT observations at 608 MHz to
constrain the spectral indices of these new radio sources, and of the emission
that was already studied in the literature at higher frequency. We find that
the spectrum of the radio halo and of the relic at LOFAR frequency follows the
same power law as observed at higher frequencies. The radio bridge, the radio
arc, and the radio bar all have steep spectra, which can be used to constrain
the particle acceleration mechanisms. We argue that the radio bridge could be
caused by the re-acceleration of electrons by shock waves that are injected
along the filament during the cluster mass assembly. Despite the sensitivity
reached by our observations, the emission from the radio halo does not trace
the emission of the gas revealed by X-ray observations. We argue that this
could be due to the difference in the ratio of kinetic over thermal energy of
the intra-cluster gas, suggested by X-ray observations.
