The Planck clusters in the LOFAR sky. I. LoTSS-DR2: new detections and
  sample overview
  Relativistic electrons and magnetic fields permeate the intra-cluster medium
(ICM) and manifest themselves as diffuse sources of synchrotron emission
observable at radio wavelengths, namely radio halos and radio relics. Although
there is broad consensus that the formation of these sources is connected to
turbulence and shocks in the ICM, the details of the required particle
acceleration, the strength and morphology of the magnetic field in the cluster
volume, and the influence of other sources of high-energy particles are poorly
known. Sufficiently large samples of radio halos and relics, which would allow
us to examine the variation among the source population and pinpoint their
commonalities and differences, are still missing. At present, large numbers of
these sources are easiest to detect at low radio frequencies, where they shine
brightly. We examined the low-frequency radio emission from all 309 clusters in
the second catalog of Planck Sunyaev Zel'dovich detected sources that lie
within the 5634 deg$^2$ covered by the Second Data Release of the LOFAR
Two-meter Sky Survey (LoTSS-DR2). We produced LOFAR images at different
resolutions, with and without discrete sources subtracted, and created overlays
with optical and X-ray images before classifying the diffuse sources in the
ICM, guided by a decision tree. Overall, we found 83 clusters that host a radio
halo and 26 that host one or more radio relics (including candidates). About
half of them are new discoveries. The detection rate of clusters hosting a
radio halo and one or more relics in our sample is $30\pm11$% and $10\pm6$%,
respectively. Extrapolating these numbers, we anticipate that once LoTSS covers
the entire northern sky it will provide the detection of $251\pm92$ clusters
hosting a halo and $83\pm50$ clusters hosting at least one relic from Planck
clusters alone.
