Discovery of a radio halo (and relic) in a $M_{500} < 2 \times 10^{14}$
  M$_\odot$ cluster
  Radio halos are diffuse synchrotron sources observed in dynamically unrelaxed
galaxy clusters. Current observations and models suggest that halos trace
turbulent regions in the intra-cluster medium where mildly relativistic
particles are re-accelerated during cluster mergers. Due to the higher
luminosities and detection rates with increasing cluster mass, radio halos have
been mainly observed in massive systems ($M_{500} \gtrsim 5 \times10^{14}$
M$_\odot$). Here, we report the discovery of a radio halo with a largest linear
scale of $\simeq$750 kpc in PSZ2G145.92-12.53 ($z=0.03$) using LOFAR
observations at 120$-$168 MHz. With a mass of $M_{500} = (1.9\pm0.2) \times
10^{14}$ M$_\odot$ and a radio power at 150 MHz of $P_{150} = (3.5 \pm 0.7)
\times 10^{23}$ W/Hz, this is the least powerful radio halo in the least
massive cluster discovered to date. Additionally, we discover a radio relic
with a mildly convex morphology at $\sim$1.7 Mpc from the cluster center. Our
results demonstrate that LOFAR has the potential to detect radio halos even in
low-mass clusters, where the expectation to form them is very low ($\sim$5%)
based on turbulent re-acceleration models. Together with the observation of
large samples of clusters, this opens the possibility to constrain the low end
of the power-mass relation of radio halos.
