GRB 120422A/SN 2012bz: Bridging the Gap between Low- And High-Luminosity
  GRBs
  At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered
with peak luminosities ($L_{\rm iso} < 10^{48.5}~\rm{erg\,s}^{-1}$)
substantially lower than the average of the more distant ones ($L_{\rm iso} >
10^{49.5}~\rm{erg\,s}^{-1}$). The properties of several low-luminosity
(low-$L$) GRBs indicate that they can be due to shock break-out, as opposed to
the emission from ultrarelativistic jets. Owing to this, it is highly debated
how both populations are connected, and whether there is a continuum between
them. The burst at redshift $z=0.283$ from 2012 April 22 is one of the very few
examples of intermediate-$L$ GRBs with a $\gamma$-ray luminosity of
$L\sim10^{48.9}~\rm{erg\,s}^{-1}$ that have been detected up to now. Together
with the robust detection of its accompanying supernova SN 2012bz, it has the
potential to answer important questions on the origin of low- and high-$L$ GRBs
and the GRB-SN connection. We carried out a spectroscopy campaign using medium-
and low-resolution spectrographs at 6--10-m class telescopes, covering the time
span of 37.3 days, and a multi-wavelength imaging campaign from radio to X-ray
energies over a duration of $\sim270$ days. Furthermore, we used a tuneable
filter centred at H$\alpha$ to map star formation in the host galaxy and the
surrounding galaxies. We used these data to extract and model the properties of
different radiation components and incorporate spectral-energy-distribution
fitting techniques to extract the properties of the host galaxy. Modelling the
light curve and spectral energy distribution from the radio to the X-rays
revealed the blast-wave to expand with an initial Lorentz factor of
$\Gamma_0\sim60$, low for a high-$L$ GRB, and that the afterglow had an
exceptional low peak luminosity-density of
$\lesssim2\times10^{30}~\rm{erg\,s}^{-1}\,\rm{Hz}^{-1}$ in the sub-mm.
[Abridged]
