Context. At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with luminosities that are substantially
lower (Liso ≲ 1048.5 erg s-1) than the average of more distant ones (Liso ≳ 1049.5 erg s-1). It has been suggested that the
properties of several low-luminosity (low-L) GRBs are due to shock break-out, as opposed to the emission from ultrarelativistic
jets. This has led to much debate about how the populations are connected.
Aims. The burst at redshift z = 0.283
from 2012 April 22 is one of the very few examples of intermediate-L GRBs with a γ-ray luminosity of Liso ~ 1049.6−49.9 erg
s-1 that have been detected up to now. 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.
Methods. We carried
out a spectroscopy campaign using medium- and low-resolution spectrographs with 6-10-m class telescopes, which covered a time
span of 37.3 days, and a multi-wavelength imaging campaign, which ranged from radio to X-ray energies over a duration of ~270
days. Furthermore, we used a tuneable filter that is centred at Hα to map star-formation in the host and the surrounding galaxies.
We used these data to extract and model the properties of different radiation components and fitted the spectral energy distribution
to extract the properties of the host galaxy.
Results. Modelling the light curve and spectral energy distribution
from the radio to the X-rays revealed that the blast wave expanded with an initial Lorentz factor of Γ0 ~ 50, which is a low
value in comparison to high-L GRBs, and that the afterglow had an exceptionally low peak luminosity density of ≲2 × 1030 erg
s-1 Hz-1 in the sub-mm. Because of the weak afterglow component, we were able to recover the signature of a shock break-out
in an event that was not a genuine low-L GRB for the first time. At 1.4 hr after the burst, the stellar envelope had a blackbody
temperature of kBT ~ 16 eV and a radius of ~7 × 1013 cm (both in the observer frame). The accompanying SN 2012bz reached a
peak luminosity of MV = −19.7 mag, which is 0.3 mag more luminous than SN 1998bw. The synthesised nickel mass of 0.58 M⊙,
ejecta mass of 5.87 M⊙, and kinetic energy of 4.10 × 1052 erg were among the highest for GRB-SNe, which makes it the most
luminous spectroscopically confirmed SN to date. Nebular emission lines at the GRB location were visible, which extend from
the galaxy nucleus to the explosion site. The host and the explosion site had close-to-solar metallicity. The burst occurred
in an isolated star-forming region with an SFR that is 1/10 of that in the galaxy’s nucleus.
Conclusions. While the
prompt γ-ray emission points to a high-L GRB, the weak afterglow and the low Γ0 were very atypical for such a burst. Moreover,
the detection of the shock break-out signature is a new quality for high-L GRBs. So far, shock break-outs were exclusively
detected for low-L GRBs, while GRB 120422A had an intermediate Liso of ~1049.6−49.9 erg s-1. Therefore, we conclude that GRB
120422A was a transition object between low- and high-L GRBs, which supports the failed-jet model that connects low-L GRBs
that are driven by shock break-outs and high-L GRBs that are powered by ultra-relativistic jets.