At low redshift , a handful of gamma-ray bursts ( GRBs ) have been discovered with peak luminosities ( L _ { iso } < 10 ^ { 48.5 } ~ { } erg s ^ { -1 } ) substantially lower than the average of the more distant ones ( L _ { iso } > 10 ^ { 49.5 } ~ { } 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 \sim 10 ^ { 48.9 } ~ { } 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 \sim 270 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 } \sim 60 , low for a high- L GRB , and that the afterglow had an exceptional low peak luminosity-density of \lesssim 2 \times 10 ^ { 30 } ~ { } erg s ^ { -1 } Hz ^ { -1 } in the sub-mm .
Because of the weak afterglow component , we were for the first time able to recover the signature of a shock break-out that was not a genuine low- L GRB .
At 1.4 hours after the burst , the stellar envelope had a blackbody temperature of k _ { B } T \sim 16 ~ { } eV and a radius of \sim 7 \times 10 ^ { 13 } cm .
The accompanying SN 2012bz reached a peak luminosity of M _ { V } = -19.7 mag , 0.3 mag more luminous than SN 1998bw .
The synthesised nickel mass of 0.58 ~ { } M _ { \odot } , ejecta mass of 5.87 ~ { } M _ { \odot } , and kinetic energy of 4.10 \times 10 ^ { 52 } ~ { } erg were among the highest recorded values for GRB-SNe , making it the most luminous spectroscopically confirmed SN to data .
Nebular emission lines at the GRB location were visible , extending from the galaxy nucleus to the explosion site .
The host and the explosion site had close to solar metallicities .
The burst occurred in an isolated star-forming region with a SFR that is 1/10th of that in the galaxy ’ s nucleus .
While the prompt \gamma -ray emission points to a high- L GRB , the weak afterglow and the low \Gamma _ { 0 } 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 L _ { iso } of \sim 10 ^ { 48.9 } ~ { } erg s ^ { -1 } .
Therefore , we conclude that GRB 120422A was a transition object between low- and high- L GRBs , supporting the failed-jet model that connects shock-break-out driven low- L and high- L GRBs that are powered by ultra-relativistic jets .