GRB 120422A/SN 2012bz: Bridging the gap between low- and high-luminosity gamma-ray bursts

International audienceContext. 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