GRB 120422A/SN 2012bz: Bridging the gap between low- and high-luminosity gamma-ray bursts
Abstract
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.
Origin : Publication funded by an institution
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