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

Schulze, S and Malesani, D and Cucchiara, A and Tanvir, NR and Kruhler, T and de Ugarte Postigo, A and Leloudas, G and Lyman, J and Bersier, D and Wiersema, K and Perley, DA and Schady, P and Gorosabel, J and Anderson, JP and Castro-Tirado, AJ and Cenko, SB and De Cia, A and Ellerbroek, LE and Fynbo, JPU and Greiner, J and Hjorth, J and Kann, DA and Kaper, L and Klose, S and Levan, AJ and Martin, S and O'Brien, PT and Page, KL and Pignata, G and Rapaport, S and Sanchez-Ramirez, R and Sollerman, J and Smith, IA and Sparre, M and Thoene, CC and Watson, DJ and Xu, D and Bauer, EE and Bayliss, M and Bjornsson, G and Bremer, M and Cano, Z and Covino, S and D'Elia, V and Frail, DA and Geier, S and Goldoni, P and Hartoog, OE and Jakobsson, P and Korhonen, H and Lee, KY and Milvang-Jensen, B and Nardini, M and Guelbenzu, AN and Oguri, M and Pandey, SB and Petitpas, G and Rossi, A and Sandberg, A and Schmidl, S and Tagliaferri, G and Tilanus, RPJ and Winters, JM and Wright, D and Wuyts, E (2014) GRB 120422A/SN 2012bz: Bridging the gap between low- and high-luminosity gamma-ray bursts. Astronomy and Astrophysics, 566. ISSN 0004-6361

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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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Q Science > QD Chemistry
Divisions: Astrophysics Research Institute
Publisher: EDP Sciences
Related URLs:
Date Deposited: 09 Feb 2017 10:13
Last Modified: 08 Sep 2017 02:29
DOI or Identification number: 10.1051/0004-6361/201423387
URI: http://researchonline.ljmu.ac.uk/id/eprint/5480

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