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

¹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
e-mail: sschulze@astro.puc.cl
² Millennium Center for Supernova Science  
³ Centre for Astrophysics and Cosmology, Science and, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁵ Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
⁶ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁷ Instituto de Astrofísica de Andalucía Consejo Superior de Investigaciones Científicas (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁸ The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova University Centre, 10691 Stockholm, Sweden
⁹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park Liverpool L3 5RF, UK
¹⁰ Department of Astronomy, California and of Technology, MC 249-17, 1200 East California Blvd, Pasadena CA 91125, USA
¹¹ Hubble Fellow  
¹² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
¹³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt MD 20771, USA
¹⁵ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹⁶ Department of Particle Physics and Astrophysics, Faculty of Physics, Weizmann Institute of Science, 76100 Rehovot, Israel
¹⁷ Astronomical and Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹⁸ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁹ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
²⁰ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 19001, Santiago 19, Chile
²¹ Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile
²² Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia
²³ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova University Centre, 10691 Stockholm, Sweden
²⁴ Department of Physics and Astronomy, Rice University, 6100 South Main MS-108, Houston, TX 77005-1892, USA
²⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
²⁶ Department of Physics, Harvard University, 17 Oxford Street, Cambridge, MA 02138, USA
²⁷ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46 23807 Merate, Italy
²⁸ Institute de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
²⁹ ASI-Science Data Center, via del Politecnico, 00133 Roma, Italy
³⁰ National Radio Astronomy Observatory, PO Box O, Socorro NM 87801, USA
³¹ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
³² APC, Astroparticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
³³ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³⁴ Università degli studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
³⁵ Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
³⁶ Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU, WPI), University of Tokyo, 277-8583 Chiba, Japan
³⁷ Aryabhatta Research and of observational-sciences, Manora peak, Nainital 263 129, India
³⁸ James Clerk Maxwell Telescope, Joint Astronomy Centre, 660 North A’ohoku Place, University Park, Hilo, HI 96720, USA
³⁹ Netherlands Organization for Scientific Research, Laan van Nieuw Oost-Indie 300, 2509 AC The Hague, The Netherlands
⁴⁰ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
⁴¹ Department of Astronomy & Astrophysics, The University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
⁴² Kavli Institute for Cosmological Physics at the University of Chicago, USA
⁴³ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁴⁴ Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País Vasco UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
⁴⁵ Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
⁴⁶ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy

Received: 8 January 2014
Accepted: 7 March 2014

Abstract

Context. At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with luminosities that are substantially lower (L_iso ≲ 10^48.5 erg s^-1) than the average of more distant ones (L_iso ≳ 10^49.5 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 L_iso ~ 10^49.6−49.9 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 k_BT ~ 16 eV and a radius of ~7 × 10¹³ cm (both in the observer frame). The accompanying SN 2012bz reached a peak luminosity of M_V = −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 L_iso of ~10^49.6−49.9 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.