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.
Bibliographical noteFunding Information:
We thank Shri Kulkarni (Caltech) for obtaining the Keck spectrum. S.S. thanks Tsvi Piran (The Hebrew University, Israel), Nir Sapir, Eli Waxman (Weizmann Institute of Science, Israel), Milena Bufano (Universidad Andrés Bello, Chile), Maryam Modjaz (New York University, USA), and the anonymous referee for many productive and valuable discussions. S.S. acknowledges support by a Grant of Excellence from the Icelandic Research Fund, from the University of Iceland Research Fund, from the Dark Cosmology Centre, where part of this study was performed, and from CONICYT through FONDECYT grant 3140534. We acknowledge support from Basal-CATA PFB-06/2007 (FEB, SS), Iniciativa Cientifica Milenio grant P10-064-F (Millennium Center for Supernova Science), by Project IC120009 “Millennium Institute of Astrophysics (MAS)” of Iniciativa Científica Milenio del Ministerio de Economía, Fomento y Turismo de Chile, with input from “Fondo de Innovación para la Competitividad, del Ministerio de Economía, Fomento y Turismo de Chile” (F.E.B., G.P., J.A.P., S.S.), CONICYT-Chile FONDECYT 1101024 (FEB). J.A.P. acknowledges support by CONICYT through FONDECYT grant 3110142. D.M. acknowledges the Instrument Center for Danish Astrophysics for support. T.K. and H.K. acknowledge support by the European Commission under the Marie Curie Intra-European Fellowship Programme in FP7. J.P.U.F., B.M.J., and D.X. acknowledge support from the ERC-StG grant EGGS-278202. K.L.P. acknowledges financial support by the UK Space Agency for the Swift project. G.L. is supported by the Swedish Research Council through grant No. 623-2011-7117. J.L. acknowledges the UK Science and Technology Facilities Council for research studentship support. The research activity of AdUP, C.T., and J.G. is supported by Spanish research project AYA2012-39362-C02-02. A.d.U.P. acknowledges support by the European Commission under the Marie Curie Career Integration Grant programme (FP7-PEOPLE-2012-CIG 322307). A.J.C.T. acknowledges support from the Spanish research project AYA2009-14000-C03-01 and AYA2012-39727-C03-01 D.A.K. acknowledges support by the DFG cluster of excellence “Origin and Structure of the Universe” and funding by the Thüringer Landessternwarte Tautenburg. A.R. acknowledges support by the Thüringer Landessternwarte Tautenburg. P.S. acknowledges support through the Sofja Kovalevskaja Award from the Alexander von Humboldt Foundation of Germany. A.N.G. and S.K. acknowledge support by DFG KL 766/16-1. S. Schmidl acknowledges support by the Thüringer Ministerium für Bildung, Wissenschaft und Kultur under FKZ 12010-514. The Dark Cosmology Centre is funded by the Danish National Research Foundation. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This research has made use of the GHostS database (www.grbhosts.org), which is partly funded by Spitzer/NASA grant RSA Agreement No. 1287913. Based in part on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of the program 089.A-0067, the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a co-operative agreement with the NSF on behalf of the Gemini partnership, as part of the programs GN-2012A-Q-9, GN-2012A-Q-39, GN-2012B-Q-5, GS-2012A-Q-30, GS-2012A-Q-38, GS-2012A-Q-30, and GN-2012B-Q-5, the Nordic Optical Telescope (NOT), operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias, as part of the program P45-002 (PI: Jakobsson) and ITP10-04 (PI: Kotak, QUB), the Gran Telescopio Canarias (GTC), installed in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma, with Magellan as part of CN2012A-059, with the IRAM Plateau de Bure Interferometer, the James Clerk Maxwell Telescope, as part of the program M12AI12, with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member Statesand NASA. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W.M. Keck Foundation. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK The James Clerk Maxwell Telescope is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the United Kingdom, the National Research Council of Canada, and the Netherlands Organisation for Scientific Research. Additional funds for the construction of SCUBA-2 were provided by the Canada Foundation for Innovation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Support for CARMA construction was derived from the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the James S. McDonnell Foundation, the Associates of the California Institute of Technology, the University of Chicago, the states of California, Illinois, and Maryland, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement, and by the CARMA partner universities. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, and the US Department of Energy Office of Science. The SDSS-III web site is http://www.sdss3.org/ . SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory, Carnegie Mellon University, University of Florida, the French Participation Group, the German Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University, Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia, University of Washington, and Yale University.
- Dust, extinction
- Galaxies: ISM
- Galaxies: individual: GRB 120422A
- Gamma-ray burst: individual: GRB 120422A
- Supernovae: individual: SN 2012bz