The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration γ-ray bursts (GRBs)1,2,3,4. Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft γ-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations5,6,7 and searches8. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c. Independently, a second mildly relativistic supernova has been reported9. Contrary to the radio data, optical observations10,11,12,13 of SN 2007gr indicate a typical type Ic supernova with ejecta velocities ∼6,000 km s-1, much lower than in GRB-associated supernovae. We conclude that in SN 2007gr a small fraction of the ejecta produced a low-energy mildly relativistic bipolar radio jet, while the bulk of the ejecta were slower and, as shown by optical spectropolarimetry14, mildly aspherical.
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Z.P. acknowledges support from the Hungarian Scientific Research Fund (OTKA, grant K72515). We are grateful to A. Soderberg for providing the VLA coordinates of SN 2007gr in August 2007, which helped us to confirm our initial e-VLBI detection and thus enabled our follow-up VLBI observations. e-VLBI development in Europe is supported by the EC DG-INFSO funded Communication Network Developments project 'EXPReS', contract no. 02662. The EVN is a joint facility of European, Chinese, South African and other radio astronomy institutes funded by their national research councils. The WSRT is operated by ASTRON (Netherlands Institute for Radio Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). ParselTounge was developed in the context of the ALBIUS project, funded by the European Community’s sixth Framework Programme under RadioNet R113CT 2003 5058187. J.G. acknowledges a Royal Society Wolfson Research Merit Award. A.J.v.d.H. was supported by the NASA Postdoctoral Program at the MSFC, administered by Oak Ridge Associated Universities through a contract with NASA.
Author Contributions Z.P., G.B.T. and M.B. were responsible for data analysis, C.K. for project initiation and manuscript writing, E.R.-R. and J.G. for theoretical interpretation. All other authors have significantly contributed to the data analysis and interpretation.
The authors declare no competing financial interests.
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Paragi, Z., Taylor, G., Kouveliotou, C. et al. A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr. Nature 463, 516–518 (2010). https://doi.org/10.1038/nature08713
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