Long γ-ray bursts are associated with energetic, broad-lined, stripped-envelope supernovae1,2 and as such mark the death of massive stars. The scarcity of such events nearby and the brightness of the γ-ray burst afterglow, which dominates the emission in the first few days after the burst, have so far prevented the study of the very early evolution of supernovae associated with γ-ray bursts3. In hydrogen-stripped supernovae that are not associated with γ-ray bursts, an excess of high-velocity (roughly 30,000 kilometres per second) material has been interpreted as a signature of a choked jet, which did not emerge from the progenitor star and instead deposited all of its energy in a thermal cocoon4. Here we report multi-epoch spectroscopic observations of the supernova SN 2017iuk, which is associated with the γ-ray burst GRB 171205A. Our spectra display features at extremely high expansion velocities (around 115,000 kilometres per second) within the first day after the burst5,6. Using spectral synthesis models developed for SN 2017iuk, we show that these features are characterized by chemical abundances that differ from those observed in the ejecta of SN 2017iuk at later times. We further show that the high-velocity features originate from the mildly relativistic hot cocoon that is generated by an ultra-relativistic jet within the γ-ray burst expanding and decelerating into the medium that surrounds the progenitor star7,8. This cocoon rapidly becomes transparent9 and is outshone by the supernova emission, which starts to dominate the emission three days after the burst.
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The optical spectra obtained with GTC/OSIRIS and VLT/X-shooter are available in the GRBspec repository at http://grbspec.iaa.es. The optical spectra obtained with VLT/X-shooter are also available in the WISEREP repository at https://wiserep.weizmann.ac.il/object/7496. The optical data shown in the plots and tables and the Python codes used for the data analysis are available from the corresponding author on reasonable request. The entire photometric dataset is available at https://osf.io/apq3d/. Swift XRT and UVOT data are public (https://heasarc.gsfc.nasa.gov/docs/archive.html). The open-source code TARDIS used for the spectrum synthesis is available at https://tardis.readthedocs.io/en/latest/.
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We acknowledge A. S. Esposito for the rendering of the figures presented in this work. L.I. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2016-30940. L.I., A.d.U.P., C.C.T. and D.A.K. acknowledge support from the Spanish research project AYA2017-89384-P. A.d.U.P. acknowledges support from funding associated with Ramón y Cajal fellowship RyC-2012-09975. C.C.T. acknowledges support from funding associated with Ramón y Cajal fellowship RyC-2012-09984. D.A.K. acknowledges support from funding associated with Juan de la Cierva Incorporacion fellowship IJCI-2015-26153. K.M. acknowledges support from JSPS Kakenhi grants (18H05223, 18H04585 and 17H02864). S. Schmidl acknowledges support from grant DFG Klose 766/16-3 and discussions with S. Klose. R.L.C.S. acknowledges funding from STFC. M.J.M. acknowledges the support of the National Science Centre, Poland, through POLONEZ grant 2015/19/P/ST9/04010; this project has received funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement number 665778. R.S.-R. acknowledges support from ASI (Italian Space Agency) through contract number 2015-046-R.0 and from the European Union’s Horizon 2020 programme under the AHEAD project (grant agreement number 654215). The Cosmic Dawn Center is funded by the DNRF. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). G.L. was supported by a research grant from VILLUM FONDEN (project number 19054). K.E.H. acknowledges support by a Project Grant (162948–051) from The Icelandic Research fund. J.J. and L.K. acknowledge support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy.
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