It is difficult to establish the properties of massive stars that explode as supernovae1,2. The electromagnetic emission during the first minutes to hours after the emergence of the shock from the stellar surface conveys important information about the final evolution and structure of the exploding star3,4,5,6. However, the unpredictable nature of supernova events hinders the detection of this brief initial phase7,8,9. Here we report the serendipitous discovery of a newly born, normal type IIb supernova (SN 2016gkg)10, which reveals a rapid brightening at optical wavelengths of about 40 magnitudes per day. The very frequent sampling of the observations allowed us to study in detail the outermost structure of the progenitor of the supernova and the physics of the emergence of the shock. We develop hydrodynamical models of the explosion that naturally account for the complete evolution of the supernova over distinct phases regulated by different physical processes. This result suggests that it is appropriate to decouple the treatment of the shock propagation from the unknown mechanism that triggers the explosion.
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We are grateful to P. Brown for providing information about the photometry of the early Swift/UVOT data of SN 2006aj. M.C.B. acknowledges support from the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) through grant PICT-2015-3083 ‘Progenitores de Supernovas de Colapso Gravitatorio’ and from the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence ‘Origin and Structure of the Universe’. M.C.B., G.F. and O.G.B. acknowledge support from grant PIP-2015-2017-11220150100746CO of CONICET ‘Estrellas Binarias y Supernovas’. G.F. further acknowledges support from ANPCyT grant PICT-2015-2734 ‘Nacimiento y Muerte de Estrellas Masivas: Su relación con el Medio Interestelar’. K.M. acknowledges support from JSPS KAKENHI grant 17H02864. Partial support for this work was provided by NASA through programmes GO-14115 and AR-14295 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. M.O. acknowledges support from grant PI UNRN40B531. A.V.F. is also grateful for financial assistance from the Christopher R. Redlich Fund, the TABASGO Foundation and the Miller Institute for Basic Research in Science (University of California Berkeley). We thank the University of California Berkeley undergraduate students S. Channa, G. Halevy, A. Halle, M. de Kouchkovsky, J. Molloy, T. Ross, S. Stegman and S. Yunus for their effort in collecting Lick/Nickel data, and T.d.J. for help with some of the Keck observations. The Lick and Keck Observatory staff provided excellent assistance. A major upgrade of the Kast spectrograph on the Shane 3-m telescope at Lick Observatory was made possible through gifts from William and Marina Kast as well as the Heising-Simons Foundation. Research at Lick Observatory is partially supported by a gift from Google. KAIT and its on-going operation were made possible by donations from Sun Microsystems, Inc., the Hewlett-Packard Company, AutoScope Corporation, Lick Observatory, the NSF, the University of California, the Sylvia and Jim Katzman Foundation and the TABASGO Foundation. Some of the data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among California Institute of Technology, the University of California and NASA; the observatory was made possible by financial support from the W. M. Keck Foundation. O.G.B. is a member of the Carrera del Investigador Científico de la Comisión de Investigaciones Científicas de la Provincia de Buenos Aires (CIC), Argentina.
The authors declare no competing financial interests.
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Extended data figures and tables
The image is a combination of the final 21-image series obtained at discovery. We show only the relevant region, containing the supernova (red circle), its host and the comparison stars for photometry (indicated with numbers on the upper left of each star; see Extended Data Table 1). Image obtained by V.B.
The supernova location is indicated in all panels with a white circle. North is up and east is to the left. The bar in a indicates a scale of 20″. a, A combination of 40 exposures obtained before the detection of the supernova. b–l, Sequence of images obtained during the initial rise as combinations of five or six individual exposures. Labels on the lower left of each panel indicate the mean ut time of the images. Photometry from the latter set of images is shown with blue diamonds in Fig. 1. Images obtained by V.B.
Extended Data Figure 3 Follow-up observations of SN 2016gkg compared with those of other type IIb supernovae.
a, BVRI light curves for SN 2016gkg (symbols) obtained with KAIT and the Nickel telescope. V-band data from V.B. and J.L.S. converted from the clear band and data from Atlas, ASAS, Swift and LCOGT17 are also included. Open symbols are unfiltered data from KAIT, transformed to the R band. Data of type IIb supernovae SN 1993J (dashed lines)70 and SN 2011dh (solid lines)34,38,71 are included for comparison. MJD, modified Julian date. b, Optical spectra of SN 2016gkg (black) compared with data of the type IIb supernovae SN 1993J (blue)36,37 and SN 2011dh (red)38 at similar epochs.
a, b, Model (lines) bolometric light curve (a) and photospheric velocity evolution (b) compared with observations (points) during the 56Ni-dominated phase. No attempt was made to reproduce the initial light-curve decline (before day 4). c, d, Absolute V-band light-curve models (lines) compared with observations (points) during the SBO and post-shock cooling phases, for different progenitor radii (R⊙; c) and explosion energies (foe; d). Error bars are 1σ and are shown only when they are larger than the data points.
Absolute V-band magnitude of our preferred model (solid line), a similar model including some CSM (dashed line) and a model with approximately four times larger explosion energy (dotted line), compared with the early-time observations (points). The CSM is not necessarily in hydrostatic and thermal equilibrium. The presence of the CSM material reduces the slope during the SBO phase, making it even more compatible with the observations, without affecting the evolution at times later than about 1 day. Even assuming an extreme explosion energy, the resulting cooling-peak slope is substantially smaller than that during the SBO. Error bars are 1σ and are shown only when they are larger than the data points.
a, The HST WFC3/UVIS F555W image mosaic from 2016 October 10. b, A portion of the HST WFPC2 F606W image mosaic from 2001 August 21. The candidate position of the progenitor is indicated by tick marks. c, Stellar-atmosphere SED fit (line) to the candidate HST photometry (red squares). An H ii region of which we obtained a Keck DEIMOS spectrum is seen about 8.6″ north of the progenitor. Error bars are 1σ. d, Evolutionary tracks on the HRD of our progenitor binary model (primary star in black; secondary star in cyan, magenta and green for different accretion efficiencies). Large circles indicate the end points of both stars, with final masses labelled, the red square shows the progenitor candidate location and the blue line is the zero-age main sequence with masses indicated. e, Spectrum of a bright H ii region seen in b, 8.6″ north of SN 2016gkg.
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Bersten, M., Folatelli, G., García, F. et al. A surge of light at the birth of a supernova. Nature 554, 497–499 (2018). https://doi.org/10.1038/nature25151
Nature Astronomy (2019)
Nature Astronomy (2018)