A surge of light at the birth of a supernova

Abstract

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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Figure 1: Photometry of SN 2016gkg at discovery.
Figure 2: Luminosity versus rise rate for objects with early optical detections.
Figure 3: Hydrodynamical model of the V-band light curve of SN 2016gkg.

References

  1. 1

    Langer, N. Presupernova evolution of massive single and binary stars. Annu. Rev. Astron. Astrophys. 50, 107–164 (2012)

    CAS  Article  ADS  Google Scholar 

  2. 2

    Smartt, S. J. Observational constraints on the progenitors of core-collapse supernovae: the case for missing high-mass stars. Publ. Astron. Soc. Aust. 32, e016 (2015)

    Article  ADS  Google Scholar 

  3. 3

    Falk, S. W. & Arnett, W. D. Radiation dynamics, envelope ejection, and supernova light curves. Astrophys. J. Suppl. Ser. 33, 515–562 (1977)

    CAS  Article  ADS  Google Scholar 

  4. 4

    Ensman, L. & Burrows, A. Shock breakout in SN 1987A. Astrophys. J. 393, 742–755 (1992)

    Article  ADS  Google Scholar 

  5. 5

    Matzner, C. D. & McKee, C. F. The expulsion of stellar envelopes in core-collapse supernovae. Astrophys. J. 510, 379–403 (1999)

    CAS  Article  ADS  Google Scholar 

  6. 6

    Tominaga, N. et al. Shock breakout in type II plateau supernovae: prospects for high-redshift supernova surveys. Astrophys. J. Suppl. Ser. 193, 20 (2011)

    Article  ADS  Google Scholar 

  7. 7

    Morokuma, T. et al. Kiso Supernova Survey (KISS): survey strategy. Publ. Astron. Soc. Jpn 66, 114 (2014)

    CAS  Article  ADS  Google Scholar 

  8. 8

    Förster, F. et al. The High Cadence Transient Survey (HITS). I. Survey design and supernova shock breakout constraints. Astrophys. J. 832, 155 (2016)

    Article  ADS  Google Scholar 

  9. 9

    Tanaka, M. et al. Rapidly rising transients from the Subaru Hyper Suprime-Cam Transient Survey. Astrophys. J. 819, 5 (2016)

    Article  ADS  Google Scholar 

  10. 10

    Otero, S. & Buso, V. Discovery Certificate for Object 2016gkg. TNS Astronomical Transient Report No. 5381, https://wis-tns.weizmann.ac.il/object/2016gkg/discovery-cert (Transient Name Server, 2016)

  11. 11

    Campana, S. et al. The association of GRB 060218 with a supernova and the evolution of the shock wave. Nature 442, 1008–1010 (2006)

    CAS  PubMed  Article  ADS  Google Scholar 

  12. 12

    Modjaz, M. et al. From shock breakout to peak and beyond: extensive panchromatic observations of the type Ib supernova 2008D associated with Swift X-ray transient 080109. Astrophys. J. 702, 226–248 (2009)

    CAS  Article  ADS  Google Scholar 

  13. 13

    Gal-Yam, A. et al. A Wolf-Rayet-like progenitor of SN 2013cu from spectral observations of a stellar wind. Nature 509, 471–474 (2014)

    CAS  PubMed  Article  ADS  Google Scholar 

  14. 14

    Garnavich, P. M. et al. Shock breakout and early light curves of type II-P supernovae observed with Kepler. Astrophys. J. 820, 23 (2016)

    Article  ADS  Google Scholar 

  15. 15

    Yaron, O. et al. Confined dense circumstellar material surrounding a regular type II supernova. Nat. Phys. 13, 510–517 (2017)

    CAS  Article  Google Scholar 

  16. 16

    Tartaglia, L. et al. The progenitor and early evolution of the type IIb SN 2016gkg. Astrophys. J. 836, L12 (2017)

    Article  ADS  Google Scholar 

  17. 17

    Arcavi, I. et al. Constraints on the progenitor of SN 2016gkg from its shock-cooling light curve. Astrophys. J. 837, L2 (2017)

    Article  ADS  Google Scholar 

  18. 18

    Kilpatrick, C. D. et al. On the progenitor of the type IIb supernova 2016gkg. Mon. Not. R. Astron. Soc. 465, 4650–4657 (2017)

    CAS  Article  ADS  Google Scholar 

  19. 19

    Colgate, S. A. & McKee, C. Early supernova luminosity. Astrophys. J. 157, 623 (1969)

    CAS  Article  ADS  Google Scholar 

  20. 20

    Falk, S. W. Shock steepening and prompt thermal emission in supernovae. Astrophys. J. 225, L133–L136 (1978)

    Article  ADS  Google Scholar 

  21. 21

    Klein, R. I. & Chevalier, R. A. X-ray bursts from type II supernovae. Astrophys. J. 223, L109–L112 (1978)

    Article  ADS  Google Scholar 

  22. 22

    Tolstov, A. et al. Multicolor light curve simulations of population III core-collapse supernovae: from shock breakout to 56Co decay. Astrophys. J. 821, 124 (2016)

    Article  ADS  Google Scholar 

  23. 23

    Bersten, M. C. et al. The type IIb supernova 2011dh from a supergiant progenitor. Astrophys. J. 757, 31 (2012)

    Article  ADS  Google Scholar 

  24. 24

    Tolstov, A. G., Blinnikov, S. I. & Nadyozhin, D. K. Coupling of matter and radiation at supernova shock breakout. Mon. Not. R. Astron. Soc. 429, 3181–3199 (2013)

    Article  ADS  Google Scholar 

  25. 25

    Sapir, N., Katz, B. & Waxman, E. Non-relativistic radiation mediated shock breakouts. III. Spectral properties of supernova shock breakout. Astrophys. J. 774, 79 (2013)

    CAS  Article  ADS  Google Scholar 

  26. 26

    Bellm, E. The Zwicky transient facility. In The Third Hot-wiring the Transient Universe Workshop (eds Wozniak, P. R. et al. 27–33 (2014)

  27. 27

    Drout, M. R. et al. Rapidly evolving and luminous transients from Pan-STARRS1. Astrophys. J. 794, 23 (2014)

    Article  ADS  CAS  Google Scholar 

  28. 28

    Šimon, V., Pizzichini, G. & Hudec, R. Evolution of the color indices in SN 2006aj associated with GRB 060218. Astron. Astrophys. 523, A56 (2010)

    Article  ADS  Google Scholar 

  29. 29

    Harris, W. E. A comment on image detection and the definition of limiting magnitude. Publ. Astron. Soc. Pacif. 102, 949–953 (1990)

    Article  ADS  Google Scholar 

  30. 30

    Filippenko, A. V ., Li, W. D ., Treffers, R. R. & Modjaz, M. The Lick Observatory supernova search with the Katzman Automatic Imaging Telescope. ASP Conf. Ser. 246, 121–130 (2001)

    ADS  Google Scholar 

  31. 31

    Ganeshalingam, M. et al. Results of the Lick Observatory supernova search follow-up photometry program: BVRI light curves of 165 type Ia supernovae. Astrophys. J. Suppl. Ser. 190, 418–448 (2010)

    Article  ADS  Google Scholar 

  32. 32

    Shivvers, I. et al. The nearby type Ibn supernova 2015G: signatures of asymmetry and progenitor constraints. Mon. Not. R. Astron. Soc. 471, 4381–4397 (2017)

    CAS  Article  ADS  Google Scholar 

  33. 33

    Oke, J. B. et al. The Keck Low-Resolution Imaging Spectrometer. Publ. Astron. Soc. Pacif. 107, 375–385 (1995)

    Article  ADS  Google Scholar 

  34. 34

    Faber, S. M. et al. The DEIMOS spectrograph for the Keck II telescope: integration and testing. Proc. SPIE 4841, 1657–1669 (2003)

    Article  ADS  Google Scholar 

  35. 35

    Filippenko, A. V. The importance of atmospheric differential refraction in spectrophotometry. Publ. Astron. Soc. Pacif. 94, 715–721 (1982)

    Article  ADS  Google Scholar 

  36. 36

    Barbon, R. et al. SN 1993J in M 81: one year of observations at Asiago. Astron. Astrophys. Suppl. Ser. 110, 513–519 (1995)

    CAS  ADS  Google Scholar 

  37. 37

    Matheson, T. et al. Optical spectroscopy of supernova 1993J during its first 2500 days. Astron. J. 120, 1487–1498 (2000)

    Article  ADS  Google Scholar 

  38. 38

    Ergon, M. et al. Optical and near-infrared observations of SN 2011dh – the first 100 days. Astron. Astrophys. 562, A17 (2014)

    Article  CAS  Google Scholar 

  39. 39

    Yaron, O. & Gal-Yam, A. WISeREP—an interactive supernova data repository. Publ. Astron. Soc. Pacif. 124, 668–681 (2012)

    Article  ADS  Google Scholar 

  40. 40

    Branch, D. et al. Direct analysis of spectra of type Ib supernovae. Astrophys. J. 566, 1005–1017 (2002)

    CAS  Article  ADS  Google Scholar 

  41. 41

    Bersten, M. C., Benvenuto, O. & Hamuy, M. Hydrodynamical models of type II plateau supernovae. Astrophys. J. 729, 61 (2011)

    Article  ADS  Google Scholar 

  42. 42

    Nomoto, K. & Hashimoto, M. Presupernova evolution of massive stars. Phys. Rep. 163, 13–36 (1988)

    CAS  Article  ADS  Google Scholar 

  43. 43

    Drout, M. R. et al. The first systematic study of type Ibc Supernova multi-band light curves. Astrophys. J. 741, 97 (2011)

    Article  ADS  Google Scholar 

  44. 44

    Lyman, J. D. et al. Bolometric light curves and explosion parameters of 38 stripped-envelope core-collapse supernovae. Mon. Not. R. Astron. Soc. 457, 328–350 (2016)

    CAS  Article  ADS  Google Scholar 

  45. 45

    Taddia, F . et al. The Carnegie Supernova Project I: analysis of stripped-envelope supernova light curves. Astron. Astrophys. https://doi.org/10.1051/0004-6361/201730844 (2017)

    Article  ADS  Google Scholar 

  46. 46

    Van Dyk, S. D. et al. The progenitor of supernova 2011dh has vanished. Astrophys. J. 772, L32 (2013)

    Article  ADS  Google Scholar 

  47. 47

    Piro, A. L. et al. Numerically modeling the first peak of the type IIb SN 2016gkg. Astrophys. J. 846, 94 (2017)

    Article  ADS  Google Scholar 

  48. 48

    Khazov, D. et al. Flash spectroscopy: emission lines from the ionized circumstellar material around <10-day-old type II supernovae. Astrophys. J. 818, 3 (2016)

    Article  ADS  Google Scholar 

  49. 49

    Moriya, T. J., Yoon, S.-C., Gräfener, G. & Blinnikov, S. I. Immediate dense circumstellar environment of supernova progenitors caused by wind acceleration: its effect on supernova light curves. Mon. Not. R. Astron. Soc. 469, L108–L112 (2017)

    CAS  Article  ADS  Google Scholar 

  50. 50

    Soderberg, A. M. et al. An extremely luminous X-ray outburst at the birth of a supernova. Nature 453, 469–474 (2008)

    CAS  PubMed  Article  ADS  Google Scholar 

  51. 51

    Mazzali, P. A. et al. The metamorphosis of supernova SN 2008D/XRF 080109: a link between supernovae and GRBs/hypernovae. Science 321, 1185–1188 (2008)

    CAS  PubMed  Article  ADS  Google Scholar 

  52. 52

    Ghisellini, G., Ghirlanda, G. & Tavecchio, F. Did we observe the supernova shock breakout in GRB 060218? Mon. Not. R. Astron. Soc. 382, L77–L81 (2007)

    Article  ADS  Google Scholar 

  53. 53

    Li, L.-X. The X-ray transient 080109 in NGC 2770: an X-ray flash associated with a normal core-collapse supernova. Mon. Not. R. Astron. Soc. 388, 603–610 (2008)

    Article  ADS  Google Scholar 

  54. 54

    Brown, P. J ., Breeveld, A. A ., Holland, S ., Kuin, P. & Pritchard, T. SOUSA: the Swift Optical/Ultraviolet Supernova Archive. Astrophys. Space Sci. 354, 89–96 (2014)

    Article  ADS  Google Scholar 

  55. 55

    Rubin, A. & Gal-Yam, A. Exploring the efficacy and limitations of shock-cooling models: new results for type II supernovae observed by the Kepler mission. Astrophys. J. 848, 8 (2017)

    Article  ADS  CAS  Google Scholar 

  56. 56

    Fruchter, A. S., Hack, W., Dencheva, N., Droettboom, M. & Greenfield, P. BetaDrizzle: a redesign of the MultiDrizzle package. In 2010 Space Telescope Science Institute Calibration Workshop 382–387 (2010)

  57. 57

    Gonzaga, S., Hack, W., Fruchter, A. & Mack, J. (eds) The DrizzlePac Handbook (STScI, 2012)

  58. 58

    Dolphin, A. E. WFPC2 stellar photometry with HSTPHOT. Publ. Astron. Soc. Pacif. 112, 1383–1396 (2000)

    Article  ADS  Google Scholar 

  59. 59

    Kurucz, R. ATLAS9 Stellar Atmosphere Programs and 2 km/s Grid. Kurucz CD-ROM No. 13 (Smithsonian Astrophysical Observatory, 1993)

  60. 60

    Benvenuto, O. G. & De Vito, M. A. A code for stellar binary evolution and its application to the formation of helium white dwarfs. Mon. Not. R. Astron. Soc. 342, 50–60 (2003)

    Article  ADS  Google Scholar 

  61. 61

    Benvenuto, O. G., Bersten, M. C. & Nomoto, K. A binary progenitor for the type IIb supernova 2011dh in M51. Astrophys. J. 762, 74 (2013)

    Article  ADS  CAS  Google Scholar 

  62. 62

    Paczýnski, B. Evolutionary processes in close binary systems. Annu. Rev. Astron. Astrophys. 9, 183 (1971)

    Article  ADS  Google Scholar 

  63. 63

    Phillips, M. M. et al. On the source of the dust extinction in type Ia supernovae and the discovery of anomalously strong Na I absorption. Astrophys. J. 779, 38 (2013)

    Article  ADS  CAS  Google Scholar 

  64. 64

    Koopmann, R. A. & Kenney, J. D. P. An atlas of H and R images and radial profiles of bright isolated spiral galaxies. Astrophys. J. Suppl. Ser. 162, 97–112 (2006)

    CAS  Article  ADS  Google Scholar 

  65. 65

    Schlafly, E. F. & Finkbeiner, D. P. Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD. Astrophys. J. 737, 103 (2011)

    Article  ADS  Google Scholar 

  66. 66

    Kewley, L. J. & Dopita, M. A. Using strong lines to estimate abundances in extragalactic H II regions and starburst galaxies. Astrophys. J. Suppl. Ser. 142, 35–52 (2002)

    CAS  Article  ADS  Google Scholar 

  67. 67

    Pilyugin, L. S. & Grebel, E. K. New calibrations for abundance determinations in H II regions. Mon. Not. R. Astron. Soc. 457, 3678–3692 (2016)

    CAS  Article  ADS  Google Scholar 

  68. 68

    Curti, M. et al. New fully empirical calibrations of strong-line metallicity indicators in starforming galaxies. Mon. Not. R. Astron. Soc. 465, 1384–1400 (2017)

    CAS  Article  ADS  Google Scholar 

  69. 69

    Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009)

    CAS  Article  ADS  Google Scholar 

  70. 70

    Richmond, M. W. et al. UBVRI photometry of SN 1993J in M81: the first 120 days. Astron. J. 107, 1022–1040 (1994)

    Article  ADS  Google Scholar 

  71. 71

    Arcavi, I. et al. SN 2011dh: discovery of a type IIb supernova from a compact progenitor in the nearby galaxy M51. Astrophys. J. 742, L18 (2011)

    Article  ADS  Google Scholar 

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Acknowledgements

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.

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Contributions

M.C.B., hydrodynamical models and interpretation. G.F., supernova and pre-supernova data analysis and interpretation. F.G., supernova data analysis and interpretation. S.V.D.D., supernova and pre-supernova data analysis and interpretation. O.G.B., binary evolution models. M.O., early data comparisons. M.T. and K.M., shock-breakout interpretation. V.B., supernova discovery. J.L.S., early supernova observations. A.V.F., Lick and Keck Observatory data and paper editing. W.Z., T.G.B., T.d.J., I.S., S.K. and N.S., observations and reductions. T.J.M., circumstellar material interpretation. K.N., pre-supernova models. S.B.C. and D.A.P., spectral reductions.

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Correspondence to M. C. Bersten or G. Folatelli.

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Extended data figures and tables

Extended Data Figure 1 Image of SN 2016gkg in NGC 613.

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.

Extended Data Figure 2 Series of discovery images of SN 2016gkg.

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. bl, 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.

Extended Data Figure 4 Hydrodynamical modelling of SN 2016gkg.

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.

Extended Data Figure 5 Modelling of the initial rise of SN 2016gkg.

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.

Extended Data Figure 6 The progenitor candidate and environment of SN 2016gkg.

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.

Extended Data Table 1 Comparison stars in the field of SN 2016gkg
Extended Data Table 2 Discovery imaging description and photometry of SN 2016gkg
Extended Data Table 3 Follow-up BVRI and clear photometry of SN 2016gkg
Extended Data Table 4 H ii region line fluxes

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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

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