For decades, optical time-domain searches have been tuned to find ordinary supernovae, which rise and fall in brightness over a period of weeks. Recently, supernova searches have improved their cadences and a handful of fast-evolving luminous transients have been identified1,2,3,4,5. These have peak luminosities comparable to type Ia supernovae, but rise to maximum in less than ten days and fade from view in less than one month. Here we present the most extreme example of this class of object thus far: KSN 2015K, with a rise time of only 2.2 days and a time above half-maximum of only 6.8 days. We show that, unlike type Ia supernovae, the light curve of KSN 2015K was not powered by the decay of radioactive elements. We further argue that it is unlikely that it was powered by continuing energy deposition from a central remnant (a magnetar or black hole). Using numerical radiation hydrodynamical models, we show that the light curve of KSN 2015K is well fitted by a model where the supernova runs into external material presumably expelled in a pre-supernova mass-loss episode. The rapid rise of KSN 2015K therefore probes the venting of photons when a hypersonic shock wave breaks out of a dense extended medium.

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This work is partially supported by NASA K2 cycle 4 grant NNH15ZDA001N and cycle 5 grant NNX17AI64G. We acknowledge support from the Australian Research Council Centre of Excellence for All-sky Astrophysics through project number CE110001020.

Author information


  1. Space Telescope Science Institute, Baltimore, MD, USA

    • A. Rest
    •  & G. Strampelli
  2. Department of Physics, University of Notre Dame, Notre Dame, IN, USA

    • P. M. Garnavich
  3. Department of Astronomy, University of California, Berkeley, CA, USA

    • D. Khatami
    •  & D. Kasen
  4. Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    • D. Khatami
    •  & D. Kasen
  5. The Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Australian National University, Canberra, Australian Capital Territory, Australia

    • B. E. Tucker
  6. Australian Research Council, Centre of Excellence for All-sky Astrophysics, Sydney, New South Wales, Australia

    • B. E. Tucker
  7. Astronomy Department, University of Maryland, College Park, MD, USA

    • E. J. Shaya
    • , R. P. Olling
    •  & R. Mushotzky
  8. Cerro Tololo Inter-American Observatory, La Serena, Chile

    • A. Zenteno
    •  & R. C. Smith
  9. Gemini Observatory, La Serena, Chile

    • S. Margheim
  10. Harvard–Smithsonian Center for Astrophysics, Cambridge, MA, USA

    • D. James
    •  & V. A. Villar
  11. Center for Mathematical Modeling, University of Chile, Santiago, Chile

    • F. Förster


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A.R., P.M.G., B.E.T. and D. Kasen contributed to the scientific analysis. D. Khatami compared the data with theoretical models. E.J.S. discovered the KSN 2015K event and, along with R.P.O. and R.M., reduced the K2 light curve data. A.Z., G.S., D.J. and R.C.S. obtained and reduced the DECam data. S.M. and B.E.T. obtained and reduced the spectra. F.F. and V.A.V. contributed the light-curve fitting. All authors contributed to the scientific text.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to A. Rest.

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