How Bright Can Supernovae Get?

Supernovae — enormous explosions associated with the end of a star’s life — come in a variety of types with different origins. A new study has examined how the brightest supernovae in the Universe are produced, and what limits might be set on their brightness.

Ultra-Luminous Observations

Recent observations have revealed many ultra-luminous supernovae, which have energies that challenge our abilities to explain them using current supernova models. An especially extreme example is the 2015 discovery of the supernova ASASSN-15lh, which shone with a peak luminosity of ~2*1045 erg/s, nearly a trillion times brighter than the Sun. ASASSN-15lh radiated a whopping ~2*1052 erg in the first four months after its detection.

How could a supernova that bright be produced? To explore the answer to that question, Tuguldur Sukhbold and Stan Woosley at University of California, Santa Cruz, have examined the different sources that could produce supernovae and calculated upper limits on the potential luminosities of each of these supernova varieties.

Explosive Models

Sukhbold and Woosley explore multiple different models for core-collapse supernova explosions, including:

  1. Prompt explosion
    A star’s core collapses and immediately explodes.
  2. Pair instability
    Electron/positron pair production at a massive star’s center leads to core collapse. For high masses, radioactivity can contribute to delayed energy output.
  3. Colliding shells
    Previously expelled shells of material around a star collide after the initial explosion, providing additional energy release.
  4. Magnetar
    The collapsing star forms a magnetar — a rapidly rotating neutron star with an incredibly strong magnetic field — at its core, which then dumps energy into the supernova ejecta, further brightening the explosion.

They then apply these models to different types of stars.

Setting the Limit

ASASSN-15lh

The authors show that the light curve of ASASSN-15lh (plotted in orange) can be described by a model (black curve) in which a magnetar with an initial spin period of 0.7 ms and a magnetic field of 2*1013 Gauss deposits energy into ~12 solar masses of ejecta. Click for a closer look! [Adapted from Sukhbold&Woosley 2016]

The authors find that the maximum luminosity that can be produced by these different supernova models ranges between 5*1043 and 2*1046 erg/s, with total radiated energies of 3*1050 to 4*1052 erg. This places the upper limit on the brightness of a supernova at about 5 trillion times the luminosity of the Sun.

The calculations performed by Sukhbold and Woosley confirm that, of the options they explore, the least luminous events are produced by prompt explosions. The brightest events possible are powered by the rotational energy of a newly born magnetar at the heart of the explosion.

The energies of observed ultra-luminous supernovae are (just barely) contained within the bounds of the mechanisms explored here. This is even true of the extreme ASASSN-15lh — which, based on the authors’ calculations, was almost certainly powered by an embedded magnetar. If we were to observe a supernova more than twice as bright as ASASSN-15lh, however, it would be nearly impossible to explain with current models.

Citation

Tuguldur Sukhbold and S. E. Woosley 2016 ApJ 820 L38. doi:10.3847/2041-8205/820/2/L38

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