A team of astronomers led by Dr Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics, Germany, has demonstrated a link between the bright supernova explosion SN 2011kl and one of the brightest and longest bursts of gamma rays ever observed, GRB 111209A. The results show that SN 2011kl was not driven by radioactive decay, as expected, but was instead powered by the decaying super-strong magnetic fields around a magnetar – a small neutron star spinning hundreds of times per second and possessing a magnetic field much stronger than normal neutron stars.

GRB 111209A was discovered by NASA’s Swift space observatory on December 9, 2011.

As the afterglow from this burst faded it was studied using both the MPG/ESO 2.2-m telescope at ESO’s La Silla Observatory in Chile and the Very Large Telescope (VLT) at Paranal Observatory. The clear signature of SN 2011kl was found.

“Since a long-duration gamma-ray burst is produced only once every 10,000 – 100,000 supernovae, the star that exploded must be somehow special,” said Dr Greiner, lead author of the paper published today in the journal Nature.

“Astronomers had assumed that these GRBs came from very massive stars – about 50 times the mass of the Sun – and that they signaled the formation of a black hole.”

“But now our new observations of the supernova SN 2011kl, found after the GRB 111209A, are changing this paradigm for ultra-long duration GRBs.”

In the favored scenario of a massive star collapse, sometimes known as a collapsar, the week-long burst of optical and infrared emission from the supernova is expected to come from the decay of radioactive Nickel-56 formed in the explosion.

But in the case of GRB 111209A, the combined MPG/ESO and VLT observations showed unambiguously for the first time that this could not be the case.

The only explanation that fitted the observations of SN 2011kl following GRB 111209A was that it was being powered by a magnetar.

“The new results provide good evidence for an unexpected relation between GRBs, very bright supernovae and magnetars,” said study co-author Dr Paolo Mazzali of the Liverpool John Moores University in UK and the Max Planck Institute for Astrophysics in Germany.

“Some of these connections were already suspected on theoretical grounds for some years, but linking everything together is an exciting new development.”

Dr Greiner added: “the case of SN 2011kl and GRB 111209A forces us to consider an alternative to the collapsar scenario. This finding brings us much closer to a new and clearer picture of the workings of GRBs.”

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Greiner J. et al. 2015. A very luminous magnetar-powered supernova associated with an ultra-long gamma-ray burst. Nature 523, 189-192; doi: 10.1038/nature14579