Magnetars Drive Some Extreme Gamma-ray Bursts

A link between a very long-lasting burst of gamma rays and an unusually bright supernova explosion has been found, with the help of observations from ESO’s La Silla and Paranal Observatories in Chile.

Unexpectedly, the findings demonstrate that the supernova was not powered by radioactive decay, but rather was driven by decaying super-strong magnetic fields around an object known as a magnetar.

Gamma-ray bursts (GRBs) are first picked up by orbiting telescopes sensitive to this type of high-energy radiation (which can’t penetrate the Earth’s atmosphere), and then observed at longer wavelengths by other telescopes both in space and on the ground.

GRBs typically last only a few seconds, but in highly rare cases the gamma rays continue for hours.

One such ultra-long duration GRB was detected up by the Swift satellite on 9 December 2011. Named GRB 111209A, it was both one of the longest and brightest GRBs ever observed.

As the burst faded, it was studied using the GROND instrument on the MPG/ESO 2.2-metre telescope at La Silla and also with the X-shooter instrument on the Very Large Telescope (VLT) at Paranal.

The unmistakable signature of a supernova, later named SN 2011kl, was found. This is the first time that a supernova has been found to be associated with an ultra-long GRB.

Lead author Jochen Greiner, from the Max-Planck-Institut für extraterrestrische Physik explains:

“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. Astronomers had assumed that these GRBs came from very massive stars—about 50 times the mass of the Sun—and that they signalled 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 scenario of a massive star collapse, sometimes called a collapsar, the week-long burst of optical/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 GROND and VLT observations showed clearly that for the first time, this could not be the case. Other suggestions were also ruled out.

The only possible explanation that matched the observations of the supernova following GRB 111209A was that it was being powered by a magnetar.

A magnetar is a tiny neutron star spinning hundreds of times per second and possessing a magnetic field much stronger than normal neutron stars, which are also known as radio pulsars.

Magnetars are believed to be the most strongly magnetised objects in the known Universe. This is the first time that such an unambiguous connection between a supernova and a magnetar has been possible.

Some of these connections were already suspected on theoretical grounds for some years, said co-author Paolo Mazzali, but connecting everything together is an exciting new development.

Jochen Greiner, et al
A very luminous magnetar-powered supernova associated with an ultra-long γ-ray burst
Nature 523, 189–192 (09 July 2015) doi:10.1038/nature14579

Illustration: artist’s impression shows a supernova and associated gamma-ray burst driven by a rapidly spinning neutron star with a very strong magnetic field — an exotic object known as a magnetar. Observations from ESO’s La Silla and Paranal Observatories in Chile have for the first time demonstrated a link between a very long-lasting burst of gamma rays and an unusually bright supernova explosion. The results show that the supernova following the burst GRB 111209A was not driven by radioactive decay, as expected, but was instead powered by the decaying super-strong magnetic fields around a magnetar. Credit: ESO