New Type of Supernova
© Caltech/Robert Quimby and Nature
Before and after images of four hydrogen-poor superluminous supernovae discovered by the Palomar Transient Factory. In each case, the supernovae shine far brighter than all other stars combined in their host galaxies.

A new type of supernova that shines up to 10 times brighter than any previously recorded has been discovered by an international team of astronomers. However, the team has yet to explain the exact mechanism that drives this new type of exploding star, with existing models failing to reproduce the radiation emanating from this new class of violent events.

Supernovae - highly energetic events caused by the explosion of a star - can often shine brighter than an entire galaxy for a brief period of time. To date, three mechanisms have been used to explain the vast amount of associated radiation observed by astronomers during these events. However, a team led by Robert Quimby at the California Institute of Technology in the US has identified a batch of six supernovae with radiation properties that cannot be explained by any of the three mechanisms.

The first cause discounted by Quimby was radioactive decay. During the highly energetic explosion of a supernova the temperature skyrockets. This allows heavy elements, including 56Ni, to be synthesized. Their subsequent radioactive decay produces gamma-rays that slow down the rate at which the supernova fades away. Crucially, the explosions observed by Quimby were too short-lived. "These supernovae faded about three times as quickly as those driven by radioactive decay," he explains.

Glowing hydrogen

A second possibility is that surrounding hydrogen-rich material is heated by the energy of the explosion, causing it to radiate light. This hydrogen could have been blown off the stars at an earlier time by stellar winds. However, Quimby could not find any evidence of hydrogen. "No traces were found when we analysed the spectral lines of these supernovae. This meant we were able to rule out an interaction with hydrogen-rich circumstellar material," he says.

The elimination of hydrogen also discounted the third conventional mechanism. In this scenario the hydrogen in the atmosphere of the star is ionized as the explosion tears through it. This fog of ionized hydrogen is opaque to radiation. Over time the hydrogen recombines, the fog clears and the radiation streams outwards. But again, as no hydrogen was observed, this cannot easily explain Quimby's pool of six supernovae.

Instead, this latest research puts forward two alternatives that could explain the sextet. The first is a similar process to the heating of hydrogen-rich material surrounding the star. "Some very massive stars, around 100 times more massive that the Sun, could throw off shells of carbon and oxygen instead," Quimby explains. "If a supernova explodes within a shells, it would heat the shell up." As the shells expand and cool, the supernova gradually fades away.

Rotating neutrons

Quimby's second suggestion invokes magnetars. When a massive star dies in a supernova, it can leave behind a superdense, rapidly rotating bundle of neutrons - a neutron star. If this neutron star is highly magnetized, then it is called a magnetar. The interaction of the intense magnetic field with the surrounding ionized material could be behind the mystery supernovae. "The interaction acts as a brake, slowing down the spinning of the magnetar - a process that releases some of its rotational energy into the supernova ejecta," Quimby says. "This could supply an additional source of energy that would make it brighter than a normal supernova."

However, Quimby does not believe he has everything wrapped up just yet. "These ideas are brand new; they didn't exist 10 years ago. We definitely need to do more work to figure this out," he says. Rubina Kotak, a supernova expert at Queen's University, Belfast, who was not involved in the research, also believes it is tricky. "It is really difficult to say what is powering these explosions as we've only seen a handful of them and we don't have complete observations over the whole event," she told "We are all waiting for the next one, which hopefully we can catch early enough to monitor all aspects of it."

Meanwhile, Quimby is using the Hubble Space Telescope (HST) to probe the known supernovae further. "I am using the HST to look at their ultraviolet spectra," he explains. "Hopefully, we can get a better idea of what materials are in the ejecta and place better constraints on how the events evolve over time. This could allow us to work out which of our models is applicable."

The research is published in Nature 10.1038/nature10095.