Unusual supernova is 'missing link' in gamma-ray burst mystery

Scientists have found that supernovae with lighter particles and strong polar ejections create bursts of gamma rays.
By Kathy Fey | Apr 28, 2015
Astrophysicists have long wondered why some supernovae emit bursts of gamma rays while others do not. Recent research has finally answered that question by showing how an unusual supernova first spotted in 2012 could emit no gamma rays when its characteristics should have led to gamma-ray bursts (GRBs) based on former predictions.

According to Astronomy Magazine, the team used the National Science Foundation's Very Large Array (VLA) to find the "missing link" in the gamma-ray burst mystery.

"This is a striking result that provides a key insight about the mechanism underlying these explosions," Sayan Chakraborti of the Harvard-Smithsonian Center for Astrophysics said. "This object fills in a gap between GRBs and other supernovae of this type, showing us that a wide range of activity is possible in such blasts."

The "missing link" space object is called Supernova 2012ap and falls into the category of core-collapse supernovae. This type of supernova results when a massive star collapses in on itself when its core can no longer maintain the energy needed to support the star's weight. The result of the collapse is an extremely dense neutron star or a black hole. The remaining star material is ejected into space via a supernova explosion.

A core-collapse supernova typically creates a bubble-shaped blast that radiates in all directions at far less than the speed of light, and produces no GRBs. Rarely, a supernova of this type will blast the collapsed material outward from a swiftly rotating disk of material at its core, resulting in the material being ejected from the supernova's poles in powerful jets approaching the speed of light and creating GRBs.

This type of supernova which has a swirling disk of material and jets is termed an "engine-driven" supernova, and until this recent research would have been assumed to produce GRBs. Supernova 2012ap confirmed that engine-driven supernovae can break the mold. "This supernova had jets moving at nearly the speed of light, and those jets were quickly slowed down, just like the jets we see in gamma-ray bursts," team member Alicia Soderberg said.

The team concluded that supernovae can produce jets without also producing GRBs if the material in the jets is made of heavier particles. It is not the mere presence of jets that creates GRBs, but rather the nature of the jets.

"This object shows that the nature of the engine plays a central role in determining the characteristics of this type of supernova explosion," Soderberg said.

"What we see is that there is a wide diversity in the engines in this type of supernova explosion. Those with strong engines and lighter particles produce gamma-ray bursts, and those with weaker engines and heavier particles don't," Chakraborti said.

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