Gamma-ray bursts (GRBs) are extremely energetic and quick releases of powerful radiation caused by cataclysmic events. Some are associated with hypernovae, the explosive end of huge stars, but not all hypernovae produce gamma-ray bursts (GRBs). Researchers now know why.

Researchers led by a team at the Institute of Astrophysics of Andalusia have worked out how GRBs are produced by hypernovae. A hypernova happens when a star over 25 times the mass of the Sun collapses on itself after it has run out of fuel. The core of the star becomes either a neutron star or a black hole due to gravity and two jets of material are released. Around the jets, there’s a cocoon of material that expands and it’s this interplay that determines if a gamma-ray burst is produced.

As the jet moves forward it loses energy and sometimes it is stopped before it can break free from the stellar material (the so-called envelope), which is ejected in the explosion. If the jet of material reaches the surface of the envelope the GRB will be emitted, but if it’s choked we will “only” see the hypernova. The observations of GRB 171205A and its associated hypernova led the team to these conclusions. The findings are reported in Nature.

“This work has allowed us to find the missing link between these two types of hypernova through the detection of an additional component: A sort of hot cocoon generated around the jet, as it propagates through the outer layers of the progenitor star,” lead author Dr Luca Izzo said in a statement. “The jet transfers a significant part of its energy to the cocoon and, if it manages to reach the surface of the star, will produce the gamma-ray emission that we know as a GRB.”   

In the specific case of GRB 171205A, researchers were lucky enough to catch the hypernova very early on and as it has a relatively weak jet, they were able to perform many detailed observations at the onset of the GRB.

“Such events occur on average every ten years, so we immediately started an intense observing campaign to observe the emerging hypernova from the very early phases on," added co-author Dr Christina Thöne. “In fact, with our early observations we managed to obtain the earliest detection of a hypernova to date, less than one day after the collapse of the star.”

The discovery suggests that models of hypernovae (and maybe even supernovae) should be revised to include the cocoon component and take into account how the jet interacts with the stellar material.