Violent stellar explosion provides new knowledge about the universe

It is not a discreet event when giant stars explode in deep space. Some of the most powerful of these explosions are called hypernovae.

They produce extremely powerful energy bursts at enormous velocities, and sometimes trigger gamma radiation flashes in the form of a jet. There is a good theoretical basis suggesting that this jet can form a ‘cocoon’ within the star. And now, such a cocoon has been identified for the first time by observing a hypernova called SN 2017iuk. The new findings have just been published in the scientific journal Nature.

“We had a short window for observing this cocoon, as their signature only remains visible for a few days after the event has taken place,” explains Giorgos Leloudas, Senior Researcher at DTU Space and co-author of the Nature article.

“However, we were well prepared, and we have been observing it intensely since we saw the first gamma-rays or gamma-ray photons from the hypernova in 2017 and subsequently were able to locate and examine it via terrestrial telescopes.”

500 million light years from Earth

The hypernova is located 500 million light years from Earth. This may seem far away, but it is actually the third-closest gamma-ray burst observed in 20 years.

“Our observations will improve our understanding of the chemical processes taking place in connection with these stellar explosions, and how their material is dissipated. This offers us an overall better picture of the internal structure of these enormous stars, which in turn brings us new insights into their birth and demise,” says Giorgos Leloudas.

It all ends with a rapidly rotating black hole

Hypernovae have a mass about 25 times larger than that of the Sun and constitute a special type of energetic supernova, i.e. collapsing star. When they explode, vast amounts of energy are emitted in the form of gamma radiation – long gamma-ray bursts (GRBs) – and material moving at about 99 per cent of the speed of light.

“However, material is also ejected to the sides inside the star during the event, in the form of e.g. calcium and silicon, also at extremely high energy levels and moving at about one-third of the speed of light. During this process, a cocoon-shaped structure is formed inside the star, and this has now been demonstrated through observations,” explains Luca Izzo from the Spanish Instituto de Astrofísica de Andalucía (IAA-CSIC), senior researcher and primary author of the Nature article.

“We also believe that the cocoon can only arise because this type of explosion creates a black hole rotating at an extreme velocity.”

The star has multiple layers, and while the core turns into a black hole, the remaining material is ejected into space. This interstellar material in the form of e.g. calcium, silicon and gases again forms part in the formation of new stars.

Discovered by satellite and imaged from Earth

The hypernova was discovered in December 2017. Prior to this, in December that year, the Swift satellite recorded a gamma-ray burst, designated GRB 171205A, and it was assumed that this must originate from a hypernova.

The hypernova could then be localized through observations from Earth using the Very Large Telescope (VLT) at the European Observatory in Chile and the Spanish Gran Telescopio CANARIAS. Subsequently it was named SN 2017iuk.

(Morten Garly Andersen)