The Missing Link Between Gamma-Ray Bursts and Hypernovae

  • Astronomers found a new component that reveals interesting insights into the death of massive stars. 
  • It’s a sort of hot cocoon accompanying jets of matter at the surface of the progenitor star. 

The death of a star depends on what type of star it is. Low mass stars like our Sun die in a tranquil manner, whereas massive stars suffer extreme explosive events. In some cases, these explosions can outshine the brightness of entire host galaxy.

Although these processes have been going on for billions of years in the Universe, astronomers weren’t able to detect a superluminous supernova (also known as Hypernova) followed by a gamma-ray burst until 1998. This discovery made it clear that there is a connection between the supernova and gamma-ray burst, but how these two phenomena are related to each other is still a mystery.

Recently, an international team of astronomers detected a new component in such events and unveiled interesting insights into the death of massive stars that generate gamma-ray burst and hypernovae.

What’s The Additional Component?

In this study, researchers observed a star that is 25 times more massive than our Sun. Such a massive star collapses when there is no nuclear fuel left (hydrogen and helium) and the outer layer explodes as a supernova.

What’s left over after a supernova explosion is an extremely dense core of a star which turns either into a neutron star or a black hole (if there is a sufficient mass), and two polar jets of matter are emitted at the same moment.

The jets are powerful enough to penetrate the star’s external layer, and once they get out of the star, they generate gamma-rays that can be detected from Earth. Then external layers are ejected, producing an extremely bright hypernova explosion.

So far, astronomers have detected numerous hypernovae that don’t have associated gamma-ray bursts. And the reason behind this occurrence hasn’t been explained until now.

The new study suggests a presence of an additional component: A kind of hot cocoon produced around jets, as they propagate through the star’s outer layers. These jets transfer a substantial amount of its energy to the cocoon.

An artistic impression of hypernova | Credit: Anna Serena Esposito

If jets pierce the star’s external layers, they generate the gamma-ray emission. Whereas, in some cases, they fail to reach the surface of the star and never get into the circumstellar medium due to lack of sufficient energy. In such cases, astronomers only observe a hypernova (without any gamma-ray bursts).

Reference: Nature | doi:10.1038/s41586-018-0826-3 | IAA-CSIC

The cocoon is the link between two types of hypernovae that has been explored so far, and the observed differences would be explained by chocked jets.

The Event Observation

Astronomers studied the gamma-ray bursts (GRB 171205A) that were detected on 5th  December 2017 in a galaxy located about 500 million light-years away. This was the 4th closest long gamma-ray bursts ever detected. The details of the hypernova were captured using the Gran Telescopio Canarias (a 10.4m reflecting telescope) on the La Palma island, Spain.

GRB 171205A during the maximum brightness of the event 

They observed an unusual component that was expanding at astonishing velocities and had chemical abundances different than those of previously recorded in similar events. These velocities and chemical compositions matched the existence of cocoon accompanying jets of matter at the star’s surface.

As expected, the cocoon dragged the matter out from the star’s interior. Initially, it emitted more energy than that of the gamma-ray burst, indicating that jets transmitted a huge portion of its energy to the cocoon. After a few days, this cocoon faded away.

Read: How Gravity Affects Shape Of Matter Near A Black Hole?

Overall, these observations showed that the energy of gamma-ray bursts relies on the interaction between the stellar material and jet, as well as on the cocoon. The study also suggests that the standard model of supernovae needs to be revised.

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