Observations of nearby supernova and associated jet cocoon provide new insights on gamma-ray bursts

Gamma-ray bursts are the most powerful explosions in the cosmos. These explosions last several seconds and emit the same amount of light as nearly all the stars in the universe.

Such extreme amounts of energy can only be released during catastrophic events like the death of a very massive star, and also produce visible supernovae or hypernovae, the latter being 5 to 50 times more energetic than supernovae.

While the connection between GRBs and hypernovae has been well established, it is still unclear why some hypernovae do not have associated GRBs.

Through the detailed observation of a rare, nearby hypernova, an international team of researchers including Chryssa Kouveliotou, a professor of physics at the George Washington University, discovered the missing link connecting hypernovae to GRBs in the form of a hot cocoon around the jets of matter expelled by the central engine as these spread through the outer layers of the progenitor star.

“For a handful of hypernovae not accompanied by GRBs, an excess of high velocity material had been observed in their optical spectra, which had been attributed to a thermal cocoon, namely a jet which never managed to break out of the interior to the surface of its progenitor star,” Dr. Kouveliotou said.

“With this recent event, we showed the jet provided a significant part of its energy to the cocoon, which allowed both the jet and the gamma rays to break out of the surface of the star. This was the first time we actually got to peek directly at the core of a collapsing massive star!”

The new discovery was announced in the study, “Signatures of a jet cocoon in early spectra of a supernova associated with a γ-ray burst,” published today in the journal Nature. The international team was led by Luca Izzo, a researcher at the Institute of Astrophysics of Andalusia in Spain. Dr. Kouveliotou was one of two researchers on the team based in the United States.

On December 5, 2017, the researchers discovered a long-duration GRB in a spiral galaxy about 500 million light-years from Earth, the fourth closest long-duration GRB ever observed. Since this GRB, labeled 171205A, was so close to Earth and detected so early, they were able to monitor the evolution of the source daily with the largest telescopes on the globe. This collaborative effort allowed researchers to capture an unprecedented level of information about the event in many different wavelengths over time.

Within the first days after the GRB detection, evidence of the presence of a hypernova was reported, but the findings were very peculiar.

Hypernovae are characterized by high expansion velocities on the order of 30,000 kilometers per second, but the hypernova the researchers observed–designated SN 2017iuk–had a much higher velocity, more than 100,000 kilometers per second in the first few hours after the explosion.

A detailed analysis of the multi-wavelength emission observed in the very early days of SN 2017iuk led to surprising results.

“We first noticed a peculiar component, which showed very high velocity and unusual chemical composition not seen previously in similar events,” Dr. Kouveliotou explained. “These features fit perfectly with the assumption that we observe material from the central engine escaping from the GRB progenitor star.

The team did in fact observe a hot cocoon taking material from the interior of the star to the outer layers. After about three days, the hot cocoon faded away and the hypernova began behaving similarly to those observed previously.

The researchers indicated the energy emitted by the cocoon in the earliest days of the hypernova was larger than that of the GRB, implying the jet deposited the bulk of its energy into the cocoon. These results demonstrate choked jets are indeed the reason some hypernovae do not seem to be associated with GRBs.

The researchers noted the findings have interesting consequences for how supernova and hypernova explosion models are constructed in the future.