Astronomers target gravitational waves coming from dying star 'cocoons'

To reach their findings, a team of researchers simulated the collapse of a massive dying star.
Chris Young
An artist's impression of a cocoon of stellar debris.
An artist's impression of a cocoon of stellar debris.

Ore Gottlieb / CIERA / Northwestern University 

To date, astrophysicists have only detected gravitational waves originating from black hole and neutron star mergers, which are categorized as binary systems.

However, Northwestern University researchers have suggested a new non-binary candidate: cocoons made from the remnants of massive dying stars.

In search of a non-binary gravitational wave source

When stars die, they go supernova in a massive cataclysmic explosion reverberating far and wide throughout the cosmos. In the process, they also form cocoons of debris surrounding the region of space that was once the location of a living star.

Using state-of-the-art simulations, the researchers showed that these cocoons could emit gravitational waves that should be detectable by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

"As of today, LIGO has only detected gravitational waves from binary systems, but one day it will detect the first non-binary source of gravitational waves," Northwestern’s Ore Gottlieb, who led the study, explained in a press statement. "Cocoons are one of the first places we should look to for this type of source."

In their simulations, the researchers modeled the collapse of a massive star turning into a black hole that then blasts powerful jets that travel near the speed of light out into space.

Surprising results in supernova simulation

The researchers initially set out to determine whether the accretion discs of black holes might emit detectable gravitational waves. Surprisingly, though, they came out with another result altogether.

"When I calculated the gravitational waves from the vicinity of the black hole, I found another source disrupting my calculations — the cocoon," Gottlieb said. "I tried to ignore it. But I found it was impossible to ignore. Then I realized the cocoon was an interesting gravitational wave source."

These cocoons are formed when the jets emitted by the black hole crash into the collapsing layers of the dying star, forming a bubble-like structure around the jet.

"A jet starts deep inside of a star and then drills its way out to escape," Gottlieb continued. "It's like when you drill a hole into a wall. The spinning drill bit hits the wall, and debris spills out of the wall. The drill bit gives that material energy. Similarly, the jet punches through the star, causing the star’s material to heat up and spill out. This debris forms the hot layers of a cocoon."

Now that LIGO has finally started its fourth observation run, with new updates that reportedly double its sensitivity, the gravitational wave observatory could be able to detect waves originating from these cocoons.

"Our study is a call to action to the community to look at cocoons as a source of gravitational waves," Gottlieb explained. "We also know cocoons to emit electromagnetic radiation, so they could be multi-messenger events. By studying them, we could learn more about what happens in the innermost part of stars, the properties of jets and their prevalence in stellar explosions."

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