Until now, gravitational waves have been detected by astrophysicists only in binary systems – the merger of two black holes, two neutron stars or one of them. Theoretically, gravitational waves emanating from a single non-binary source could be detected, but such elusive signals have yet to be detected.

Researchers at Northwestern University now suggest that these elusive signals can be sought in a new, unexpected and completely unexplored area: cocoons of violent, energetic debris surrounding dying massive stars.

For the first time in history, researchers have used modern simulations to show that these cocoons can emit gravitational waves. And unlike gamma-ray burst jets, cocoon gravitational waves must be within the frequency range that the Laser Interferometer Gravitational-Wave Observatory (LIGO) can detect.

“To date, LIGO has only detected gravitational waves from binary systems, but it will one day detect the first non-binary source of gravitational waves,” said Ore Gottlieb of Northwestern, who led the study. “Cocoons are one of the first places we should look for such sources.”

Gottlieb recently presented the research at a virtual press briefing at the 242nd meeting of the American Astronomical Society.

New source “was impossible to ignore”

To conduct the study, Gottlieb and his colleagues used a new state-of-the-art simulation to simulate the collapse of a massive star. When massive stars collapse into black holes, they can create powerful streams (or jets) of particles that move at near the speed of light. Gottlieb’s simulations modeled this process – from the moment the star collapsed into a black hole to the exit of the jet.

First, he wanted to see if the accretion disk forming around the black hole could emit visible gravitational waves. But something unexpected always emerged from his data.

Evolution of the jet cocoon from birth into a black hole to exit from a star (color map is logarithm of off-axis strain amplitude and sound represents GW frequency). Credit: Ore Gottlieb/CIERA/ Northwestern University

“When I calculated the gravitational waves around the black hole, I discovered another source that turned my calculations upside down – the cocoon,” Gottlieb said. “I tried to ignore it. But I found it impossible to ignore. Then I realized that the cocoon is an interesting source of gravitational waves.”

When the jets collide with the collapsed layers of a dying star, a bubble or “cocoon” forms around the jet. Cocoons are turbulent places where hot gases and debris mix in a chaotic manner and expand in all directions away from the jet. Gottlieb explained that as the energy bubble accelerates from the jet, it distorts space-time, creating ripples of gravitational waves.

“The jet starts deep in the star and then pierces its way out to escape,” Gottlieb said. “It’s like drilling a hole in a wall. The rotating drill hits the wall and rubble pours out of the wall. The drill energizes this material. Similarly, the jet pierces the star, causing the star’s material to heat up and explode. This debris forms the hot layers of the cocoon.”

Call to action to look at the cocoons

If the cocoons produce gravitational waves, LIGO could detect them in future studies, Gottlieb said. Researchers have often looked for single-source gravitational waves from gamma-ray bursts or supernovas, but astrophysicists doubt LIGO can detect them.

“Both jets and supernovas are very energetic explosions,” Gottlieb said. Said. “But we can only detect gravitational waves from higher frequency asymmetrical bursts. Supernovas are highly spherical and symmetrical, so spherical explosions do not change the balanced distribution of mass in the star to emit gravitational waves. Gamma-ray bursts last for tens of seconds, so the frequency is very low – below the frequency range that LIGO is sensitive to.”

Instead, Gottlieb asks astrophysicists to direct their attention to both asymmetrical and high-energy cocoons.

“Our work is a call to action for the community to look at cocoons as the source of gravitational waves,” he said. “We also know that cocoons emit electromagnetic radiation, so they can be multiple message events. By studying them, we can learn more about what happens in the deepest parts of stars, the properties of jets and their prevalence in stellar explosions.” Source