December 6, 2023

So far, we have only detected gravitational waves originating from binary star systems, black holes or neutron star mergers. But Northwestern University researchers may have discovered a new source of non-binary gravitational waves: a debris “cocoon” that forms around a dying, massive star.

Gravitational waves are invisible, but extremely fast ripples caused by some of the most violent, energetic processes in space-time. Gravitational waves travel at the speed of light, squeezing and stretching anything they pass by.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States was specifically designed to detect gravitational waves. Like Italy’s Virgo Interferometer and Japan’s Kamioka Gravitational-Wave Explorer (KAGRA), LIGO allows scientists to use the information they gather to analyze our universe. So far, gravitational waves have been observed from binary star systems, where two celestial bodies — such as black holes or neutron stars — get close enough that their gravity causes them to orbit each other.

Now, researchers at Northwestern University in Illinois say we can measure gravitational waves originating from a single, non-dual source: the debris “cocoon” around a dying massive star.

“As of today, LIGO has only detected gravitational waves from binary star systems, but someday it will detect the first non-binary source of gravitational waves,” said Ore Gottlieb, lead author of the study. “Cocoons are one of the first places we should be looking for such sources.”

Core-collapse supernovae are massive stars that have undergone gravitational collapse and are in the final stages of their lives. Its collapse is associated with prolonged gamma-ray bursts that manifest as energetic jets that cause stellar material to form an hourglass-shaped cocoon as it is forced away from the star.

“The jet starts deep inside the star and drills its way out,” Gottlieb said. “It’s like you’re drilling a hole in a wall. The rotating drill hits the wall and debris spills out of the wall. The drill imparts energy to the matter. Similarly, a jet passes through a star, causing the star’s material to heat up and spill out. These debris Forms the thermal layer of the cocoon.”

The researchers used simulations to simulate the collapse of a massive star, from the star’s collapse into a black hole to the jet’s escape. They demonstrated for the first time that cocoons can emit detectable gravitational waves. That’s preferable to looking for gravitational waves from gamma-ray bursts or supernovae, which occur at frequencies that LIGO might not be able to detect, the researchers said.

“Supernovae are fairly spherical and symmetric, so a spherical explosion would not change the equilibrium mass distribution in the star to emit gravitational waves,” Gottlieb said. “Gamma-ray bursts last tens of seconds, so the frequency is very small — below the frequency band LIGO is sensitive to.”

Cocoons, on the other hand, are asymmetrical and highly energetic, which the researchers say should mean LIGO could pick them up. They may provide additional information.

“Our research is a call to action for the community to consider cocoons as a source of gravitational waves,” Gottlieb said. “We also know that cocoons emit electromagnetic radiation, so they may be multi-messenger events. By studying them, we can learn more about what happens in the innermost part of the star, the properties of the jets and their prevalence in stellar explosions.”

The study was published in the journal ArXiv The video below, produced by Northwestern University, shows how the cocoon of a dying star generates gravitational waves.

Cocoons of dying stars emit gravitational waves

source: Northwest University