Gravitational Waves: Novel Detection Source Found in Dying Stars’ Debris Clouds

Jet emissions from collapsed stars penetrate clouds of stellar remnants. Credit: Ore Gottlieb/CIERA/Northwestern University

Historically, detection of gravitational waves has been limited to binary systems, involving the fusion of either two black holes, two neutron stars, or a combination of the two. The theory suggests the possibility of detecting these waves from a singular, non-binary source, however, no such signals have been found yet.

A team of researchers at Northwestern University have suggested a new, unexplored potential source of these elusive signals: the turbulent and energetic debris clouds that envelop dying, massive stars.

Utilizing advanced simulation technologies for the first time, the researchers have demonstrated that these stellar debris clouds, or cocoons, can produce gravitational waves. Moreover, unlike gamma-ray burst jets, these waves from cocoons fall within the frequency band that the Laser Interferometer Gravitational-Wave Observatory (LIGO) can detect.

“Up until now, LIGO has only identified gravitational waves from binary systems. However, we believe it will eventually detect the first non-binary source,” said the study’s lead, Ore Gottlieb of Northwestern University. “We suggest cocoons as the primary location to search for this kind of source.”

The study findings were recently presented by Gottlieb during a virtual press briefing at the 242nd meeting of the American Astronomical Society.

The new source was ‘unavoidable’

The research, led by Gottlieb, involved state-of-the-art simulations to model the collapse of a massive star. When these stars collapse into black holes, they often trigger the release of high-speed particle outflows or jets. Gottlieb’s simulations imitated this process from the moment of star collapse to the jet escape.

Initially, the focus was on determining if the accretion disk forming around a black hole could emit detectable gravitational waves. However, Gottlieb continually observed an unexpected result.

“When I computed the gravitational waves near the black hole, I encountered an interfering source — the cocoon,” Gottlieb said. “I attempted to disregard it, but it proved to be unignorable. It then dawned on me that the cocoon could be a valuable source of gravitational waves.”

When jets crash into the collapsing layers of the dying star, a “cocoon” forms around the jet. Cocoons are areas of turbulence, where heated gases and debris scatter randomly, expanding in all directions from the jet. The energetic bubble’s acceleration from the jet distorts space-time, creating gravitational waves, as Gottlieb elaborated.

“A jet initiates deep inside a star and then bores its way out,” said Gottlieb. “It resembles drilling a hole in a wall, where the drill bit impacts the wall, and debris spills out. Similarly, the jet breaks through the star, heating up the star’s material, which then spills out, forming the hot layers of a cocoon.”

Call for renewed focus on cocoons

If cocoons indeed produce gravitational waves, LIGO should be able to detect them in its future runs, according to Gottlieb. Researchers have typically hunted for single-source gravitational waves from gamma-ray bursts or supernovae, but there’s skepticism about LIGO’s ability to detect such signals.

“Both jets and supernovae are extremely energetic explosions,” said Gottlieb. “However, we can only detect gravitational waves from high-frequency, asymmetrical explosions. Supernovae are generally spherical and symmetrical, so spherical explosions don’t disrupt the balanced mass distribution in the star enough to emit gravitational waves. Gamma-ray bursts last for tens of seconds, so their frequency is too low — outside of LIGO’s sensitivity range.”

Instead, Gottlieb urges astrophysicists to shift their focus to cocoons, which are both asymmetrical and highly energetic.

“Our research is a call to action for the community to consider cocoons as potential sources of gravitational waves,” he said. “Cocoons are also known to emit electromagnetic radiation, indicating potential multi-messenger events. By studying them, we could deepen our understanding of the innermost part of stars, the properties of jets, and their occurrence in stellar explosions.”

Reference: “Jetted and Turbulent Stellar Deaths: New LVK-detectable Gravitational-wave Sources” by Ore Gottlieb et al., 10 July 2023, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ace03a

Gottlieb is a CIERA Fellow at Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). Co-authors of the study from Northwestern include professors Vicky Kalogera and Alexander Tchekovskoy, postdoctoral associates Sharan Banagiri and Jonatan Jacquemin-Ide, and graduate student Nick Kaaz.

The study received support from the National Science Foundation, NASA, and the Fermi Cycle 14 Guest Investigator program. The Department of Energy’s Oak Ridge National Laboratory supercomputer Summit and National Energy Research Scientific Computing Center’s supercomputer Perlmutter made the advanced simulations possible through the ASCR Leadership Computing Challenge computational time award.

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More about Gravitational Waves

• Northwestern University’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA)
• The Astrophysical Journal Letters
• Laser Interferometer Gravitational-Wave Observatory (LIGO)
• National Science Foundation
• NASA
• Oak Ridge National Laboratory
• National Energy Research Scientific Computing Center