These gravitational waves are produced when stars, 15 to 20 times more massive than the sun, exhaust their fuel and undergo a catastrophic collapse, leading to an explosion. This process, known as a collapsar event, leaves behind a black hole encircled by a rapidly rotating disk of residual material. As this material spirals into the black hole, the intense motion distorts the surrounding space-time, creating gravitational waves that ripple across the universe.

Through advanced simulations, researchers found that these waves might be detected by instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO), which first observed gravitational waves from merging black holes in 2015. Detecting waves from collapsars could offer new insights into the complex processes within these massive stars and the formation of black holes.

"Currently, the only gravitational wave sources that we have detected come from a merger of two compact objects - neutron stars or black holes," explained Ore Gottlieb, a research fellow at the Flatiron Institute's Center for Computational Astrophysics (CCA) in New York City. "One of the most interesting questions in the field is: What are the potential non-merger sources that could produce gravitational waves that we can detect with current facilities? One promising answer is now collapsars."

In collaboration with CCA visiting scholar and Columbia professor Yuri Levin, and Tel Aviv University professor Amir Levinson, Gottlieb simulated the aftermath of a massive star's collapse, including factors like magnetic fields and cooling rates. Their results suggest that the gravitational waves from collapsars could be detected from up to 50 million light-years away. While this distance is much shorter than the range for waves from mergers of black holes or neutron stars, it still surpasses the strength of any non-merger event simulated to date.

Gottlieb noted that these findings were unexpected. Previously, scientists believed that the chaotic nature of a star's collapse would produce a mix of gravitational waves too disordered to distinguish from the cosmic background noise. However, the new simulations indicate that the rotating disks around collapsars can generate coherent waves, similar to the clear signals from merging compact objects.

"I thought that the signal would be much messier because the disk is a continuous distribution of gas with material spinning in different orbits," Gottlieb stated. "We found that the gravitational waves from these disks are emitted coherently, and they're also rather strong."

Gottlieb's calculations also suggest that some of these events might already be present in existing LIGO datasets. With proposed detectors like the Cosmic Explorer and Einstein Telescope, dozens of such events could potentially be observed each year.

While the gravitational wave community is eager to search for these signals, identifying them is challenging. The team simulated a limited number of potential collapsar events, but with stars varying widely in mass and rotation, the corresponding gravitational wave signals could differ significantly.

"In principle, we would ideally simulate 1 million collapsars to be able to create a generic template, but unfortunately, these are very expensive simulations," Gottlieb explained. "So, for now, we have to pick other strategies."

One approach involves searching historical data for events similar to those modeled in the simulations. However, due to the diversity of star signals, finding a direct match is unlikely. Another method would involve looking for other signals, like supernovae or gamma-ray bursts, from nearby collapsar events and then checking gravitational wave data from the same region.

Identifying gravitational waves from collapsars could provide invaluable information about the inner workings of stars and black holes, areas that remain largely mysterious.

"These are things that we can otherwise not detect," Gottlieb emphasized. "The only way for us to study these inner stellar regions around the black hole is through gravitational waves."