A Black Hole Merger Caught in October 2024 Had a Fast-Spinning 19.6-Solar-Mass Primary. Physicists Used Its 'Shape' to Rule Out a Whole Class of Black Hole Impostors.

In general relativity, a black hole is the simplest object in the universe: tell me its mass and its spin, and I can tell you everything else about it, including exactly how much its rotation flattens it. Exotic alternatives such as boson stars, hypothetical compact objects built from particles that can pile into the same quantum state, would deform differently when they spin. A team of physicists has now used that difference to test a real gravitational-wave signal, and the result, published in Physical Review Letters, rules out a large family of black hole look-alikes for one of the two objects involved.

The signal is GW241011, recorded by the LIGO Hanford detector in Washington state and the Virgo detector in Italy in October 2024. It came from the merger of two compact objects with masses of about 19.6 and 5.9 times the sun. The heavier one was spinning fast, with a dimensionless spin of roughly 0.78, and the signal was loud, with a signal-to-noise ratio of about 36 across the detector network. That combination of a lopsided mass ratio, a rapidly rotating primary and a clean signal is exactly what the method needs.

"The paper builds on a method we originally proposed in 2017 to use gravitational-wave observations to test whether compact objects are truly black holes," said N. V. Krishnendu, co-first author and corresponding author, whose collaborators include researchers at the University of Birmingham, the Perimeter Institute for Theoretical Physics and the Canadian Institute for Theoretical Astrophysics. "The idea was motivated by a fundamental question: black holes are completely characterized by their mass and spin in general relativity, whereas exotic compact objects, such as boson stars, can have additional structure that changes their multipole moments."

The property in question is the spin-induced quadrupole moment, a number describing how far an object's rotation pushes its mass distribution away from a perfect sphere. That distortion leaves a fingerprint in the gravitational waves emitted as two objects spiral together. The team used Bayesian parameter estimation with waveform models that let each object's quadrupole moment float freely, rather than assuming the black hole value from the start, and then compared the measured value against predictions for different boson-star and exotic-fluid-star models, keeping only those that numerical relativity simulations show are stable enough to exist for long.

"We find that large classes of exotic compact objects, including rotating boson stars with quartic self-interactions, cannot explain the observed properties of the primary," Krishnendu said. "At the same time, sufficiently compact exotic objects, with compactness C≳0.24, remain viable possibilities." In plain terms, the heavier object in GW241011 is consistent with a Kerr black hole, and one specific kind of impostor is now excluded, but the data do not yet prove it is a black hole.

"Our result does not prove that the object is a black hole, as sufficiently compact exotic objects can still be consistent with the data," Krishnendu added. "I think this distinction is important. We are constraining the space of possible alternatives rather than simply labeling the object as a black hole." Co-first author Aditya Vijaykumar cautioned that the constraint applies only to this one binary, not to the population of compact objects as a whole.

The team expects the method to sharpen quickly. The LIGO-Virgo-KAGRA network is preparing its fifth observing run with substantially better sensitivity around 2029, third-generation ground detectors are being designed, and the space-based LISA mission would open up a far larger and more varied catalog of mergers. "What began as a theoretical proposal in 2017 is therefore becoming a practical observational tool for testing the nature of compact objects," Krishnendu said.