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GW241011's fast-spinning primary matches a Kerr black hole's predicted deformation
A signal LIGO Hanford and Virgo recorded in October 2024 was loud and lopsided enough to pin down how spin deforms its 19.6-solar-mass object. Rotating boson stars with quartic self-interactions cannot fit it.
The Scientist · Science desk
What happened
- The LIGO Hanford detector in the United States and the Virgo detector in Italy recorded GW241011, a signal linked to the merger of two compact objects interpreted as black holes.
- The two merging objects had masses of about 19.6 and 5.9 solar masses, and the heavier one carried a dimensionless spin of roughly 0.78.
- The Physical Review Letters paper reports that the more massive object is consistent with a Kerr black hole, whose properties are set entirely by its mass and spin.
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Why it matters
- capability Whether a merging object is a black hole can now be answered with a fitted number on a specific event, so a model builder can be told which objects the data excludes.
- constraint The test only bites when a fast-spinning heavy primary, a large mass ratio and a loud signal show up together, so most catalogue detections will not deliver a comparable bound.
- exposure Boson-star models with quartic self-interactions now have a measurement to answer to, and any surviving alternative has to reach a compactness the data still allows.
- precedent A proposal made in 2017 waited for the right event to become a competitive constraint. Other proposed tests of compact-object nature should expect the same pattern.
A spin-induced quadrupole moment is a shape measurement: rotation pulls a body's mass out of spherical symmetry [7]. For a rotating black hole in general relativity, the size of that deformation is fixed once mass and spin are known [6]. Boson stars and other exotic compact objects carry internal structure a black hole does not, and that structure can move their multipole moments off the black hole value [17]. Two mergers can look alike in a detector and differ in that one number [19].
"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, the paper's co-first author and corresponding author [9][18]. That 2017 proposal was also published in Physical Review Letters [8].
The measurement needed a signal of a particular shape. Krishnendu said "the method would be particularly powerful for an event with a rapidly spinning primary, a significant mass asymmetry, and a high signal-to-noise ratio, because these conditions make the spin-induced multipole moment much easier to measure" [13]. GW241011's two objects differ in mass by a factor of about 3.3, dividing 19.6 by 5.9 [15]. "This made GW241011 an ideal system for applying the method we developed almost eight years earlier," he said [14].
The exclusion is stated in the paper's own terms. "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 [10]. Objects that are compact enough, at a compactness of roughly 0.24 or above, are still allowed by the data [11].
A consistency result of this kind bounds alternatives without measuring the Kerr description to arbitrary precision. Krishnendu said the observations made so far "have been consistent with the predictions of general relativity for binary black holes" [12]. The constraint says nothing about the lighter 5.9-solar-mass object, or about typical detections: it covers one parameter of the heavier body in one event [6], and the three properties that made it measurable arrived together in a single signal [13]. Another bound of this quality waits on another event with all three.
The Birmingham, Perimeter Institute and Canadian Institute for Theoretical Astrophysics team published the analysis in Physical Review Letters [5].
What to watch
- Whether a later observing run yields another primary with spin near 0.78 and a mass ratio above 3, which sets how fast the quadrupole bound tightens.
- Whether boson-star modellers publish quadrupole predictions for the compactness range above 0.24 that the data still allows.
- Whether the LIGO-Virgo-KAGRA collaboration publishes a catalogue-wide combined constraint on the spin-induced quadrupole.