Fig 1: Signal GW231123, very brief and atypical, in the data of Ligo Hanford (left) and Livingston (right) detectors.
The upper panels show the amplitude of the data over time (gray traces). The shaded blue band indicates our real signal estimate. The lower panels are spectrograms, also called time-frequency cards, which show the amplitude of the signal over time (horizontal axis) and depending on the frequencies (vertical axis). The brighter colors represent a stronger signal.
On November 23, 2023 at 1:54 pm: 30 UTC, Ligo-Virgo-Kagra collaboration (LVK) detected GW231123, a gravitational wave signal probably due to the merger of two black holes whose combined mass was the highest ever observed by LVK collaboration. These black holes would have turned on themselves at an incredible speed, and their individual masses seem to be in a fork which questions existing theories on the evolution and end of life of massive stars.
Signal detection
This gravitational wave was observed by the two Advanced Ligo detectors from Hanford and Livingston during the first part of the fourth LVK observation campaign (O4A). The consistency between the two observatories was essential for reliable detection. As Figure 1 shows, the signal lasted approximately a tenth of a second, but it clearly emerges from the noise, about 20 times stronger than the typical noise of a detector. To ensure that it was not a random anomaly in the data, we carried out meticulous statistical checks. Thanks to techniques simulating thousands of years of false data, we have found that the probability that random noise imitates GW231123 is less than once every 10,000 years! This gives us absolute confidence in the non -terrestrial origin of the signal, and therefore in the reality of this gravitational wave signal.
Source of the signal
The data strongly suggest that this signal comes from the violent merger of two black holes. To find out more about these black holes, including their mass and their speed of rotation, we used several models based on the theory of general relativity of Einstein in order to simulate the appearance of such a signal for different pairs of black holes.
By comparing the data to these models, we found that these black holes weighed approximately 137 and 103 times the mass of the sun. Given all the uncertainties, their total mass was probably between 190 and 265 solar masses, dethroning GW190521 as the most massive black holes observed to date. As if it was not already impressive enough, the two black holes probably turned almost as quickly as their maximum theoretical speed, which makes GW231123 not only the most massive black holes binary, but also that whose black holes had the highest spin ever detected with certainty by gravitational waves.
The merger produced a black hole whose mass is probably between 182 and 251 solar masses. It thus belongs to a rare category of black holes called intermediate mass holes: heavier than those from stellar collapse, but much lighter than the supermassive black holes that are hidden in the center of the galaxies. The remains of the merger of GW231123 and GW190521 are distinguished as the most clear gravitational waves of these elusive medium -sized black holes.
Why are these properties interesting?
Current theories on stellar evolution suggest that black holes whose mass is between 60 and 130 solar masses should be rare, even non -existent. This “prohibited” mass beach, called “mass interval of black holes”, would result from particular types of explosions which tear the heavy stars (supernovae with pairs instability) or eject an important part of their mass before their collapse (supernovae with instability of pulses), thus preventing the formation of a heavy black hole.
However, GW231123 questions this hypothesis. The lightest black hole is almost certainly in the mass interval, with 83 % chance of falling there, while the heaviest black hole has 26 % of chance. This suggests that traditional stellar evolution may not fully explain their origins.
An interesting possibility is that one of these black holes, or both, have formed following previous mergers of black holes. This would explain their high estimated masses and spins, and suggest that they lived in an extremely dense astrophysical environment, like a cluster of nuclear stars or an active galactic nucleus, where black holes are more likely to collide.
These dense environments can also lead to black holes orbiting around each other according to more elongated or eccentric trajectories. However, to limit their complexity, our models currently assume that black holes revolve in a spiral inward on almost spherical orbits which are gradually shrinking as they emit gravitational waves. However, if the orbits are strongly eccentric, especially just before fusion, this could affect the wave forms emitted in a way that our models do not take into account. For GW231123, this possibility remains open and requires more advanced model tests.
Other scenarios that have been able to produce a signal like this, such as gravitational lenses, primordial black holes, supernova with heart collapse, Bosons stars and cosmic strings, are astrophysically less likely than those mentioned above.
The last moments of the merger
For most black holes observed by the LVK – nearly 300 when writing this scientific summary – detectors are more sensitive to the first phases of the signal, when the black holes intersect in a spiral before finally merging. However, thanks to its large mass, GW231123 provided us with the clearest vision of its peak: fusion and decay phase, when the newly formed black hole radiates energy by gravitational waves, vibrates and ends up reaching a stable state, a bit like a bell ringing towards silence.
We compared this last part of the signal to the predictions of general relativity concerning the decrease of a black hole, and have noted a strong concordance between theory and our data. However, the extreme properties of GW231123 repel our models to their limits, leaving certain subtle characteristics unexplained and indicating areas where our wavelems can be improved.
Conclusion
A few years ago, in our summary of GW190521, we claim that the records were made to be broken, and GW231123 did it. With properties that could include black holes in the mass gap and spins close to the theoretical limit, this event is both extraordinary and difficult to interpret. It pushes us to explore other ways of formation of black holes, beyond the only traditional stellar evolution, and highlights the limits of our current models of wave forms. While we continue to listen to the universe through the gravitational waves, GW231123 forcefully reminds us that the Cosmos still has many surprises in store for us, and that we only start to discover them. To find out more:
Consult our websites:
- ligo.org
- www.virgo-gw.eu
- gwcenter.icrr.u-tokyo.ac.jp/en/
- Read a free prepublication of the full scientific article here or on Arxiv.org
- Publication of data from the Open Sciences Center on Gravitational Waves for GW231123 available here