October and November 2024 proved to be particularly productive for gravitational wave astronomy. Within the two months, the LIGO-Virgo-KAGRA collaboration detected two black hole mergers with such unusual properties that they're changing our understanding of how they form and evolve. Both events feature rapidly spinning black holes in unequal mass pairs, properties that point toward a violent history of previous collisions rather than a quiet stellar origin.
The first merger, with the catchy name GW241011, occurred roughly 700 million light years away when black holes weighing 20 and 6 solar masses spiraled together. What makes this event extraordinary is the spin of the larger black hole, one of the fastest rotating black holes ever observed through gravitational waves. Just one month later, GW241110 was detected at 2.4 billion light years away, involving black holes of 17 and 8 solar masses. This merger revealed a primary black hole spinning in the opposite direction to its orbit, a configuration never directly observed before.
A computer generated image of the collision of two black holes released after the event was detected for the first time by Ligo in 2016 (Credit : LIGO Laboratory)
These spin properties carry fundamental implications for understanding the origin of black holes. When massive stars collapse and die, they typically leave behind black holes with modest spins aligned with their original orbital motion. The dramatic spins observed in both GW241011 and GW241110 suggest these aren't first generation black holes formed directly from stellar collapse. Instead, they're likely products of earlier mergers, second generation black holes born from previous collisions that left them spinning rapidly and sometimes in unexpected directions.
In both cases, the larger black hole was nearly double the mass of its companion, a size difference more consistent with hierarchical mergers than with binary stars that formed together. This pattern suggests these systems assembled in dense stellar environments like globular clusters, where black holes frequently encounter one another and merge repeatedly over time. Each collision adds mass and can dramatically alter spin, building up the unusual properties observed in these observations.
Image of the black hole at the centre of Messier 87 (Credit : Event Horizon Telescope)
The fabulous clarity of the GW241011 signal allowed astronomers to verify Einstein's general relativity with remarkable precision. The rapid rotation of the primary black hole causes the object to deform slightly, an effect predicted by mathematician Roy Kerr's solution for rotating black holes. The gravitational waves carry this deformation as a distinctive signature that matches theoretical predictions almost perfectly. The signal also contains higher harmonics, overtones similar to those in musical instruments, confirming another prediction from Einstein's theory.
As detector sensitivity continues improving, more discoveries like GW241011 and GW241110 will hopefully emerge, revealing even more diverse environments where black holes collide and help us to refine the fundamental laws governing these most extreme objects in our universe.
Source : Pair of Distinct Black Hole Mergers Sheds New Light on Nature of Their Formation and Evolution
