Second generation black holes observed by LIGO, Virgo and KAGRA

Illustrated: Visualization – merging (fusion) of black holes of different masses (ChatGPT/DALL·E, OpenAI).

In a new paper published recently in The Astrophysical Journal Letters, the international LIGO-Virgo-KAGRA Collaboration reports on the detection of two gravitational wave events in October and November of last year with unusual black hole spins. An observation that adds an important new piece to our understanding of the most elusive phenomena in the universe.

Gravitational waves are 'ripples' in space-time that result from cataclysmic events in deep space, with the strongest waves produced by the collision of black holes. Using sophisticated algorithmic techniques and mathematical models, researchers are able to reconstruct many physical features of the detected black holes from the analysis of gravitational signals, such as their masses and the distance of the event from Earth, and even the speed and direction of their rotation around their axis, called spin.

The first merger detected on Oct. 11, 2024 (GW241011), occurred roughly 700 million light years away and resulted from the collision of two black holes weighing in at around 17 and 7 times the mass of our Sun. The larger of the two black holes in GW241011 was measured to be one of the fastest rotating black holes observed to date. Almost one month later, GW241110 was detected on Nov. 10, coming from around 2.4 billion light years away and involving the merger of black holes roughly 16 and 8 times the mass of the Sun. While most observed black holes spin in the same direction as their orbit, the primary black hole of GW241110 was noted to be spinning in a direction opposite its orbit – a first of its kind.

Both detections, interestingly, point toward the possibility of “second-generation” black holes. GW241011 and GW241110 are among the most novel events among the several hundred that the LIGO-Virgo-KAGRA network has observed. And with both events having one black hole which is both significantly more massive than the other and rapidly spinning, they provide tantalizing evidence that these black holes were formed from previous black hole mergers. Scientists point to certain clues, including the size differential between the black holes in each merger – the larger was nearly double the size of the smaller – and the spin orientations of the larger of the black holes in each event. A natural explanation for these peculiarities is that the black holes are the result of earlier coalescences. This process, called a hierarchical merger, suggests that these systems formed in dense environments, in regions like star clusters, where black holes are more likely to run into each other and merge again and again.