In a paper published 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 2024 with unusual black hole spins. This observation adds an important new piece to our understanding of the most elusive phenomena in the universe.
We had seen merges of two first generation black holes but never of this type which would have had at least one other merger prior to this merger detected (either four merged into two merged into one or some other configuration like that). That’s what they mean by hierarchical mergers.
Ah, so the differing spin (and mass) of the merging black holes they just detected indicate that at least one of them was already a second generation black hole, and is evidence for multi-generation hierarchical mergers. That makes sense.