Accretion is All You Need: Black Hole Spin Alignment in Merger GW231123 Indicates Accretion Pathway

Abstract GW231123 represents the most massive binary–black hole merger detected to date, lying firmly within, or even above, the pair-instability mass gap. The component spins are both exceptionally high ( a 1 = 0.9 0 − 0.19 + 0.10 , a 2 = 0.8 0 − 0.51 + 0.20 ), which is difficult to explain with repeated mergers. Here we show that the black hole spin vectors are closely aligned with each other while significantly tilted relative to the binary’s orbital angular momentum, pointing to a common accretion-driven origin. We examine astrophysical formation channels capable of producing near-equal, high-mass, and mutually aligned spins consistent with GW231123—particularly binaries…

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  National Aeronautics and Space Administration

  Awards: 80NSSC24K0440, 80NSSC22K0822