Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway

article Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway published yes Imre Bartos author Zoltán Haiman author 7c006e8c-cc0d-11ee-8322-cb904ef76f360000-0003-3633-5403 ZoHa department 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 (a1 = 0.90 +0.10/-0.19, a2 = 0.80 +0.20/-0.51), 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 embedded in AGN disks and Population III remnants, which grew via coherent misaligned gas accretion. We further argue that other high-mass, high-spin events, e.g., GW190521, may share a similar evolutionary pathway. These findings underscore the critical role of sustained, coherent accretion in shaping the most extreme black hole binaries. https://research-explorer.ista.ac.at/download/21713/21788/2026_AstrophysicalJourLetters_Bartos.pdf application/pdfno https://creativecommons.org/licenses/by/4.0/ IOP Publishing2026 eng The Astrophysical Journal Letters 2041-8205 2041-8213 2508.0855810.3847/2041-8213/ae2bff 9962 Bartos, I., & Haiman, Z. (2026). Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway. The Astrophysical Journal Letters. IOP Publishing. <a href="https://doi.org/10.3847/2041-8213/ae2bff">https://doi.org/10.3847/2041-8213/ae2bff</a> Bartos I, Haiman Z. Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway. The Astrophysical Journal Letters. 2026;996(2). doi:<a href="https://doi.org/10.3847/2041-8213/ae2bff">10.3847/2041-8213/ae2bff</a> I. Bartos and Z. Haiman, “Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway,” The Astrophysical Journal Letters, vol. 996, no. 2. IOP Publishing, 2026. I. Bartos, Z. Haiman, The Astrophysical Journal Letters 996 (2026). Bartos, Imre, and Zoltán Haiman. “Accretion Is All You Need: Black Hole Spin Alignment in Merger GW231123 Indicates Accretion Pathway.” The Astrophysical Journal Letters, vol. 996, no. 2, L44, IOP Publishing, 2026, doi:<a href="https://doi.org/10.3847/2041-8213/ae2bff">10.3847/2041-8213/ae2bff</a>. Bartos I, Haiman Z. 2026. Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates accretion pathway. The Astrophysical Journal Letters. 996(2), L44. Bartos, Imre, and Zoltán Haiman. “Accretion Is All You Need: Black Hole Spin Alignment in Merger GW231123 Indicates Accretion Pathway.” The Astrophysical Journal Letters. IOP Publishing, 2026. <a href="https://doi.org/10.3847/2041-8213/ae2bff">https://doi.org/10.3847/2041-8213/ae2bff</a>. 217132026-04-12T22:01:49Z2026-05-04T09:54:18Z