---
DOAJ_listed: '1'
OA_place: publisher
OA_type: gold
_id: '21713'
abstract:
- lang: eng
  text: 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.
acknowledgement: The authors thank Davide Gerosa and Matthew Mould for valuable suggestions.
  We are grateful for support by the National Science Foundation under grant No. PHY-2309024
  (I.B.) and by NASA under grants 80NSSC22K0822 and 80NSSC24K0440 (Z.H.). We used
  OpenAI’s ChatGPT (OpenAI 2025) during the preparation of this manuscript. This material
  is based upon work supported by NSF’s LIGO Laboratory, which is a major facility
  fully funded by the National Science Foundation.
article_number: L44
article_processing_charge: Yes
article_type: original
arxiv: 1
author:
- first_name: Imre
  full_name: Bartos, Imre
  last_name: Bartos
- first_name: Zoltán
  full_name: Haiman, Zoltán
  id: 7c006e8c-cc0d-11ee-8322-cb904ef76f36
  last_name: Haiman
  orcid: 0000-0003-3633-5403
citation:
  ama: 'Bartos I, Haiman Z. Accretion is all you need: Black Hole spin alignment in
    merger GW231123 indicates accretion pathway. <i>The Astrophysical Journal Letters</i>.
    2026;996(2). doi:<a href="https://doi.org/10.3847/2041-8213/ae2bff">10.3847/2041-8213/ae2bff</a>'
  apa: 'Bartos, I., &#38; Haiman, Z. (2026). Accretion is all you need: Black Hole
    spin alignment in merger GW231123 indicates accretion pathway. <i>The Astrophysical
    Journal Letters</i>. IOP Publishing. <a href="https://doi.org/10.3847/2041-8213/ae2bff">https://doi.org/10.3847/2041-8213/ae2bff</a>'
  chicago: 'Bartos, Imre, and Zoltán Haiman. “Accretion Is All You Need: Black Hole
    Spin Alignment in Merger GW231123 Indicates Accretion Pathway.” <i>The Astrophysical
    Journal Letters</i>. IOP Publishing, 2026. <a href="https://doi.org/10.3847/2041-8213/ae2bff">https://doi.org/10.3847/2041-8213/ae2bff</a>.'
  ieee: 'I. Bartos and Z. Haiman, “Accretion is all you need: Black Hole spin alignment
    in merger GW231123 indicates accretion pathway,” <i>The Astrophysical Journal
    Letters</i>, vol. 996, no. 2. IOP Publishing, 2026.'
  ista: '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.'
  mla: 'Bartos, Imre, and Zoltán Haiman. “Accretion Is All You Need: Black Hole Spin
    Alignment in Merger GW231123 Indicates Accretion Pathway.” <i>The Astrophysical
    Journal Letters</i>, 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>.'
  short: I. Bartos, Z. Haiman, The Astrophysical Journal Letters 996 (2026).
corr_author: '1'
date_created: 2026-04-12T22:01:49Z
date_published: 2026-01-10T00:00:00Z
date_updated: 2026-05-04T09:54:18Z
day: '10'
ddc:
- '520'
department:
- _id: ZoHa
doi: 10.3847/2041-8213/ae2bff
external_id:
  arxiv:
  - '2508.08558'
file:
- access_level: open_access
  checksum: ac46ba3d13f0150ccbc42665bed3ae47
  content_type: application/pdf
  creator: dernst
  date_created: 2026-05-04T09:49:53Z
  date_updated: 2026-05-04T09:49:53Z
  file_id: '21788'
  file_name: 2026_AstrophysicalJourLetters_Bartos.pdf
  file_size: 866725
  relation: main_file
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file_date_updated: 2026-05-04T09:49:53Z
has_accepted_license: '1'
intvolume: '       996'
issue: '2'
language:
- iso: eng
month: '01'
oa: 1
oa_version: Published Version
publication: The Astrophysical Journal Letters
publication_identifier:
  eissn:
  - 2041-8213
  issn:
  - 2041-8205
publication_status: published
publisher: IOP Publishing
quality_controlled: '1'
scopus_import: '1'
status: public
title: 'Accretion is all you need: Black Hole spin alignment in merger GW231123 indicates
  accretion pathway'
tmp:
  image: /images/cc_by.png
  legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode
  name: Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)
  short: CC BY (4.0)
type: journal_article
user_id: 2DF688A6-F248-11E8-B48F-1D18A9856A87
volume: 996
year: '2026'
...