Rethinking Thorne-Żytkow Object Formation: The Fate of X-ray Binary LMC X-4 and Implications for Ultra-long Gamma-ray Bursts

• Published in: arXiv.org 2023-11
• Main Authors: Hutchinson-Smith, Tenley, Rosa Wallace Everson, Batta, Aldo, Yarza, Ricardo, Twum, Angela A, Law-Smith, Jamie A P, Ramirez-Ruiz, Enrico
• Format: Article
• Language: English
• Subjects: Accretion disks; Angular momentum; Binary stars; Black holes; Companion stars; Gamma ray astronomy; Gamma ray bursts; Luminosity; Neutron stars; Neutrons; Rotating spheres; Stellar cores; X ray binaries; X ray stars; X-ray astronomy
• ISSN: 2331-8422

We present a start-to-end simulation aimed at studying the long-term fate of high mass X-ray binaries and whether a Thorne-Żytkow object (TŻO) might ultimately be produced. We analyze results from a 3D hydrodynamical simulation that models the eventual fate of LMC X-4, a compact high mass X-ray binary system, after the primary fills its Roche lobe and engulfs the neutron star companion. We discuss the outcome of this engulfment within the standard paradigm of TŻO formation. The post-merger angular momentum content of the stellar core is a key ingredient, as even a small amount of rotation can break spherical symmetry and produce a centrifugally supported accretion disk. Our findings suggest the inspiraling neutron star, upon merging with the core, can accrete efficiently via a disk at high rates (\(\approx 10^{-2}M_\odot/{\rm s}\)), subsequently collapsing into a black hole and triggering a bright transient with a luminosity and duration typical of an ultra-long gamma-ray burst. We propose that the canonical framework for TŻO formation via common envelope needs to be revised, as the significant post-merger accretion feedback will unavoidably unbind the vast majority of the surrounding envelope.