Current predictions suggest that non-inspiral gravitational wave (GW) sources are unlikely to be detected in LIGO-Virgo-KAGRA (LVK) runs. We conduct the first numerical study of GWs from collapsar disks, by using state-of-the-art 3D general-relativistic magnetohydrodynamic simulations of collapsing stellar cores, with a fixed Kerr metric of the central black hole (BH) and simple prescriptions for the disk cooling. We find that strongly cooled disks with a scale height ratio of H/R > 0.1 induce Rossby instability in compact, high-density rings. The trapped Rossby vortices generate vigorous coherent emission regardless of disk magnetization and BH spin. For BH mass of ~ 10 Msun, the GW spectrum peaks at ~100 Hz with some breadth due to various non-axisymmetric modes. The spectrum broadens further over time due to the disk's viscous spreading and the increasing circularization radius of the infalling gas. Weaker-cooled disks with H/R~0.3 form a low-density extended structure of spiral arms, resulting in a broader, lower-amplitude spectrum. If collapsar disks are strongly cooled, LVK could detect a few events annually, suggesting that some events may already be hidden in observed data. Third-generation GW detectors could detect dozens to thousands of collapsar disks annually, depending on the cooling strength. GWs from collapsar disks are ~2 orders of magnitude brighter with a substantially higher event rate than those expected from core-collapse supernovae, making them potentially the most promising burst-type GW class for detection in LVK and Cosmic Explorer. This highlights the importance of further exploration and modeling of GWs from collapsar disks, promising insights into the physics of collapsing stars.

3D GRMHD simulations of disk formation in a collapsing star

GW spectrum evolution - model C
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Logarithmic mass density (c.g.s.) - spin 0.8 (model C)
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Logarithmic mass density (c.g.s.) - spin 0.8 - 0-12.8 s
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Strain volume density for on-axis observers - spin 0.8
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Logarithmic mass density (c.g.s.) - spin 0.8 - precession
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Logarithmic mass density (c.g.s.) - spin 0.1
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Logarithmic mass density (c.g.s.) - spin 0.1 - artist version
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Logarithmic mass density (c.g.s.) - weak cooling
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Logarithmic mass density (c.g.s.) - mild cooling
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3D GRMHD simulations of disk formation in a BNS post-merger

Logarithmic mass density (c.g.s.)
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