Gravitational Waves from Cooled Accretion Disks

In LIGO's sight? Vigorous Coherent Gravitational Waves from Cooled Collapsar Disks (2024)

We present the first numerical study of gravitational waves (GWs) from collapsar disks, using state-of-the-art 3D general-relativistic magnetohydrodynamic simulations of collapsing stellar cores. These simulations incorporate a fixed Kerr metric for the central black hole (BH) and employ simplified prescriptions for disk cooling. We find that cooled disks with an expected scale height ratio of H/R>0.1 at ~10 gravitational radii 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 shifts toward lower frequencies as the disk viscously spreads and the circularization radius of the infalling gas increases. Weaker-cooled disks with H/R~0.3 form a low-density extended structure of spiral arms, resulting in a broader, lower-amplitude spectrum. Assuming an optimistic detection threshold with a matched-filter SNR of 20 and a rate similar to supernovae Ib/c, LIGO-Virgo-KAGRA (LVK) could detect ~1 event annually, suggesting that GW events may already be hidden in observed data. Third-generation GW detectors could detect dozens to hundreds of collapsar disks annually, depending on the cooling strength and the disk formation rate. The GW amplitudes from collapsar disks are >100 higher with a substantially greater 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.