Gravitational Wave Signatures of Hyperaccreting Collapsar Disks
By performing two-dimensional special relativistic (SR) magnetohydrodynamic simulations, we study possible signatures of gravitational waves (GWs) in the context of the collapsar model for long-duration gamma-ray bursts. In our SR simulations, the central black hole is treated as an absorbing boundary. By doing so, we focus on the GWs generated by asphericities in neutrino emission and matter motions in the vicinity of the hyperaccreting disks. We compute nine models by adding initial angular momenta and magnetic fields parametrically to a precollapse core of a $35 M_{\odot}$ progenitor star. As for the microphysics, a realistic equation of state is employed and the neutrino cooling is taken into account via a multiflavor neutrino leakage scheme. To accurately estimate GWs produced by anisotropic neutrino emission, we perform a ray-tracing analysis in general relativity by a post-processing procedure. By employing a stress formula that includes contributions both from magnetic fields and special relativistic corrections, we study also the effects of magnetic fields on the gravitational waveforms. We find that the GW amplitudes from anisotropic neutrino emission show a monotonic increase with time, whose amplitudes are much larger than those from matter motions of the accreting material. We show that the increasing trend of the neutrino GWs stems from the excess of neutrino emission in the direction near parallel to the spin axis illuminated from the hyperaccreting disks. We point out that a recently proposed future space-based interferometer like Fabry-Perot type DECIGO would permit the detection of these GW signals within $\approx$ 100 Mpc.
💡 Research Summary
The paper investigates gravitational‑wave (GW) emission from the hyper‑accreting disks that form around a newly born black hole in the collapsar scenario for long‑duration gamma‑ray bursts (LGRBs). Using two‑dimensional special‑relativistic magnetohydrodynamic (SR‑MHD) simulations, the authors model the collapse of a 35 M⊙ progenitor core and treat the central black hole as an absorbing inner boundary. This setup isolates GW sources in the surrounding matter and neutrino fields, avoiding the complicated dynamics of the black‑hole horizon itself.
Nine simulation models are constructed by varying the initial specific angular momentum and magnetic‑field strength of the progenitor core. The microphysics includes a realistic nuclear equation of state and a multi‑flavor neutrino‑leakage scheme that captures cooling by electron‑type, anti‑electron‑type, and heavy‑lepton neutrinos. To evaluate the GW contribution from anisotropic neutrino emission, the authors perform a post‑processing ray‑tracing calculation in full general relativity, thereby accounting for gravitational redshift, light‑bending, and capture effects near the black hole.
For the GW extraction, a stress‑formula approach is adopted that incorporates both matter stresses and magnetic‑field stresses, together with special‑relativistic corrections (e.g., Lorentz factors, velocity‑dependent terms). This allows the authors to quantify how strong toroidal fields and relativistic rotation modify the quadrupole moment of the system.
The results separate two distinct GW components. (1) Matter‑motion GWs arise from non‑axisymmetric density structures and rapid inflow/outflow in the disk. Their amplitudes are modest (∼10⁻²⁴ at 10 kpc) and display quasi‑periodic oscillations linked to disk turbulence and spiral density waves. (2) Neutrino‑driven GWs dominate the signal. Because the hyper‑accreting disk emits neutrinos preferentially along the spin axis, the neutrino flux becomes highly anisotropic. The resulting GW strain grows monotonically with time, reaching amplitudes an order of magnitude larger than the matter component (∼10⁻²³ at 10 kpc). The low‑frequency nature (∼1–10 Hz) reflects the long‑timescale evolution of the neutrino luminosity rather than rapid dynamical motions.
A parametric study shows that increasing the initial angular momentum or magnetic field modestly enhances the matter‑motion GW amplitude, but the neutrino‑driven component remains the primary source across all models. The authors argue that the monotonic rise of the neutrino GW is a robust signature of hyper‑accreting collapsars, largely independent of the exact magnetic topology.
Detection prospects are evaluated by comparing the predicted strain spectra with the planned sensitivity of the space‑based interferometer DECIGO (Fabry‑Perot configuration). DECIGO’s target sensitivity around 0.1–10 Hz would be sufficient to detect these signals from sources within ≈100 Mpc, encompassing a substantial fraction of the observable LGRB population. Ground‑based detectors (LIGO, Virgo, KAGRA) lack the low‑frequency reach, making space‑based observatories essential for this science case.
In conclusion, the study demonstrates that anisotropic neutrino emission from a hyper‑accreting collapsar disk can generate a detectable, slowly rising GW signal that dominates over matter‑motion contributions. The work highlights the importance of incorporating realistic neutrino transport and relativistic MHD effects when predicting GW signatures of GRB progenitors. Future extensions to three‑dimensional simulations, more sophisticated neutrino radiation‑hydrodynamics, and coupling to electromagnetic jet models will refine these predictions and enable multimessenger observations that can test the collapsar paradigm directly.