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Circular polarization of gravitational waves from magnetorotational supernovae

Shota Shibagaki, Tomoya Takiwaki, Kei Kotake, Takami Kuroda, Tobias Fischer

Abstract

Context. Gravitational waves (GW) provide a unique probe of the explosion mechanism of massive stars and the evolution of nascent proto-neutron stars (PNS). Magnetorotational explosions are one of the promising non-canonical core-collapse supernova scenarios, possibly linked to magnetar formation and energetic supernova explosions. However, the GW signatures of such events remain incompletely understood presently. Aims. This study investigates the origin and nature of gravitational-wave polarization arising from a magnetorotational core-collapse model and examines its potential detectability by current gravitational-wave observatories. Methods. We perform a three-dimensional simulation of general-relativistic magnetohydrodynamics of a rapidly rotating, strongly magnetized 20 M$_\odot$ progenitor, including multi-energy neutrino transport. The polarization states of the GW signals are analyzed with Stokes parameters. Results. We find that strong circular polarization emerges along the rotation axis during the early post-bounce phase (<230 ms after core bounce). The characteristic GW spectrum peaks at ~90 Hz, consistent with the emission at twice the local angular velocity (~45 Hz) around the PNS surface at cylindrical radii of ~50 km. These features are attributed to the low-T/|W| instabilities and non-axisymmetric motions near the PNS, rather than to the magnetohydrodynamic jets themselves. The polarization signals lie within the sensitivity bands of current GW detectors. Conclusions. Our study demonstrates that models launching magnetorotationally driven jets can produce circularly polarized GW signals originating from the inner PNS region. This provides an observational signature that complements previous findings from non-magnetized rotating models. Thus, our novel findings establish that the GW polarization is a promising diagnostic of non-canonical core-collapse supernovae.

Circular polarization of gravitational waves from magnetorotational supernovae

Abstract

Context. Gravitational waves (GW) provide a unique probe of the explosion mechanism of massive stars and the evolution of nascent proto-neutron stars (PNS). Magnetorotational explosions are one of the promising non-canonical core-collapse supernova scenarios, possibly linked to magnetar formation and energetic supernova explosions. However, the GW signatures of such events remain incompletely understood presently. Aims. This study investigates the origin and nature of gravitational-wave polarization arising from a magnetorotational core-collapse model and examines its potential detectability by current gravitational-wave observatories. Methods. We perform a three-dimensional simulation of general-relativistic magnetohydrodynamics of a rapidly rotating, strongly magnetized 20 M progenitor, including multi-energy neutrino transport. The polarization states of the GW signals are analyzed with Stokes parameters. Results. We find that strong circular polarization emerges along the rotation axis during the early post-bounce phase (<230 ms after core bounce). The characteristic GW spectrum peaks at ~90 Hz, consistent with the emission at twice the local angular velocity (~45 Hz) around the PNS surface at cylindrical radii of ~50 km. These features are attributed to the low-T/|W| instabilities and non-axisymmetric motions near the PNS, rather than to the magnetohydrodynamic jets themselves. The polarization signals lie within the sensitivity bands of current GW detectors. Conclusions. Our study demonstrates that models launching magnetorotationally driven jets can produce circularly polarized GW signals originating from the inner PNS region. This provides an observational signature that complements previous findings from non-magnetized rotating models. Thus, our novel findings establish that the GW polarization is a promising diagnostic of non-canonical core-collapse supernovae.
Paper Structure (7 sections, 3 equations, 5 figures)

This paper contains 7 sections, 3 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Results
  3. Conclusions
  4. Numerical setup
  5. Simulation code and initial condition
  6. Gravitational wave analysis
  7. Emission Region

Figures (5)

  • Figure 1: 3D entropy plot for MHD jets (green region) at $t_{\mathrm{pb}}=100$ ms. The red and blue region around the center is the normalized density deviation from the angle-averaged density on the equatorial plane. The central light yellow sphere is the PNS.
  • Figure 2: GW strains of plus (red solid) and cross (blue dashed) modes (top), and spectrogram of its $V$-mode power spectrum (bottom) seen along the pole at a source distance of 10 kpc.
  • Figure 3: Amplitude spectral densities of the I-mode (red solid) and V-mode (blue dotted) GW strains for a polar observer at 10 kpc. Also shown are the shaded sensitivity bands of current detectors: LIGO (light blue), Virgo (green), and KAGRA (magenta), bounded by the achieved O4 and target O5 sensitivities (dashed).
  • Figure 4: Color map of normalized $m=1$ mode amplitude for density deviation $\rho_m / \rho_0$ as a function of time and cylindrical radius.
  • Figure 5: Relative contributions from each rectangular box defined by $|z|<z_0$ to the total $V$-mode GW spectrogram (seen from the pole). We color only the $t-f$ domains with $|V_{\rm tot}|>10^{-46}$. The $z_0$ is denoted on the upper right corner of each panel. As a reference, the bottom right panel shows the total $V$-mode GW spectrogram.