Convection signatures in early-time gravitational waves from core-collapse supernovae

TL;DR

This paper investigates how prompt convection shapes the early-time gravitational-wave signal from core-collapse supernovae. Using 29 axisymmetric simulations of a $16.5\,M_\odot$ red supergiant with varied rotation and magnetic-field configurations, the authors apply ensemble empirical mode decomposition to separate core and convection contributions, finding that the first six intrinsic mode functions dominate the early waveform. A persistent low-frequency component tied to convection emerges across all models, and strong magnetic fields can slow core rotation and modify resonance with epicyclic modes, potentially triggering jets and a memory offset in the GW signal. The work demonstrates that prompt convection can yield amplitudes comparable to or larger than the bounce signal, with implications for detectability by current and next-generation detectors and for inferring interior core properties from GW observations.

Abstract

Gravitational waves emitted from core-collapse supernova explosions are critical observables for extracting information about the dynamics and properties of both the progenitor and the post-bounce~evolution of the system. They are prime targets for current interferometric searches and represent a key milestone for the capabilities of next-generation interferometers. This study aims to characterize how the gravitational waveform associated with prompt stellar convection depends on the rotational rate and magnetic field topology of the progenitor star. We carry out a series of axisymmetric simulations of a $16.5\,\mathrm{M}_\odot$ red supergiant with five configurations of initial magnetic fields and varying degrees of initial rotation. We then analyze the contribution of early-time convection and the proto-neutron star core to the waveform using ensemble empirical mode decomposition, alongside spectral and Fourier analyses, to facilitate comparison and interpretation of the results. Our simulations reveal that early post-bounce gravitational waves signals are dominated by the first six intrinsic mode functions, with variations due to rotation and magnetic fields influencing the signal strength. Strong magnetic fields decelerate core rotation, affecting mode excitation. Regardless of the initial rotation, convection consistently drives a low-frequency mode that lasts throughout the evolution. Additionally, our results show that the bounce signal is not consistently the strongest component of the waveform. Instead, we find that prompt convection generates a post-bounce signal of comparable or even greater amplitude.

Figures (11)

• Figure 1: Evolution of amplitude for models s0.0-1 (A), s1.0-1 (B), and s2.0-1 (C). Panels from top to bottom show the time evolution of the amplitude for the whole simulation domain, core, sonic envelope, and outer layers. Finally, the bottom row shows the space-time evolution of $\mathcal{D}h(t, r)$. Dashed, solid, and dotted lines represent the core, sonic envelope, and average radius, respectively. Grey shades mark regions where $N^2<0$, i.e. approximately, regions where convection takes place. Numbered ellipses identify modes in each region (see text).
• Figure 2: Time evolution of the total strain and the six sIMF for model s1.2-1. The dashed blue line on the first panel represents the full strain extracted on the whole simulation domain, and the solid red line the strain as a sum of the sIMF.
• Figure 3: Panels A and B show the difference between the highest and lowest points of the signal, $\Delta h$, against the rotational kinetic energy to gravitational energy ratio for bounce and post-bounce signals, and for core ($\Delta h_{\rm core}$) and convection ($\Delta h_{\rm conv}$) signals, respectively, for models with magnetic field configuration 1. Panels C and D display the frequencies of the previous intensity peaks in relation to the same quantity on the x axis for the same pool of models as in the previous two panels.
• Figure 4: Panel A: Snapshot of the entropy at [15]ms after bounce, showing the convective bubbles surrounding the core. Panel B: Snapshot of the radial velocity at [65.7]ms, showing the propagation (here at $\unit[50]{km}$) of a wave generated by oscillations of the core and propagating outwards. Both snapshots are for model s1.0-1.
• Figure 5: amplitude (top row) and corresponding spectrograms for models s0.4-1 (panel A, representative of the class of slowly rotating models), s0.9-1 (B, prototype of intermediately rotating cases) and s2.4-1 (C, representative of the class of fast-rotating models), calculated with a time window of $\unit[10]{ms}$. The red line corresponds to the convection frequency (Equation (\ref{['eq:conv_f']})) at the sonic envelope surface, the blue lines show the epicyclic frequency (solid) and its first overtone (dotted) in the $\pi/4$ direction at the core surface, while the green line is the fundamental quadrupolar mode frequency ($^2f$) computed with the quasi-universal relation in TorresForne19b.