Core-Collapse Supernovae and their Gravitational Wave Signals: The Status of Theory and Modeling

Abstract

The detection of gravitational waves from a core-collapse supernova in the Milky Way or its vicinity represents a unique opportunity to probe the inner workings of these explosions. In this review, I briefly summarize our current understanding of the supernova explosion mechanism and then outline the physical processes that shape the supernova gravitational wave signal. The review highlights how the various components of the signal have the potential to constrain the progenitor rotation, the proto-neutron star structure, the nuclear equation of state, the nature of hydrodynamic instabilities, and the violence of turbulent motions in the supernova core. I also highlight some open questions and uncertainties in the theory of supernova gravitational wave astronomy as well as challenges for further progress. Specifically, there is a need to develop large model databases, systematic uncertainty quantification and methods for evidence assessment to prepare for multi-messenger observations from a Galactic supernova.

Figures (5)

• Figure 1: Sketch of the structure of the supernova core highlighting the instabilities and oscillatory motions that occur in different regions. Convective motions are denoted by circular arrows and oscillatory motions are denoted by short black arrows. At the center of the proto-neutron star, there is a convectively stable low-entropy core (blue), surrounded by the convective mantle (indigo). Further out, there is a convectively stable atmosphere (brick red). A fraction of neutrinos from the atmosphere and the outer layers of the mantle deposit their energy in the heating region (light red). The heating region may be unstable to convection due to neutrino energy deposition, or to SASI shock oscillations. The buoyancy frequency is particularly high in the proto-neutron star atmosphere and the steep entropy gradient in the outer region of the core (blue annulus).
• Figure 2: Illustration of the typical structure of the core-collapse supernova GW signal from a relativistic 3D simulations of a non-rotating $20\,\mathrm{M}_\odot$ star yoon_17. The plot shows the distance-normalized amplitude $A_\times$ for one observer direction. The first convection is a signal from the shock and proto-neutron star oscillations triggered by prompt convection. This is followed by a more quiet period, until the high-frequency signal develops. The inset shows that this signal has a clear quasi-periodic structure despite the stochastic amplitude modulations. After the explosion, asymmetric shock expansion produces a slowly growing offset in the GW signal. Note that the neutrino memory signal, which typically dominates the matter memory signal, is not shown here.
• Figure 3: Typical spectrogram of a predicted core-collapse supernova GW signal, from the same $20\,\mathrm{M}_\odot$ model as shown in Figure \ref{['fig:gw_template']}. The most prominent feature in the spectrogram is the high-frequency ramp-up signal, which rises from a few hundred Hz to above 1 kHz in this simulation. There is some haze of $p$-modes above this dominant emission band, in particular when the GW amplitudes are highest. The signal from prompt convection contributes some power at frequencies of $\sim 100\,\mathrm{Hz}$ at early times.
• Figure 4: Predicted GW amplitude for a non-exploding, SASI-dominated supernova model of an $18\,\mathrm{M}_\odot$ star for one observer direction. Note the distinctly different periodicity from the high-frequency signal in Figure \ref{['fig:gw_template']} that can be seen in the inset.
• Figure 5: Spectrogram corresponding to the waveform of the SASI-dominated $18\,\mathrm{M}_\odot$ model in Figure \ref{['fig:sasi_ampl']}. The high-frequency signal is present from about $0.1\,\mathrm{s}$ to $0.3\,\mathrm{s}$ after bounce. In addition there is a low-frequency signal that rises from $100\texttt{-}200\,\mathrm{Hz}$ initially to more than $200\,\mathrm{Hz}$ after $0.4\,\mathrm{s}$.