The Gravitational Wave Signature of Core-Collapse Supernovae (0809.0695v2)

Abstract: We review the ensemble of anticipated gravitational-wave (GW) emission processes in stellar core collapse and postbounce core-collapse supernova evolution. We discuss recent progress in the modeling of these processes and summarize most recent GW signal estimates. In addition, we present new results on the GW emission from postbounce convective overturn and protoneutron star g-mode pulsations based on axisymmetric radiation-hydrodynamic calculations. Galactic core-collapse supernovae are very rare events, but within 3-5 Mpc from Earth, the rate jumps to 1 in ~2 years. Using the set of currently available theoretical gravitational waveforms, we compute upper-limit optimal signal-to-noise ratios based on current and advanced LIGO/GEO600/VIRGO noise curves for the recent SN 2008bk which exploded at ~3.9 Mpc. While initial LIGOs cannot detect GWs emitted by core-collapse events at such a distance, we find that advanced LIGO-class detectors could put significant upper limits on the GW emission strength for such events. We study the potential occurrence of the various GW emission processes in particular supernova explosion scenarios and argue that the GW signatures of neutrino-driven, magneto-rotational, and acoustically-driven core-collapse SNe may be mutually exclusive. We suggest that even initial LIGOs could distinguish these explosion mechanisms based on the detection (or non-detection) of GWs from a galactic core-collapse supernova.

• The paper examines core-collapse supernovae GW emissions, revealing that rotating collapse produces type-I waveforms potentially detectable by advanced LIGO.
• It evaluates postbounce dynamics, including rotational instabilities and protoneutron star pulsations, which critically influence the GW frequency bands.
• The study highlights that anisotropic neutrino emissions and convective overturn add complexity to the GW signals, refining future detection strategies.

Gravitational Wave Emissions from Core-Collapse Supernovae: An Overview

The paper authored by Christian D. Ott provides an extensive review of the gravitational wave (GW) emission processes anticipated during core-collapse supernovae (SNe), particularly focusing on the stages from stellar core collapse to the postbounce supernova evolution. The research examines the current state of modeling these GWs, presenting signal estimates from various processes including PNS pulsations and convection, and evaluates the potential detectability of such events with gravitational wave detectors like LIGO.

Key Processes and Phenomena

1. Rotating Core Collapse and Bounce: The literature on GW emissions from rotating core collapse is rich, owing to the dramatic changes in the quadrupole moment from rotating neutron stars at core bounce. Ott highlights the formation of PNSs post-collapse, noting that rapidly rotating stars with significant angular momentum can lead to strong GW emissions, detectable by future advanced detectors. Typically, GWs from rapidly spinning progenitors show type-I waveform signals and might be detectable by advanced LIGO if occurring within the local group.
2. Rotational Instabilities: The review discusses both dynamical and secular rotational instabilities, which may add complexity (and amplitude) to the GW signal from PNSs. Particularly, low-$\beta$ instabilities may emerge postbounce, generating emissions strongest in the lower-frequency band (around hundreds of Hz), potentially increasing the GW energy signature.
3. Postbounce Convection and SASI: Convective overturn in the postshock region is triggered by the negative entropy gradients, leading to GW emissions predominately in the low-frequency regime. Although the SASI further excites such convection, pinpointing GW characteristics from this process remains a challenge due to their stochastic nature.
4. Protoneutron Star Pulsations: Notably, the paper emphasizes the mechanoacoustic GW emissions from PNS $g$-mode pulsations, which play a pivotal role in the proposed acoustic mechanism of supernova explosions. The GW signals from PNS modes, generally peaking at higher frequencies (hundreds of Hz), could potentially be observed by advanced GW detectors and provide insights into the underlying physics of core-collapse supernovae.
5. Anisotropic Neutrino Emissions: Anisotropic emission of neutrinos also contributes to GWs, though these emissions are typically at lower frequencies, making their detection less probable with ground-based interferometers.
6. Other Emissions: The paper further explores GW contributions from magnetic fields, aspherical outflows, and potential postbounce transition to a black hole, suggesting these processes add layers to the rich GW tapestry observable from core-collapse supernovae.

Implications and Future Directions

The paper speculates that merely detecting or not detecting GWs from a nearby supernova could significantly constrain our understanding of supernova explosion mechanisms, indirectly indicating whether the neutrino mechanism, magneto-rotational mechanism, or the acoustic mechanism is predominant in these astronomical events. The mutual exclusivity of GW signatures from these mechanisms provides a compelling avenue for future research.

While initial LIGO detectors may only marginally observe such GWs within the Milky Way, the advent of advanced detectors heralds the potential for more detailed astronomical insights. Hence, this review underscores the importance of continued refinement and expansion of gravitational wave detection capabilities, positioning them as critical tools in astrophysics to unravel the complexities of stellar demise and the birth of neutron stars.

In summary, each potential GW emission mechanism reviewed by Ott presents unique observational signatures, with ongoing and future advancements in detector sensitivities promising pivotal contributions to the fundamental understanding of core-collapse supernovae and their role in the cosmic landscape.