Identifying a first-order phase transition in neutron star mergers through gravitational waves (1809.01116v2)

Abstract: We identify an observable imprint of a first-order hadron-quark phase transition at supranuclear densities on the gravitational-wave (GW) emission of neutron star mergers. Specifically, we show that the dominant postmerger GW frequency f_peak may exhibit a significant deviation from an empirical relation between f_peak and the tidal deformability if a strong first-order phase transition leads to the formation of a gravitationally stable extended quark matter core in the postmerger remnant. A comparison of the GW signatures from a large, representative sample of microphysical, purely hadronic equations of state indicates that this imprint is only observed in those systems which undergo a strong first-order phase transition. Such a shift of the dominant postmerger GW frequency can be revealed by future GW observations, which would provide evidence for the existence of a strong first-order phase transition in the interior of neutron stars.

• The paper identifies a first-order phase transition from hadronic to quark matter by detecting deviations in the dominant postmerger gravitational wave frequency.
• It employs temperature-dependent hadron-quark hybrid EOS models, notably the DD2F-SF class, to simulate and distinguish merger outcomes.
• The study provides observational criteria that enable gravitational wave detectors to constrain neutron star equations of state and probe QCD transitions.

Identifying a First-Order Phase Transition in Neutron Star Mergers through Gravitational Waves

The research by Bauswein et al. presents a significant advancement in the paper of neutron star mergers and their implications for understanding quantum chromodynamics (QCD) phase transitions. This paper identifies a distinct gravitational wave signature that could signal a first-order phase transition from hadronic to quark matter within neutron star mergers. This paper correlates this transition with deviations in postmerger gravitational wave frequencies, offering an observable phenomenon that could be crucial in interpreting astrophysical data from such catastrophic cosmic events.

Summary of Findings

The paper emphasizes the detection of a strong first-order phase transition in neutron stars using gravitational wave (GW) signatures. The authors demonstrate that the dominant post-merger GW frequency, denoted as $f_{\text{peak}}$, may exhibit notable deviations from expected empirical relationships with the tidal deformability $\Lambda$ of the neutron star. Specifically, these deviations occur when a dense quark matter core forms following the merger, leading to significant changes in certain types of equations of state (EOS) reflective of such a transition.

The research employs a variety of temperature-dependent, microscopic hadron-quark hybrid EOS models, specifically, the DD2F-SF class among others, to simulate neutron star mergers. These models are crucial in distinguishing between different EOS with and without phase transitions by focusing on observables like $f_{\text{peak}}$ and $\Lambda$. Among the findings, a key result is that a strong first-order phase transition leads to noticeable increases in the dominant postmerger frequency, distinct from the expected values for purely hadronic EOS. The DD2F-SF models, in particular, demonstrated frequencies in the range of 3.3 kHz to 3.7 kHz, contrasted with a purely hadronic peak frequency of around 3.1 kHz.

Practical and Theoretical Implications

The implications of these findings are multifaceted:

1. Observational Evidence for QCD Phase Transitions: This paper facilitates the potential identification of a first-order hadron-quark phase transition in neutron star environments. Such a phase transition is a fundamental prediction of QCD under high-density conditions, an understanding of which remains ambiguous due to limitations in experimental access to such extremes.
2. Gravitational Wave Astronomy Development: The results accentuate particular measurements crucial for gravitational wave astronomy, advancing it as a tool for probing fundamental nuclear physics. It also underscores the role of next-gen detectors that can achieve the high precision required to detect these deviations.
3. Constraints on Neutron Star Models: The correlation between postmerger signatures and specific EOS models highlights the critical need for precise modeling in predicting neutron star structures, particularly hybrid star configurations with quark cores.
4. Consistency and Differentiation among EOS Models: The work delineates how first-order phase transitions in EOS models can be differentiated using GW signals, with significant potential for constraining various proposed nuclear matter EOS under high-density conditions.

Future Directions

The paper suggests several areas for future research including:

• Expanding EOS Models: The continuation of this research requires analyzing a broader class of EOS models to determine the generality of the observed GW signatures.
• Multimessenger Astronomy: Integrating electromagnetic counterparts and neutrino detections will enhance understanding and provide complementary data validating the nature of these phase transitions.
• Higher-Order Phase Transition Research: Consideration of softer transitions and other complexities in EOS is pivotal to refining current models and widening the understanding of matter under extreme conditions.

In conclusion, this research by Bauswein et al. provides a clear methodology for leveraging gravitational waves to potentially observe a first-order phase transition in neutron stars, bridging the gap between theoretical predictions of QCD and observable astrophysical phenomena. It enhances neutron star modeling, offers pathways for future detector sensitivity calibrations, and unravels new dimensions of multimessenger astronomy.