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ABSTRACT Neutron stars in astrophysical binary systems represent exciting sources for multimessenger astrophysics. A potential source of electromagnetic transients from compact binary systems is the neutron star ocean, the external fluid layer encasing a neutron star. We present a groundwork study into tidal waves in neutron star oceans and their consequences. Specifically, we investigate how oscillation modes in neutron star oceans can be tidally excited during compact binary inspirals and parabolic encounters. We find that neutron star oceans can sustain tidal waves with frequencies between 0.01 and 20 Hz. Our results suggest that tidally resonant neutron star ocean waves may serve as a never-before studied source of precursor electromagnetic emission prior to neutron star–black hole and binary neutron star mergers. If accompanied by electromagnetic flares, tidally resonant neutron star ocean waves, whose energy budget can reach 1046 erg, may serve as early warning signs (≳1 min before merger) for compact binary mergers. Similarly, excited ocean tidal waves will coincide with neutron star parabolic encounters. Depending on the neutron star ocean model and a flare emission scenario, tidally resonant ocean flares may be detectable by Fermi and Nuclear Spectroscopic Telescope Array (NuSTAR) out to ≳100 Mpc with detection rates as high as ∼7 yr−1 for binary neutron stars and ∼0.6 yr−1 for neutron star–black hole binaries. Observations of emission from neutron star ocean tidal waves along with gravitational waves will provide insight into the equation of state at the neutron star surface, the composition of neutron star oceans and crusts, and neutron star geophysics.
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Using equation of state constraints to classify low-mass compact binary mergers
Compact objects observed via gravitational waves are classified as black holes or neutron stars primarily based on their inferred mass with respect to stellar evolution expectations. However, astrophysical expectations for the lowest mass range, ≲1.2𝑀⊙, are uncertain. If such low-mass compact objects exist, ground-based gravitational wave detectors may observe them in binary mergers. Lacking astrophysical expectations for classifying such observations, we go beyond the mass and explore the role of tidal effects. We evaluate how combined mass and tidal inference can inform whether each binary component is a black hole or a neutron star based on consistency with the supranuclear-density equation of state. Low-mass neutron stars experience a large tidal deformation; its observational identification (or lack thereof) can therefore aid in determining the nature of the binary components. Using simulated data, we find that the presence of a sub-solar mass neutron star (black hole) can be established with odds ∼100∶1 when two neutron stars (black holes) merge and emit gravitational waves at signal-to-noise ratio ∼20. For the same systems, the absence of a black hole (neutron star) can be established with odds ∼10∶1. For mixed neutron star-black hole binaries, we can establish that the system contains a neutron star with odds ≳5∶1. Establishing the presence of a black hole in mixed neutron star-black hole binaries is more challenging, except for the case of a ≲1𝑀⊙ black hole with a ≳1𝑀⊙ neutron star companion. On the other hand, classifying each individual binary component suffers from an inherent labeling ambiguity.
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- Award ID(s):
- 2207758
- PAR ID:
- 10613681
- Publisher / Repository:
- Physical Review D
- Date Published:
- Journal Name:
- Physical Review D
- Volume:
- 110
- Issue:
- 6
- ISSN:
- 2470-0010
- Subject(s) / Keyword(s):
- compact binary mergers gravitational waves nuclear equation of state
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1103/PhysRevD.110.063014
