Resonant Shattering Flares as Multimessenger Probes of Neutron Star Structure and Nuclear Symmetry

: (Alternative Format Thesis)

Abstract

Neutron stars (NSs) are stellar remnants which provide a fascinating environment in which to study fundamental nuclear physics. They contain matter on a macroscopic scale that is both cold and very dense, which are properties that terrestrial experiments are unable to achieve simultaneously. With the multimessenger detection of gravitational waves (GWs) and broad-band electromagnetic radiation from the binary NS merger GW170817, NS astronomy has moved into a new multimessenger era. The question is no longer whether we can observe such events, but what we may learn from them, both about NS structure as a whole and about its underlying nuclear physics. This thesis is focused on resonant shattering flares (RSFs), and what their coincident detection alongside GWs could tell us about NSs and the nuclear symmetry energy. RSFs are triggered when the NS crust-core interface asteroseismic mode (i-mode) is resonantly excited by the tidal field of the NS's binary partner and shatters the elastic solid NS crust. The coincident timing of GWs and a RSF during a multimessenger event would enable the measurement of the frequency at which this mode oscillates: a property which is highly sensitive to the composition of the NS crust. The nuclear symmetry energy describes the overall effect of the strong force's isospin (proton-to-neutron ratio)-dependences on the binding energy of bulk nucleonic matter. It is therefore of great interest for our understanding of fundamental physics.

Using a NS meta-model which relates a variable model of nuclear matter to NS structure, we begin by investigating the dependence of the i-mode frequency on parameters which describe the nuclear symmetry energy. We find that the i-mode frequency is highly dependent on these parameters and identify the cause of this relationship. Oscillations of the i-mode are restored by shear forces, making their frequency sensitive to the density-weighted shear speed within the NS crust. Shear speed in turn depends on the nuclear symmetry energy, as it determines the relative abundance and configuration of protons and neutrons within the crust. We also find that the i-mode frequency is insensitive to the properties of the NS core, setting it apart from more commonly measured NS properties such as their masses and radii.

We next investigate the observational prospects for RSFs. By modelling the emission of these flares as coming from collisions between fireball shells launched during resonance, we find that the prompt non-thermal gamma-ray emission may have luminosity up to a few times 10^48 erg/s, and that a broad-band afterglow could be produced. We also compute the expected rates of detectable RSFs using the BPASS population synthesis code, finding the rate of detectable RSFs to be ~0.0001-5 per year for BHNS mergers and ~0.0005-25 per year for NSNS mergers, with these bounds corresponding to different assumptions about the evolution of NS magnetic fields before merger. Observed short gamma-ray burst precursor flares are not incompatible with these findings, suggesting that they may be RSFs, which would mean that RSFs could be the most common detectable electromagnetic counterpart to GW detections of BHNS mergers.

While much effort is devoted to measuring the nuclear symmetry energy through NS observables, it is not known whether matter in the NS core remains hadronic or transitions to some exotic phase, meaning that bulk observables such as radii and tidal deformabilities may not provide reliable constraints on properties of nucleonic matter. The NS crust however can confidently be said to consist of nucleonic matter, meaning that RSFs and the i-mode are more reliable. Using a NS meta-model which consistently constructs NS crusts and outer cores from models of nucleonic matter parametrised by the symmetry energy, and which uses polytropic models for the inner cores to represent the uncertainty in the nature of matter there, we perform Bayesian analysis with a relatively uninformative prior and various combinations of an injected RSF and real astrophysical and nuclear data. We demonstrate that coincident timing of a RSF and GWs during a single binary NS inspiral would enable us to place constraints on the symmetry energy parameters that are competitive with those from current nuclear experiments. We also show that nuclear masses, RSFs and measurements of NS radii and tidal deformabilities constrain matter at different densities within the NS, providing complementary probes which show the strength of constraining nuclear matter at several different densities.

Furthering this, we investigate whether detecting multiple multimessenger events would improve the constraints we can place on the symmetry energy. Having multiple detections would allow us to probe the relationship between i-mode frequency and NS mass, but unlike the mass-radius relationship we find that this relationship is uninformative as the change in frequency with respect to mass is similar for all of our NS models. Statistical improvements are also found to be negligible, as the uncertainties in the inferred symmetry energy parameter values are dominated by their degeneracies. We therefore conclude that a single multimessenger RSF and GW event is required to fully leverage this powerful method for constraining the nuclear symmetry energy, and that these constraints will benefit from being combined with those from other sources, in order to break their degeneracies.

Overall, we find that multimessenger detections of RSFs and GWs could provide strong constraints on the nuclear symmetry energy, with only a single such detection being required for the constraints to be competitive with those from terrestrial experiments. As RSFs may already have been detected as the precursor flares observed before a fraction of short gamma-ray bursts, and upcoming GW observing runs promise increased sensitivity to binary NS mergers, such a detection may soon be obtained. By comparison to core-dependent observables such as NS mass and radius, we have shown that asteroseismic modes can allow us to probe matter within NSs at a wider range of densities, giving us more insight into the fundamental nuclear physics underpinning these compact objects.

Date of Award	13 Sept 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	David Tsang (Supervisor) & Hendrik Van Eerten (Supervisor)