arXiv subjects

N. Andersson

Publications and source records attributed to N. Andersson.

At least 19 recordsLinked to original sources

arXiv2026

Action based approach to dissipative relativistic fluid systems

We develop an action principle for a relativistic two-fluid system with dissipation. The specific constituents of the model - which serves as a proof of principle - are particles and entropy. The linchpin of the action is the assertion that a given flux is dissipative if its covariant divergence is non-zero. For our model, the particle flux is taken to be conservative while the entropy flux is dissipative. This allows for a "top-down" approach where the general question is geometric. Previous work has shown that new terms (the proper time derivative of matter space "metrics") must be included in the Lagrangian in order to produce equations of motion with terms representing bulk and shear viscosity. In addition to including these terms we show that further terms - interpreted as velocities - can be included. The new action-based model recovers known relativistic formulations of the Cattaneo equation, which results in causal heat propagation. We further advance our understanding by exploring the single-fluid limit by locking the entropy four-velocity to that of the matter component. This reduces the system to a single field equation along with a constraint equation. We show that this constraint leads to a dynamical extension of the standard Tolman red-shift condition. Finally, we provide three example actions (of increasing complexity) which demonstrate that the model is able to reproduce (in the single-fluid limit) the anticipated terms from the relativistic Navier-Stokes equations. In the general case, the action based approach allows for a much richer structure, which may be relevant for realistic models of non-linear dissipative relativistic fluid systems.

gr-qc↗

arXiv2025

Dynamical neutron-star tides: The signature of a mode resonance

Motivated by future opportunities in gravitational-wave astronomy and the ongoing effort to constrain physics under extreme conditions, we consider the signature of individual mode resonances excited during the inspiral of binary systems involving neutron stars. Specifically, we quantify how each resonant mode contributes to the effective (frequency-dependent) tidal deformability. The resonant solution is shown to be accurately represented by a new closed-form approximation, which sheds light on the involved phenomenology, and which should be useful for the development of precise waveform models and future parameter extraction efforts.

gr-qc↗

arXiv2025

Interface modes in inspiralling neutron stars: A gravitational-wave probe of first-order phase transitions

At the extreme densities in neutron stars, a phase transition to deconfined quark matter is anticipated. Yet masses, radii and tidal deformabilities offer only indirect measures of a first-order phase transition, requiring many detections to resolve or being ineffective observables if the discontinuity exists at lower densities. We report on a smoking-gun gravitational-wave signature of a first-order transition: the resonant tidal excitation of an interface mode. Using relativistic perturbation theory with an equation-of-state family informed by chiral effective field theory, we show that such a resonance may be detectable with next-generation interferometers and possibly already with LIGO A+ for sufficiently loud events.

gr-qc↗

arXiv2024

Problematic systematics in neutron-star merger simulations

Next-generation gravitational-wave instruments are expected to constrain the equation of state of dense nuclear matter by observing binaries involving neutron stars. We highlight a problematic systematic error in finite-temperature merger simulations, where shock heating associated with the neutron-star surface gives rise to elevated temperatures. We demonstrate the severe implications of this artificial heating by computing static and dynamical tidal parameters for neutron stars immersed in simulation temperature profiles. The induced systematic errors must be addressed if we want to build robust gravitational-wave signal models for neutron-star, or indeed neutron star-black hole, binaries.

gr-qc↗

arXiv2024

Covariant approach to relativistic large-eddy simulations: Lagrangian filtering

We present a proof-of-principle implementation of the first fully covariant filtering scheme applied to relativistic fluid turbulence. The filtering is performed with respect to special observers, identified dynamically as moving with the "bulk of the flow". This means that filtering does not depend on foliations of spacetime but rather on the intrinsic fibration traced out by the observers. The covariance of the approach means that the results may be lifted into an arbitrary, curved spacetime. This practical step follows theoretical work showing that the residuals introduced by filtering a fine-scale ideal fluid can be represented by a non-ideal fluid prescription at the coarse scale. We interpret such non-ideal terms using a simple first-order gradient model, which allows us to extract effective turbulent viscosities and conductivity. A statistical regression on these terms shows that the majority of their variation may be explained by the thermodynamic properties of the filtered fluid and invariants of its flow, such as the shear and vorticity. This serves as a validation of the method and enables us to fit a functional, power-law form for the non-ideal coefficients -- an approach that may be used practically to give a sub-grid closure model in large-eddy simulations.

astro-ph.HE↗

arXiv2023

Local magneto-shear instability in Newtonian gravity

The magneto-rotational instability (MRI) - which is due to an interplay between a sheared background and the magnetic field - is commonly considered a key ingredient for developing and sustaining turbulence in the outer envelope of binary neutron star merger remnants. To assess whether (or not) the instability is active and resolved, criteria originally derived in the accretion disk literature - thus exploiting the symmetries of such systems - are often used. In this paper we discuss the magneto-shear instability as a truly local phenomenon, relaxing common symmetry assumptions on the background on top of which the instability grows. This makes the discussion well-suited for highly dynamical environments such as binary mergers. We find that - although this is somewhat hidden in the usual derivation of the MRI dispersion relation - the instability crucially depends on the assumed symmetries. Relaxing the symmetry assumptions on the background we find that the role of the magnetic field is significantly diminished, as it affects the modes' growth but does not drive it. This suggests that we should not expect the standard instability criteria to provide a faithful indication/diagnostic of what "is actually going on" in mergers. We conclude by making contact with a suitable filtering operation, as this is key to separating background and fluctuations in highly dynamical systems.

astro-ph.HE↗

arXiv2023

The impact of nuclear reactions on the neutron-star g-mode spectrum

Mature neutron stars are expected exhibit gravity g-modes due to stratification caused by varying composition. These modes are affected by nuclear reactions, leading to complex (damped) mode frequencies and the suppression of high order g-modes, in contrast with the common non-dissipative analysis which leads to an infinite g-mode spectrum. Focusing on the transition between the fast and slow reaction regimes, we examine the impact of nuclear reactions on the g-mode spectrum. The general framework for the analysis is presented along with sample numerical results for the BSk21 equation of state and the standard Urca reactions.

astro-ph.HE↗

arXiv2023

Modelling Neutron-Star Ocean Dynamics

We re-visit the calculation of mode oscillations in the ocean of a rotating neutron star, which may be excited during thermonuclear X-ray bursts. Our present theoretical understanding of ocean modes relies heavily on the traditional approximation, commonly employed in geophysics. The approximation elegantly decouples the radial and angular sectors of the perturbation problem by neglecting the vertical contribution from the Coriolis force. However, as the implicit assumptions underlying it are not as well understood as they ought to be, we examine the traditional approximation and discuss the associated mode solutions. The results demonstrate that, while the approximation may be appropriate in certain contexts, it may not be accurate for rapidly rotating neutron stars. In addition, using the shallow-water approximation, we show analytically how the solutions that resemble r-modes change their nature in neutron-star oceans to behave like gravity waves. We also outline a simple prescription for lifting Newtonian results in a shallow ocean to general relativity, making the result more realistic.

astro-ph.HE↗

arXiv2022

Angular analysis of $D^0 \to π^+π^-μ^+μ^-$ and $D^0 \to K^+K^-μ^+μ^-$ decays and search for $CP$ violation

The first full angular analysis and an updated measurement of the decay-rate $CP$ asymmetry of the $D^0 \to π^+π^-μ^+μ^-$ and $D^0 \to K^+K^-μ^+μ^-$ decays are reported. The analysis uses proton-proton collision data collected with the LHCb detector at centre-of-mass energies of 7, 8 and 13 TeV. The data set corresponds to an integrated luminosity of 9 fb$^{-1}$. The full set of $CP$-averaged angular observables and their $CP$ asymmetries are measured as a function of the dimuon invariant mass. The results are consistent with expectations from the standard model and with $CP$ symmetry.

hep-ex↗

arXiv2022

Formulating bulk viscosity for neutron star simulations

In order to extract the precise physical information encoded in the gravitational and electromagnetic signals from powerful neutron-star merger events, we need to include as much of the relevant physics as possible in our numerical simulations. This presents a severe challenge, given that many of the involved parameters are poorly constrained. In this paper we focus on the role of nuclear reactions. Combining a theoretical discussion with an analysis connecting to state-of-the-art simulations, we outline multiple arguments that lead to a reactive system being described in terms of a bulk viscosity. The results demonstrate that in order to properly account for nuclear reactions, future simulations must be able to handle different regimes where rather different assumptions/approximations are appropriate. We also touch upon the link to models based on the large-eddy-strategy required to capture turbulence.

astro-ph.HE↗

arXiv2022

Tests of lepton universality using $B^0\to K^0_S \ell^+ \ell^-$ and $B^+\to K^{*+} \ell^+ \ell^-$ decays

Tests of lepton universality in $B^0\to K^0_S \ell^+ \ell^-$ and $B^+\to K^{*+} \ell^+ \ell^-$ decays where $\ell$ is either an electron or a muon are presented. The differential branching fractions of $B^0\to K^0_S e^+ e^-$ and $B^+\to K^{*+} e^+ e^-$ decays are measured in intervals of the dilepton invariant mass squared. The measurements are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of $9\, \mathrm{fb}^{-1}$. The results are consistent with the Standard Model and previous tests of lepton universality in related decay modes. The first observation of $B^0 \to K^0_S e^+ e^-$ and $B^+ \to K^{*+} e^+ e^-$ decays is reported.

hep-ex↗

arXiv2021

The physics of non-ideal general relativistic magnetohydrodynamics

We consider a framework for non-ideal magnetohydrodynamics in general relativity, paying particular attention to the physics involved. The discussion highlights the connection between the microphysics (associated with a given equation of state) and the global dynamics (from the point of view of numerical simulations), and includes a careful consideration of the assumptions that lead to ideal and resistive magnetohydrodynamics. We pay particular attention to the issue of local charge neutrality, which tends to be assumed but appears to be more involved than is generally appreciated. While we do not resolve all the involved issues, we highlight how some of the assumptions and simplifications may be tested by simulations. The final formulation is consistent, both logically and physically, preparing the ground for a new generation of models of relevant astrophysical scenarios.

astro-ph.HE↗

arXiv2021

Modelling neutron star mountains in relativity

Rapidly spinning, deformed neutron stars have long been considered potential gravitational-wave emitters. However, so far only upper limits on the size of the involved quadrupole deformations have been obtained. For this reason, it is pertinent to ask how large a mountain can be before the neutron star crust fractures. This is the question we consider in this paper, which describes how mountains can be calculated in relativistic gravity. Formally, this is a perturbative calculation that requires a fiducial force to source the mountain. Therefore, we consider three simple examples and increase their deforming amplitudes until the crust yields. We demonstrate how the derived mountains depend on the equation of state by considering a range of models obtained from chiral effective field theory. We find that the largest mountains depend sensitively on both the mechanism that sources them and the nuclear-matter equation of state.

astro-ph.HE↗

arXiv2021

A multifluid perspective on multimessenger modelling

This brief review introduces the notion of a relativistic multifluid system -- a multi-component system with identifiable relative flows -- and outlines a set of models for scenarios relevant for different astronomical observation channels. The specific problems used to illustrate the key principles include superfluid hydrodynamics (with relevance for radio and x-ray pulsar timing and gravitational-wave searches), heat flow (connecting to the problem of neutron star cooling and associated x-ray observations) and the coupling between matter and electromagnetism (linking to explosive phenomena like gamma-ray bursts and more subtle issues like the long-term evolution of a neutron star's magnetic field). We also comment on the coupling between matter and radiation, for which the multifluid approach would seem less appropriate. The main motivation of the survey is to illustrate less familiar aspects that come into play in multifluid problems, establish the relevant "language" and provide a platform for more detailed work on these issues.

gr-qc↗

arXiv2021

A gravitational-wave perspective on neutron-star seismology

We provide a bird's-eye view of neutron-star seismology, which aims to probe the extreme physics associated with these objects, in the context of gravitational-wave astronomy. Focussing on the fundamental mode of oscillation, which is an efficient gravitational-wave emitter, we consider the seismology aspects of a number of astrophysically relevant scenarios, ranging from transients (like pulsar glitches and magnetar flares), to the dynamics of tides in inspiralling compact binaries and the eventual merged object and instabilities acting in isolated, rapidly rotating, neutron stars. The aim is not to provide a thorough review, but rather to introduce (some of) the key ideas and highlight issues that need further attention.

gr-qc↗

arXiv2021

A superfluid perspective on neutron star dynamics

As mature neutron stars are cold (on the relevant temperature scale), one has to carefully consider the state of matter in their interior. The outer kilometer or so is expected to freeze to form an elastic crust of increasingly neutron-rich nuclei, coexisting with a superfluid neutron component, while the star's fluid core contains a mixed superfluid/superconductor. The dynamics of the star depend heavily on the parameters associated with the different phases. The presence of superfluidity brings new degrees of freedom -- in essence we are dealing with a complex multi-fluid system -- and additional features: Bulk rotation is supported by a dense array of quantised vortices, which introduce dissipation via mutual friction, and the motion of the superfluid is affected by the so-called entrainment effect. This brief survey provides an introduction to -- along with a commentary on our current understanding of -- these dynamical aspects, paying particular attention to the role of entrainment, and outlines the impact of superfluidity on neutron-star seismology.

astro-ph.HE↗

arXiv2021

The phenomenology of dynamical neutron star tides

We introduce a phenomenological, physically motivated, model for the effective tidal deformability of a neutron star, adding the frequency dependence (associated with the star's fundamental mode of oscillation) that comes into play during the late stages of the binary inspiral. Testing the model against alternative descriptions, we demonstrate that it provides an accurate representation of the dynamical tide up to close to merger. The simplicity of the prescription makes it an attractive alternative for a gravitational-wave data analysis implementation, facilitating an inexpensive construction of a large number of templates covering the relevant parameter space.

gr-qc↗

arXiv2020

Modelling neutron star mountains

As the era of gravitational-wave astronomy has well and truly begun, gravitational radiation from rotating neutron stars remains elusive. Rapidly spinning neutron stars are the main targets for continuous-wave searches since, according to general relativity, provided they are asymmetrically deformed, they will emit gravitational waves. It is believed that detecting such radiation will unlock the answer to why no pulsars have been observed to spin close to the break-up frequency. We review existing studies on the maximum mountain that a neutron star crust can support, critique the key assumptions and identify issues relating to boundary conditions that need to be resolved. In light of this discussion, we present a new scheme for modelling neutron star mountains. The crucial ingredient for this scheme is a description of the fiducial force which takes the star away from sphericity. We consider three examples: a source potential which is a solution to Laplace's equation, another solution which does not act in the core of the star and a thermal pressure perturbation. For all the cases, we find that the largest quadrupoles are between a factor of a few to two orders of magnitude below previous estimates of the maximum mountain size.

astro-ph.HE↗