Neutron star tidal deformability and equation of state constraints

Abstract

Despite their long history and astrophysical importance, some of the key properties of neutron stars are still uncertain. The extreme conditions encountered in their interiors, involving matter of uncertain composition at extreme density and isospin asymmetry, uniquely determine the stars' macroscopic properties within General Relativity. Astrophysical constraints on those macroscopic properties, such as neutron star masses and radii, have long been used to understand the microscopic properties of the matter that forms them. In this article we discuss another astrophysically observable macroscopic property of neutron stars that can be used to study their interiors: their tidal deformation. Neutron stars, much like any other extended object with structure, are tidally deformed when under the influence of an external tidal field. In the context of coalescences of neutron stars observed through their gravitational wave emission, this deformation, quantified through a parameter termed the tidal deformability, can be measured. We discuss the role of the tidal deformability in observations of coalescing neutron stars with gravitational waves and how it can be used to probe the internal structure of Nature's most compact matter objects. Perhaps inevitably, a large portion of the discussion will be dictated by GW170817, the most informative confirmed detection of a binary neutron star coalescence with gravitational waves as of the time of writing. Contents

Anisotropic compact stars: Constraining model parameters to account for physical features of tidal Love numbers

“…However, there is another factor regarding anisotropy in the compact stars that has been in speculation in the form of gravitational tidal effects. This is thought to be responsible for deformation and hence anisotropy in the fluid distribution in the stars (Hinderer 2008;Doneva & Yazadjiev 2012;Herrera & Barreto 2013;Biswas & Bose 2019;Rahmansyah et al 2020;Roupas & Nashed 2020;Bhar, Das & Parida 2020;Das, Parida & Sharma 2020;Chatziioannou 2020). In the present work we will emphasize on this physical ingredient of anisotropic nature of compact stars and therefore shall involved in calculating the Love numbers arising due to tidal effect.…”

“…It is argued by Pretel (2020) that the equation of state (EOS) plays a fundamental role in determining the internal structure of such stars and, consequently, in imposing stability limits. As far as the LIGO-Virgo constraints on the EOS for nuclear matter as a result of observation of the event GW170817 is concerned it is more crucial Pretel (2020); Chatziioannou (2020). According to Fattoyev et al (2020) GW170817 has provided stringent constraints on the EOS of neutron rich matter at a few times nuclear densities from the determination of the tidal deformability of a M = 1.4 M⊙ neutron star (Bauswein et al 2017;Abbott et al 2018;Annala et al 2018;Most et al 2018;Tews, Margueron & Reddy 2018;Malik et al 2018;Radice et al 2018;Tews, Margueron & Reddy 2018Capano et al 2019).…”

Tidally induced multipole moments of a nonrotating black hole vanish to all post-Newtonian orders

“…The tidal deformation of compact bodies (neutron stars and/or black holes) was revealed [1] to play an important role in the emission of gravitational waves during a binary inspiral, and to disclose important information regarding the body's internal constitution (see Ref. [2] for a recent review of these exciting developments). A measurement of the tidal deformability of a neutron star was attempted for GW170817 [3], and it delivered an upper bound of astrophysical significance [4]; this bound was used to constrain the equation of state of nuclear matter at high densities [5].…”

A new consistent neutron star equation of state from a generalized Skyrme model

“…We represent the most probable M-R region from combined observations of GW and Heavy pulsars (NICER) [30], the GW170817 event [13] and the maximum mass constraint of [29] obtained from its EM counterpart GRB170817A. Also, other constraints from NICER, chiral EFT and multimessenger observations are represented, adapted from [31] and [32].…”