Extreme Matter meets Extreme Gravity: Ultra-heavy neutron stars with crossovers and first-order phase transitions

Abstract

The speed of sound of the matter within neutron stars may contain non-smooth structure related to first-order phase transitions or crossovers. Here we investigate what are the observable consequences of structure in the speed of sound, such as bumps, spikes, step functions, plateaus, and kinks. One of the main consequences is the possibility of ultra-heavy neutron stars (with masses larger than 2.5 solar masses) and mass twins in heavy (with masses larger than 2 solar masses) and ultra-heavy neutron stars. These stars pass all observational and theoretical constraints, including those imposed by recent LIGO/Virgo gravitational-wave observations and NICER X-ray observations. We thoroughly investigate other consequences of this structure in the speed of sound to develop an understanding of how non-smooth features affect astrophysical observables, such as stellar radii, tidal deformability, moment of inertia, and Love number. Our results have important implications for future gravitational wave and X-ray observations of neutron stars and their impact in nuclear astrophysics.

Relativistic hybrid stars in light of the NICER PSR J0740+6620 radius measurement

“…The lower bound on the radius of PSR J0740 constrains a highly relevant region of the mass-radius (M-R) diagram of compact stars (CS), which in combination with the determination of the tidal deformability (TD) of a star of mass ∼ 1.4M in the GW170817 event by the LIGO-Virgo Collaboration (Abbott et al 2019) put significant constraints on the EoS of CS. These require that the stellar EoS must be moderately soft at intermediate densities (to allow for relatively small TDs) and must be stiff enough at high densities (to allow for two-solar mass CSs) [see (Tan et al 2021;Biswas 2021;Legred et al 2021;Raaijmakers et al 2021;Huth et al 2021;Zhang & Li 2021;Tang et al 2021)]. Recently, the Lead Radius Experiment Collaboration (PREX-II) reported the most precise measurement yet of the neutron skin thickness of a heavy nucleus (Adhikari et al 2021), a quantity strongly correlated with the slope of nuclear symmetric energy L sym at saturation density (ρ sat ), which constrains the nuclear EoS at densities ≥ ρ sat .…”

The slope, the hill, the drop, and the swoosh: Learning about the nuclear matter equation of state from the binary Love relations

“…Fig. 2 in [9] and [8,17,19,34,35,39,40,[45][46][47][48][49][50][51][52][61][62][63][64][65][66][67][68][69][70][71][72][73]. Reason (ii) could be related to first-order phase transitions [38,44,52,[74][75][76][77][78] or an "unphase transition", as predicted by the quarkionic model [35,40,42,48,79,80] or crossover transitions/Gibbs constructions to/with (stiff) quark matter [36,37,45,51,[81][82][83]…”

“…Methodology -We follow the formalism in [9] to model the EoS, which consists of three parts: a "crust" (loosely defined as the low density EoS n 1.5 n sat ), structure functions, and transition functions. For the crust, we use the SLy EoS [89][90][91].…”

“…Beyond that, a bump and/or plateau is introduced in c s to mimic different-order phase transitions. Transition functions are used to connect different parts of the EoS smoothly [9,92].…”

Large and massive neutron stars: Implications for the sound speed in dense QCD

“…Given the still large uncertainties involved, the consensus is that current combined data from GW, x-ray, and radio observations are not yet informative enough to identify or rule out microscopic models that exhibit first-order or crossover transitions into exotic matter at densities relevant for neutron stars (for different phase transition scenarios explored, see, e.g., Refs. [55][56][57][58][59][60][61]). We investigated in detail the enhancement in min{c 2 s,max } to reach large R 2.0 when the EOS undergoes a finite discontinuity ∆ε m at low densities, limited by the small tidal deformabilities measured in GW170817.…”