Twin Stars and the Stiffness of the Nuclear Equation of State: Ruling Out Strong Phase Transitions below 1.7 n₀ with the New NICER Radius Measurements

Abstract

We explore the connection between the stiffness of a hadronic equation of state (EoS) with a sharp phase transition to quark matter to its tidal deformability. For this we employ a hadronic relativistic mean field model with a parameterized effective nucleon mass to vary the stiffness in conjunction with a constant speed of sound EoS for quark matter. We compute multiple scenarios with phase transitions according to the four possible cases of a hybrid star EoS with a stable second branch. We demonstrate how the effective nucleon mass can be constrained by using gravitational-wave data. We find that certain values of the effective nucleon mass are incompatible with GW170817 and a phase transition simultaneously. By using the recent NICER measurements of J0030+0451 we constrain our results further and find that strong phase transitions with a visible jump in the mass–radius relation are ruled out at 1σ at densities below 1.7 times saturation density.

Neutron-star tidal deformability and equation-of-state constraints

“…In general, since GW170817 provided an upper limit onΛ, any physical effect that results in a softening of the equation of state can be consistent with the data [88,[240][241][242][243][244][245][246] As such, a strong first-order phase transition might make an equation of state model compatible with the GW170817 data, even if the hadronic part on its own is not [152,[247][248][249][250]. A number of studies have constructed models that can successfully interpret the inspiral signal from GW170817 as the coalescence of any combination of hadronic and hybrid hadronic-quark neutron stars and place corresponding constraints on the relevant model parameter space [149,247,248,[251][252][253][254][255][256][257][258][259][260].…”

Discontinuity gravity modes in hybrid stars: Assessing the role of rapid and slow phase conversions

“…Notice that all the hybrid models of Table I allow the existence of twin objects, i.e., couples of stars with the same gravitational mass but different radii. This is a very relevant signature that may be used to scrutinize the internal composition of compact objects [8,10,[51][52][53][54]. In fact, new missions probing neutron star radii such as Neutron Star Interior Composition Explorer (NICER) will be able to measure NS radii with 5%-10% uncertainty, while the future enhanced X-ray Timing and Polarimetry (eXTP) is expected to have even better precision.…”

“…Fortunately, the recent detection of very massive pulsars [43][44][45] with ≈2 M ⊙ puts stringent constraints on the nuclear EOS at supranuclear densities. Moreover, with the advent of GW observations of binary neutron star mergers [1,2,46], additional constraints are emerging [3][4][5][6][7][8][9][10].…”

“…Since their discovery more than 50 years ago, neutron stars (NS) have attracted much attention because of their extreme physical properties. Such interest has been highly boosted recently by the direct detection of gravitational waves (GWs) from the NS merger GW170817 [1] and its electromagnetic counterparts GRB170817A and AT2017gfo [2] which imposed a new set of observational constraints on some key properties of these objects [3][4][5][6][7][8][9][10].…”

The possibility of twin star solutions in a model based on lattice QCD thermodynamics

“…Instead of stiffening the quark EoS one could consider to soften the hadronic EoS at intermediate densities. A new analysis of the NICER data gives hint that an extremely soft nuclear EoS and a strong phase transition are mutually exclusive [83]. A softening of the hadronic phase is possible through the appearance of additional baryonic degrees of freedom.…”