Self-bound hybrid stars with strong phase transitions can relieve major compact star observation tensions

TL;DR

This work introduces self-bound hybrid stars with large density jumps and slow first-order phase transitions as a unified framework to reconcile anomalous compact-star observations, including HESS J1731-347, XTE J1814-338, GW190814, GW170817, and NICER results. It develops three benchmark models—HybQSs, IHSs, and HybSSs—each with a high-density CSS core coupled to an self-bound outer layer via a Maxwell construction, constrained by $E/A<930$ MeV. The slow-stable branch of these hybrids can simultaneously satisfy tidal-deformability, mass-radius, and extreme-mass observations, with Scenario 2 showing that allowing the hybrid branch to satisfy GW170817 can accommodate even stiffer nonhybrid EOSs and the heavy secondary in GW190814. The results imply distinct observational signatures (e.g., g-modes and energy-release phenomena) and motivate further exploration with alternative hadronic EOS parameterizations and Bayesian analyses to map the viable parameter space. Overall, the study demonstrates a viable, testable route to resolve major tensions in compact-star observations through a new class of self-bound hybrid stars.

Abstract

Some recent pulsar observations cannot naturally fit into the conventional picture of neutron stars: the compact objects associated with HESS J1731-347 and XTE J1814-338 have too small radii in the low-mass regime, while the secondary component of GW190814 is too massive for neutron stars to be compatible with constraints from the GW170817 event. In this study, we demonstrate that all these anomalous observations and tensions, together with other conventional ones such as recent NICER observations of PSR J0740+6620, J0030+0451, and PSR J0437-4715, can be naturally explained simultaneously by a new general type of self-bound hybrid stars with large density discontinuities, and thus are radially stable in either the slow or rapid phase transition context. As a proof of concept, we use hybrid quark stars, inverted hybrid stars, and hybrid strangeon stars as benchmark examples to explicitly demonstrate the advantage and feasibility of self-bound hybrid stars with strong phase transitions in relieving all tensions related to compact stars' masses, radii, and tidal deformabilities.

Figures (5)

• Figure 1: Mass-radius relations (left column) and squared eigenfrequencies of fundamental radial oscillation mode versus center pressure (right column) for slow stable hybrid quark stars (first row), inverted hybrid stars (second row), and hybrid strangeon stars (last row), all in Scenario 1 where the nonhybrid branches (black dot-dashed) satisfy $\Lambda_{1.4M_\odot}<800$ (GW170817). For the first row, $B=50.3\rm \, MeV/fm^3$ for the quark matter crust. Blue ($c_s^2=0.6$) and orange ($c_s^2=1$) curves darken with larger $\Delta \rho/\rho_{\rm trans}$, sampling $(1.5,1.8,2.1)$ respectively. In the second row, $B=50.3\, \rm MeV/fm^3$ for the quark matter crust. Blue ($\gamma=2.1$) and orange ($\gamma=2.3$) curves darken with increasing $\Delta\rho/\rho_{\rm trans}$, sampling (1, 1.2, 1.4) respectively. For the third row, $\epsilon/N_q=80/9$ MeV for the strangeon matter crust, fixing $n_s=0.3\,\rm fm^{-3}$. Blue ($c_s^2=0.6$) and orange ($c_s^2=1$) curves darken with larger $\Delta \rho/\rho_{\rm trans}$, sampling $(4,5,6)$ respectively. For all rows, solid and dashed line styles denote cases where the transition point is around $2\, M_{\odot}$ and a larger selected mass, where $P_{\rm trans}=125\rm\, MeV/fm^3$ and $255\rm\, MeV/fm^3$ for HybQSs, $P_{\rm trans}=55\rm\, MeV/fm^3$ and $135\rm\, MeV/fm^3$ for IHSs, $P_{\rm trans}=68\rm\, MeV/fm^3$ and $90\rm\, MeV/fm^3$ for HybSSs, respectively. Color bands show astrophysical constraints. All curves terminate where either radial stability ($\omega_0^2 \geq 0$) or causality ($c_s^2 \leq 1$) is violated.
• Figure 2: Mass-radius relations (left) and tidal deformability versus star mass (right) for slow stable HybQSs (first row) and HybSSs (second row), all in Scenario 2 where $\Lambda_{1.4M_\odot}<800$ (GW170817) is only met by the hybrid branch. For the first row, $B=30\rm \, MeV/fm^3$ for the crust quark matter. For the second row, $\epsilon/N_q=200/9$ MeV for the crust strangeon matter, fixing $n_s=0.3\,\rm fm^{-3}$. Blue ($c_s^2=0.8$) and orange ($c_s^2=1$) curves darken with larger $\Delta \rho/\rho_{\rm trans}$, sampling $(4,5,6)$ and $(8,9,10)$ for HybQSs and HybSSs, respectively. For all rows, solid and dashed line styles denote cases where the transition point is close but above the lower mass bound ($\sim 2.5 M_\odot$) of the GW190814 constraint, where $P_{\rm trans}=59\rm\, MeV/fm^3$ and $83\rm\, MeV/fm^3$ for HybQSs, $P_{\rm trans}=39 \rm\, MeV/fm^3$ and $48\rm\, MeV/fm^3$ for HybSSs, respectively. All curves are terminated at end points where $\omega_0^2= 0$. Black dot-dashed curves are nonhybrid branches. Color bands show astrophysical constraints.
• Figure A1: $M$-$R$ (left) and $\Lambda$-$M$ (right) relations for slow stable HybQSs of $B=30\rm\, MeV/fm^3$ with $P_{\rm trans}=83\rm\, MeV/fm^3$, featuring a expanded $\Delta\rho/\rho_{\rm trans}=(1,2,...,10)$ (first row) from lighter to darker depth of each color, and a more constrained range $\Delta\rho/\rho_{\rm trans}=(4,5)$ (second row) that can have viable $c_s^2$ that satisfy XTE J1814-338. All curves are truncated at the $\omega_0^2=0$ point.
• Figure A2: Similar to Fig. \ref{['extendend_B30']} but with smaller $P_{\rm trans}=59\rm\, MeV/fm^3$. The range constrained by satisfying XTE J1814-338 region (second row) becomes $\Delta\rho/\rho_{\rm trans}=(4,5,6)$.
• Figure A3: Similar to Fig. \ref{['extendend_B30']} but with smaller $B=25\rm\, MeV/fm^3$ with $P_{\rm trans}=44\rm\, MeV/fm^3$ (so that the transition occurs at the same top mass bound of GW190814). The XTE J1813-338 constrained range (second row) becomes $\Delta\rho/\rho_{\rm trans}=(5,6,7,8)$.