Hybrid stars in light of the HESS J1731-347 remnant and the PREX-II experiment

TL;DR

This study explores whether hybrid stars with stiff equations of state, supported by recent experimental and observational data, can explain the small mass-radius measurements of the HESS J1731-347 remnant and other massive pulsars.

Contribution

It introduces hybrid equations of state with density-dependent bag models and assesses their compatibility with recent astrophysical observations and experiments.

Findings

Constant bag parameter models are incompatible with 2 M_sun pulsars.

Gaussian density-dependent models can satisfy the 2 M_sun constraint.

Hybrid stars may explain the GW190814 massive compact object.

Abstract

The recent analysis on the central compact object in the HESS J1731-347 remnant suggests interestingly small values for its mass and radius. Such an observation favors soft nuclear models that may be challenged by the observation of massive compact stars. In contrast, the recent PREX-II experiment, concerning the neutron skin thickness of $^{208}$Pb, points towards stiff equations of state that favor larger compact star radii. In the present study, we aim to explore the compatibility between stiff hadronic equations of state (favored by PREX-II) and the HESS J1731-347 remnant in the context of hybrid stars. For the construction of hybrid equations of state we use three widely employed Skyrme models combined with the well-known vector MIT bag model. Furthermore we consider two different scenarios concerning the energy density of the bag. In the first case, that of a constant bag parameter, we find that the resulting hybrid equations of state are strongly disfavored by the observation of $\sim2 M_\odot$ pulsars. However, the introduction of a Gaussian density dependence yields results that are compatible with the conservative $2 M_\odot$ constraint. The utilization of recent data based on the observation of PSR J0030+0451, PSR J0952-0607 and GW190814 allows for the imposition of additional constraints on the relevant parameters and the stiffness of the two phases. Interestingly, we find that the derived hybrid equations of state do not satisfy the PSR J0030+0451 constraints in $1\sigma$ and only marginally agree with the $2\sigma$ estimations. In addition, we estimate that the observation of massive pulsars, like PSR J0952-0607, in combination with the existence of HESS J1731-347, may require a strong phase transition below $\sim 1.7n_0$. Finally, we show that the supermassive compact object involved in GW190814 could potentially be explained as a rapidly rotating hybrid star.