Chiral Invariant Mass Constraints from HESS J1731 347 in an Extended Parity Doublet Model with Isovector Scalar Meson

Abstract: The recent discovery of a central compact object (CCO) within the supernova remnant HESS J1731-347, with mass $0.77^{{+0.20}_{-0.17}} \ M_\odot$ and radius $10.4<sup>{+0.86}_{-0.78}$ km is the lightest and smallest compact object ever observed. We identify it as an ultra-light Neutron star (NS) and constrain the chiral invariant mass of nucleon $m_0$ from the observational data of NS using an extended parity doublet model with including the isovector scalar meson $a_0(980)$. We study the higher order asymmertic matter properties such as the symmetry incompressibility $K_{sym}$ and the symmetry skewness $Q_{sym}$ in the presence of $a_0$ meson. We find that $K_{sym}$ and $Q_{sym}$ is sensitive to the chiral invariant mass of nucleon $m_0$ in the presence of $a_0$ meson. We show that the equation of state in the present model satisfies all observational constraints within $2\sigma$ credible region including the HESS J1731-347 observation, as well as the constraint from $K_{sym}$ when $$740 \,\text{ MeV} \lesssim m_0 \lesssim 860 \,\text{ MeV}$$ for $L_0 =$ 57.7 MeV. Yet, the $1\sigma$ constraint from neutron stars appears to be not fully compatible with the constraint from $K_{sym}$ from the present model.