Constraints on the dense matter equation of state from young and cold isolated neutron stars

TL;DR

This study uses observational data and simulations of young, cold neutron stars to constrain the dense matter equation of state, revealing that some models cannot explain rapid cooling in certain neutron stars.

Contribution

It introduces a novel analysis of young neutron stars' cooling data combined with magneto-thermal simulations to challenge existing dense matter equations of state.

Findings

Identification of unexpectedly cold young neutron stars.

Minimal cooling models fail to explain observations.

Fast cooling processes are necessary in some mass ranges.

Abstract

Neutron stars are the dense and highly magnetic relics of supernova explosions of massive stars. The quest to constrain the Equation of State (EoS) of ultra-dense matter and thereby probe the behavior of matter inside neutron stars, is one of the core goals of modern physics and astrophysics. A promising method involves investigating the long-term cooling of neutron stars, and comparing theoretical predictions with various sources at different ages. However, limited observational data, and uncertainties in source ages and distances, have hindered this approach. In this work, re-analyzing XMM-Newton and Chandra data from dozens of thermally emitting isolated neutron stars, we have identified three sources with unexpectedly cold surface temperatures for their young ages. To investigate these anomalies, we conducted magneto-thermal simulations across diverse mass and magnetic fields, considering three different EoS. We found that the "minimal" cooling model, failed to explain the observations, regardless the mass and the magnetic field, as validated by a machine learning classification method. The existence of these young cold neutron stars suggests that any dense matter EoS must be compatible with a fast cooling process at least in certain mass ranges, eliminating a significant portion of current EoS options according to recent meta-modelling analysis.