2024NatAs...8.1020M

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 ultradense matter and thereby probe the behaviour 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, comparing theoretical predictions with various sources at different ages. However, limited observational data, and uncertainties in source ages and distances, have hindered this approach. Here, by re-analysing 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 EOSs. We found that the 'minimal' cooling model failed to explain the observations, regardless of 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.