Sebastien Guillot, Institut de Recherche en Astrophysique et Planetologie / Observatoire Midi-Pyrénées (IRAP / OMP), University of Toulouse, France — June 29, 2026

Neutrons stars are unique laboratory to study various aspects of extreme physics, among which the composition and behaviour of ultra-dense matter remains one of the open question of fundamental physics. To help answer this question, measurements of neutron stars masses and radii have helped constrained the interior and place some constraints on the dense matter equation of state – the pressure-density relation inside these ultra-dense objects. While neutron star masses are routinely obtained from radio timing of pulsars in compact binaries, the radii of neutron stars are much more difficult to measure. All existing methods, applicable to different classes of neutron stars, rely on their surface emission. Photons emerging from the surface experience extreme special and general relativistic effects in the neutron star spacetime and encode information about its compactness or gravitational redshift.  
Collecting these photons and modelling their time and/or spectral distributions has proven to be key to measurements of the radii (and
masses) of a handful  of neutron stars. In the past few years, the results obtained from the Neutron Star Interior Composition Explorer have been the hallmark of dense matter constraints, but other classes of neutron stars have also been used to this purpose. In this talk, I will present the most recent results from NICER analyses, as well as some key results using somewhat different approaches.