Constraints on the Equation-of-State of neutron stars from nearby neutron star observations

TL;DR

This paper uses multi-wavelength observations of nearby neutron stars to constrain their physical properties, aiming to improve understanding of the dense matter equation-of-state.

Contribution

It introduces a comprehensive observational approach combining X-ray, optical, and astrometric data to determine neutron star mass, radius, and compactness, providing new constraints on the EoS.

Findings

Determined the compactness of RBS1223 for the first time.

Derived neutron star radii and masses from combined observational data.

Presented new constraints on the neutron star equation-of-state.

Abstract

We try to constrain the Equation-of-State (EoS) of supra-nuclear-density matter in neutron stars (NSs) by observations of nearby NSs. There are seven thermally emitting NSs known from X-ray and optical observations, the so-called Magnificent Seven (M7), which are young (up to few Myrs), nearby (within a few hundred pc), and radio-quiet with blackbody-like X-ray spectra, so that we can observe their surfaces. As bright X-ray sources, we can determine their rotational (pulse) period and their period derivative from X-ray timing. From XMM and/or Chandra X-ray spectra, we can determine their temperature. With precise astrometric observations using the Hubble Space Telescope, we can determine their parallax (i.e. distance) and optical flux. From flux, distance, and temperature, one can derive the emitting area - with assumptions about the atmosphere and/or temperature distribution on the surface. This was recently done by us for the two brightest M7 NSs RXJ1856 and RXJ0720. Then, from identifying absorption lines in X-ray spectra, one can also try to determine gravitational redshift. Also, from rotational phase-resolved spectroscopy, we have for the first time determined the compactness (mass/radius) of the M7 NS RBS1223. If also applied to RXJ1856, radius (from luminosity and temperature) and compactness (from X-ray data) will yield the mass and radius - for the first time for an isolated single neutron star. We will present our observations and recent results.