Accurate ray tracing of realistic neutron star atmospheres for constraining their parameters

Abstract

Thermal dominated X-ray spectra of neutron stars in quiescent transient X-ray binaries and neutron stars that undergo thermonuclear bursts are sensitive to mass and radius. The mass-radius relation of neutron stars depends on the equation of state that governs their interior. Constraining this relation accurately is thus of fundamental importance to understand the nature of dense matter. In this context we introduce a pipeline to calculate realistic model spectra of rotating neutron stars with hydrogen and helium atmospheres. An arbitrarily fast rotating neutron star with a given equation of state generates the spacetime in which the atmosphere emits radiation. We use the Lorene/nrotstar code to compute the spacetime numerically and the ATM24 code to solve the radiative transfer equations self-consistently. Emerging specific intensity spectra are then ray-traced through the neutron star's spacetime from the atmosphere to a distant observer with the Gyoto code. Here, we present and test our fully relativistic numerical pipeline. To discuss and illustrate the importance of realistic atmosphere models we compare our model spectra to simpler models like the commonly used isotropic color-corrected blackbody emission. We highlight the importance of considering realistic model-atmosphere spectra together with relativistic ray tracing to obtain accurate predictions. We also insist on the crucial impact of the star's rotation on the observables. Finally, we close a controversy that has been appearing in the literature in the recent years regarding the validity of the ATM24 code.

17 pages, 13 figures; accepted in ApJ ; important updates in Section 2.2 and Fig. 2

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• Constraining the Neutron Star Mass-Radius Relation and Dense Matter Equation of State with NICER. II. Emission from Hot Spots on a Rapidly Rotating Neutron Star
• General purpose ray-tracing and polarized radiative transfer in General Relativity
• Radio Line Properties of Axion Dark Matter Conversion in Neutron Stars
• On parametrised cold dense matter equation of state inference
• Neutron stars and the dense matter equation of state: from microscopic theory to macroscopic observations
• Observational appearance of rapidly rotating neutron stars: X-ray bursts, cooling tail method, and radius determination
• Probing elastic quark phases in hybrid stars with radius measurements
• Oblate Schwarzschild approximation for polarized radiation from rapidly rotating neutron stars
• Estimating the EOS from the measurement of NS radii with 5% accuracy
• Standard cooling of rapidly rotating isolated neutron stars in 2D
• Influence of the gravitational darkening effect on the spectrum of a hot, rapidly rotating neutron star
• Influence of the gravitational darkening effect on the spectrum of a hot, rapidly rotating neutron star. II. Iron lines