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.

Probing Elastic Quark Phases in Hybrid Stars with Radius Measurements

“…This is in agreement with the more general analysis of Raposo et al (2019), who showed that even ultracompact stars are possible when larger anisotropic pressures take place. An interesting consequence for systems with ∂M/∂ρ c < 0 is that they could be more compact, which could lead to a richer phenomenology that may be probed with electromagnetic missions and relativistic ray-tracing models (see, e.g., Vincent et al 2018;Raaijmakers et al 2019).…”

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

“…A rotating NS is not perfectly spherical and the spacetime external to the star is not exactly Schwarzschild. If we wished to compute the waveforms from rotating hot spots with the greatest possible accuracy and precision, we would therefore have to solve the exact general relativistic equations for the stellar shape and exterior spacetime metric using an axisymmetric code such as RNS (Stergioulas & Friedman 1995) or LORENE/NROTSTAR (Vincent et al 2018) and then trace the rays in that spacetime.…”

A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation

“…As the star spins, the flux and spectrum of X-ray emission registered by a distant observer is modulated in a periodic manner: we can determine the rotational phase evolution of pulsars precisely and build up a pulse profile (X-ray counts per rotational phase bin per detector channel) by phase-folding X-ray events according to an ephemeris. 23 The mapping of surface emission into the pulse profile detected by a distant observer, via relativistic ray-tracing through the spacetime of a rapidly rotating (and hence oblate) star, is well understood (Pechenick et al 1983;Miller & Lamb 1998;Poutanen & Gierliński 2003;Poutanen & Beloborodov 2006;Cadeau et al 2007;Morsink et al 2007;Bauböck et al 2013;AlGendy & Morsink 2014;Nättilä & Pihajoki 2018;Vincent et al 2018). Thus, given a model for the surface emission (e.g., a geometrically thin atmosphere of some chemical and ionic composition together with a local comoving effective temperature field as a function of surface coordinates), one calculates the expected pulse profile for a given exterior spacetime solution and a given instrument.…”