Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra

By Joonas Nättilä, M. Miller, A. Steiner, J. Kajava, Valery F. Suleimanov, V. Suleimanov, J. Poutanen. 2017

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Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs) in low-mass X-ray binary systems can be used to constrain NS masses and radii. Most previous work of this type has set these constraints using Planck function fits as a proxy: both the models and the data are fit with diluted blackbody functions to yield normalizations and temperatures which are then compared against each other. Here, for the first time, we fit atmosphere models of X-ray bursting NSs directly to the observed spectra. We present a hierarchical Bayesian fitting framework that uses state-of-the-art X-ray bursting NS atmosphere models with realistic opacities and relativistic exact Compton scattering kernels as a model for the surface emission. We test our approach against synthetic data, and find that for data that are well-described by our model we can obtain robust radius, mass, distance, and composition measurements. We then apply our technique to Rossi X-ray Timing Explorer observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to all five bursts shows that the theoretical atmosphere models describe the data well but there are still some unmodeled features in the spectrum corresponding to a relative error of 1-5% of the energy flux. After marginalizing over this intrinsic scatter, we find that at 68% credibility the circumferential radius of the NS in 4U 1702-429 is R = 12.4+-0.4 km, the gravitational mass is M=1.9+-0.3 Msun, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass fraction is X < 0.09.

Subfields: Astronomy and Astrophysics, Geophysics

View Paper: https://www.semanticscholar.org/paper/65ea2db4e6f6a8541022b0f0bf4066565fe93e52

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Citations in the corpus

PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter (2019, background): “…(see Steiner et al. 2018 for a recent summary), from neutron stars during thermonuclear X-ray bursts (see Steiner et al. 2010; Özel et al. 2016; and Nättilä et al. 2017 for different perspectives), and from accretion-powered millisecond pulsars (see Salmi et al. 2018), with inferred radii…”
A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation (2019, methodology): “…constraints on radius via X-ray spectral modeling of transiently accreting and bursting NSs (see, e.g., Steiner et al. 2013; Özel et al. 2016; Nättilä et al. 2017; Shaw et al. 2018; Baillot d’Etivaux et al. 2019); we refer the reader to Miller (2013) and Özel & Freire (2016) for detailed…”
Constraining the Neutron Star Mass–Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-Ray Data Set (2019, background): “…precise measurements there are enough concerns about systematic errors that it is not yet possible to constrain the EoS significantly (see, e.g., Steiner et al. 2010, 2018; Leahy et al. 2011; Guillot et al. 2013; Heinke et al. 2014; Miller & Lamb 2016; Özel et al. 2016; Nättilä et al. 2017).”
Equation of state sensitivities when inferring neutron star and dense matter properties (2018, unknown): “…spectral modelling of neutron stars seems to favour radii in the range of 9–13 km (see e.g. Steiner, Lattimer & Brown 2013; Bogdanov et al. 2016; Nättilä et al. 2016, 2017; Özel et al. 2016; Shaw et al. 2018), although the methods are heavily affected by systematics that are still to be…”