Constraints on the Properties of the Neutron Star XTE J1814-338 from Pulse Shape Models

TL;DR

This study models the pulse shapes of neutron star XTE J1814-338 to constrain its properties, finding evidence for stiff equations of state and providing insights into neutron star structure.

Contribution

The paper introduces a comprehensive pulse shape model incorporating relativistic effects and star oblateness to constrain neutron star equations of state.

Findings

Favor stiff equations of state over softer ones.

Mass-radius constraints consistent with large neutron star masses.

Analysis supports stiff EOS incompatible with some previous softer models.

Abstract

The accretion-powered (non-X-ray burst) pulsations of XTE J1814-338 are modeled to determine neutron star parameters and their uncertainties. The model is a rotating circular hot spot and includes: (1) an isotropic blackbody spectral component; (2) an anisotropic Comptonized spectral component; (3) relativistic time-delays and light-bending; and (4) the oblate shape of the star due to rotation. This model is the simplest possible model that is consistent with the data. The resulting best-fit parameters of the model favor stiff equations of state, as can be seen from the 3-sigma allowed regions in the mass-radius diagram. We analyzed all data combined from a 23 day period of the 2003 outburst, and separately analyzed data from 2 days of the outburst. The allowed mass-radius regions for both cases only allow equations of state (EOS) that are stiffer than EOS APR (Akmal et al. 1998), consistent with the large mass that has been inferred for the pulsar NGC 6440B (Freire et al. 2008). The stiff EOS inferred by this analysis is not compatible with the soft EOS inferred from a similar analysis of SAX J1808.