DETERMINING NEUTRON STAR PROPERTIES by FITTING OBLATE-STAR WAVEFORM MODELS to X-RAY BURST OSCILLATIONS

Abstract

We describe sophisticated new Bayesian analysis methods that make it possible to estimate quickly the masses and radii of rapidly rotating, oblate neutron stars by fitting oblate-star waveform models to energy-resolved observations of the waveforms of X-ray burst oscillations produced by such stars. We find that a 25% variation of the temperature of the hot spot in the north-south direction does not significantly bias estimates of the mass M and equatorial radius derived by fitting a model that assumes a uniform-temperature spot. Our results show that fits of oblate-star waveform models to waveform data can simultaneously determine M and with 1σ uncertainties ≲7% if (1) the star's rotation rate is 600 Hz; (2) the spot center and observer's sightline are both within of the star's rotational equator; and (3) counts are collected during burst oscillations that have a fractional rms amplitude 10%. A fractional amplitude of ∼10% is realistic, and the accepted NICER and proposed LOFT and AXTAR space missions could collect counts from a single star by combining data from many X-ray bursts from the star. Uncertainties ≲7% are small enough to improve substantially our understanding of cold, ultradense matter.