http://arxiv.org/abs/1907.00996
We present a new method of matching observations of Type I (thermonuclear) X-ray bursts with models, comparing the predictions of a semi-analytic ignition model with X-ray observations of the accretion-powered millisecond pulsar SAX J1808.4–3658 in outburst. We used a Bayesian analysis approach to marginalise over the parameters of interest and determine parameters such as fuel composition, distance/anisotropy factors, neutron star mass and neutron star radius. Our study includes a treatment of the system inclination effects, inferring that the rotation axis of the system is inclined $\left(69^{+4}{-2}\right)^\circ$ from the observers line of sight, assuming the flat disk model of He & Keek (2016). This method can be applied to any accreting source that exhibits Type I X-ray bursts. We find a hydrogen mass fraction of $0.57^{+0.13}{-0.14}$ and CNO metallicity of $0.013^{+0.006}{-0.004}$ for the accreted fuel is required by the model to match the observed burst energies, for a distance to the source of $3.3^{+0.3}{-0.2}\,\mathrm{kpc}$. We infer a neutron star mass of $1.5^{+0.6}{-0.3}\,\mathrm{M}{\odot}$ and radius of $11.8^{+1.3}{-0.9}\,\mathrm{km}$ for a surface gravity of $1.88^{+0.7}{-0.4}\times10^{14}\,\mathrm{cm}\,\mathrm{s}^{-2}$ for \texttt{SAX J1808.4–3658}.
A. Goodwin, D. Galloway, A. Heger, et. al.
Wed, 3 Jul 19
11/58
Comments: 14 pages, submitted to MNRAS, comments welcome
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