Probing the Properties of Neutron Stars with Type-I X-Ray Bursts

The increase in spin frequency as the burning atmospheres of TypeI X-ray bursts cool provides a strong constraint on the radius of the underlying neutron star. If the change in spin frequency is due to a change in the thickness of the atmosphere, the radius of the star must exceed $3 G M/c^2$ for any equation of state and approximately $3.5 G M/c^2$ for most physically reasonable equations of state. This constraint arises because the direction of the Coriolis force for radial motion reverses for $R < G / c^2 (M + I / R^2)$. Furthermore, the marked change in the magnitude of the Coriolis force near compact stars provides a straightforward explanation for why the frequency of the quickly rotating bursters shifts by the same amount as the slow rotators; they are slightly more massive, 1.6M$_\odot$ versus 1.4M$_\odot$. \