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

The increase in spin frequency as the burning atmospheres of type I 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 3GM/c² for any equation of state and approximately 3.5GM/c² for most physically reasonable equations of state. This constraint arises because the direction of the Coriolis force for radial motion reverses for R<G/c²(M+I/R²). 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, 1.6 M_solar versus 1.4 M_solar.