2015MNRAS.451L..85D

Studying the reflection of X-rays off the inner edge of the accretion disc in a neutron star low-mass X-ray binary allows us to investigate the accretion geometry and to constrain the radius of the neutron star. We report on a NuSTAR observation of 4U 1608-52 obtained during a faint outburst in 2014 when the neutron star, which has a known spin frequency of ν = 620 Hz, was accreting at ≃ 1-2 percent of the Eddington limit. The 3-79 keV continuum emission was dominated by a Γ ≃ 2 power law, with an ≃ 1-2 percent contribution from a kTbb ≃ 0.3-0.6 keV blackbody component. The high-quality NuSTAR spectrum reveals the hallmarks of disc reflection; a broad iron-line peaking near 7 keV and a Compton back-scattering hump around ≃ 20-30 keV. Modelling the disc reflection spectrum points to a binary inclination of i ≃ 30°-40° and a small `coronal' height of h ≲ 8.5GM/c2. Furthermore, our spectral analysis suggests that the inner disc radius extended to Rin ≃ 7-10GM/c2, close to the innermost stable circular orbit. This constrains the neutron star radius to R ≲ 21km and the redshift from the stellar surface to z ≳ 0.12, for a mass of M = 1.5M_☉ and a spin parameter of a = 0.29.