A NuSTAR observation of disc reflection from close to the neutron star in 4U 1608-52

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 nu = 620 Hz, was accreting at similar or equal to 1-2 per cent of the Eddington limit. The 3-79 keV continuum emission was dominated by a Gamma similar or equal to 2 power law, with an similar or equal to 1-2 per cent contribution from a kT(bb) similar or equal to 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 similar or equal to 20-30 keV. Modelling the disc reflection spectrum points to a binary inclination of i similar or equal to 30 degrees-40 degrees and a small 'coronal' height of h less than or similar to 8.5GM/c(2). Furthermore, our spectral analysis suggests that the inner disc radius extended to R-in similar or equal to 7-10GM/c(2), close to the innermost stable circular orbit. This constrains the neutron star radius to R less than or similar to 21 km and the redshift from the stellar surface to z greater than or similar to 0.12, for a mass of M = 1.5 M-circle dot and a spin parameter of a = 0.29.