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

Abstract

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 per cent of the Eddington limit. The 3–79 keV continuum emission was dominated by a Γ ≃ 2 power law, with an ≃1–2 per cent contribution from a kT_(bb) ≃ 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/c^2. Furthermore, our spectral analysis suggests that the inner disc radius extended to R_(in) ≃ 7–10GM/c^2, close to the innermost stable circular orbit. This constrains the neutron star radius to R ≲21 km and the redshift from the stellar surface to z ≳0.12, for a mass of M = 1.5 M⊙ and a spin parameter of a = 0.29.