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 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 ≲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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- July 2015
- DOI:
- 10.1093/mnrasl/slv072
- arXiv:
- arXiv:1505.07112
- Bibcode:
- 2015MNRAS.451L..85D
- Keywords:
-
- accretion;
- accretion discs;
- stars: individual: 4U 1608-52;
- stars: neutron;
- X-rays: binaries;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 5 pages, 4 figures, 1 table, MNRAS Letters in press