LAUSR

We report on the NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary GX 3 + 1 performed on 2017 October 17. The source was found in a soft X-ray spectral state with 3–70 keV luminosity of LX ∼ 3 × 1037 erg s−1 (${\sim } 16{{\ \rm per\ cent}}$ of the Eddington luminosity), assuming a distance of 6 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3–70 keV NuSTAR spectral data can be described by a two-component continuum model consisting of a disc blackbody (kTdisc ∼ 1.8 keV) and a single temperature blackbody model (kTbb ∼ 2.7 keV). The spectrum shows a clear and robust indication of relativistic reflection from the inner disc which is modelled with a self-consistent relativistic reflection model. The accretion disc is viewed at an inclination of i ≃ 22°–26° and extended close to the NS, down to $R_\text{in}=(1.2\!-\!1.8) R_\text{ISCO}\:(\simeq 6.1\!-\!9.1\, R_{\mathrm{ g}}\: \text{or}\: 14\!-\!20.5$ km) which allows an upper limit on the NS radius (≤13.5 km). Based on the measured flux and the mass accretion rate, the maximum radial extension for the boundary layer is estimated to be ∼6.3 Rg from the NS surface. However, if the disc is not truncated by the boundary layer but by the magnetosphere, an estimated upper limit on the polar magnetic field would be of B ≤ 6 × 108 G.