On the disc reflection spectroscopy of NS LMXB Serpens X-1: analysis of a recent NuSTAR observation

We present NuSTAR observation of the atoll type neutron star (NS) low-mass X-ray binary (LMXB) Serpens X-1 (Ser X-1) performed on 2018 February 17. We observed Ser X-1 in a soft X-ray spectral state with 3-79 keV luminosity of L_X ∼ 0.4 × 10³⁸ erg s^-1 ( ${\sim}23{{\ \rm per\ cent}}$ of the Eddington luminosity), assuming a distance of 7.7 kpc. A positive correlation between intensity and hardness ratio suggests that the source was in the banana branch during this observation. The broad-band 3-30 keV NuSTAR energy spectrum can be well described either by a three-component continuum model consisting of a disc blackbody, a single temperature blackbody, and a power law or by a two-component continuum model consisting of a disc blackbody and a Comptonization component. A broad iron line ∼5-8 keV and the Compton back-scattering hump peaking at ∼10-20 keV band are clearly detected in the X-ray spectrum. These features are best interpreted by a self-consistent relativistic reflection model. Fits with relativistically blurred disc reflection model suggests that the inner disc radius R_in is truncated prior to the ISCO at (1.9-2.5) R_ISCO ( ${\simeq}11.4\!-\!15\, R_{g}\: \text{or}\: 26\!-\!34$ km) and the accretion disc is viewed at a low inclination of I ≃ 16°-20°. The disc is likely to be truncated either by a boundary layer or by the magnetosphere. Based on the measured flux and the mass accretion rate, the maximum radial extension for the boundary layer is estimated to be ∼6.4 R_g from the NS surface. The truncated inner disc in association with pressure from a magnetic field sets an upper limit of B ≤ 1.9 × 10⁹ G.