http://arxiv.org/abs/1907.08625
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 17 February 2018. We observed Ser~X-1 in a soft X-ray spectral state with $3-79\kev{}$ luminosity of $L_\text{X}\sim0.4\times 10^{38}$ erg~s$^{-1}$ ($\sim 23\%$ 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 broadband $3-30\kev$ \nustar{} energy spectrum can be well described by a three-component continuum model consisting of a disk blackbody, a single temperature blackbody and a power-law. A broad iron line $\sim 5-8$ keV and the Compton back-scattering hump peaking at $\sim10-20 \kev{}$ 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 $R_{in}$ is truncated prior to the ISCO at $(1.7-2.3)\;R_{ISCO}$ ($\simeq10.2-13.8\,R_{g}: \text{or}: 23-31$ km) and the accretion disc is viewed at an low inclination of $i\simeq18\degr-21\degr$. 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 $\sim6.4:R_{g}$ from the NS surface. The truncated inner disc in association with pressure from a magnetic field sets an upper limit of $B\leq1.6\times10^{9}$ G.
A. Mondal, G. Dewangan and B. Raychaudhuri
Tue, 23 Jul 19
39/72
Comments: 9 pages, 8 figures, 2 tables. To be submitted in MNRAS. arXiv admin note: substantial text overlap with arXiv:1902.02190