Relativistic Disk Reflection in the Neutron Star X-ray Binary Xte J1709-267 with Nustar

We perform the first reflection study of the soft X-ray transient and Type 1 burst source XTE J1709-267 using NuSTAR observations during its 2016 June outburst. There was an increase in flux near the end of the observations, which corresponds to an increase from ∼0.04 LEdd to ∼0.06 LEdd assuming a distance of 8.5 kpc. We have separately examined spectra from the low and high flux intervals, which were soft and show evidence of a broad Fe K line. Fits to these intervals with relativistic disk reflection models have revealed an inner disk radius of 13.8 −1.8 Rg (where Rg = GM/c ) for the low flux spectrum and 23.4 −5.4 Rg for the high flux spectrum at the 90% confidence level. The disk is likely truncated by a boundary layer surrounding the neutron star or the magnetosphere. Based on the measured luminosity and using the accretion efficiency for a disk around a neutron star, we estimate that the theoretically expected size for the boundary layer would be ∼ 0.9− 1.1 Rg from the neutron star’s surface, which can be increased by spin or viscosity effects. Another plausible scenario is that the disk could be truncated by the magnetosphere. We place a conservative upper limit on the strength of the magnetic field at the poles, assuming a∗ = 0 andMNS = 1.4M⊙, ofB ≤ 0.75− 3.70× 10 9 G, though X-ray pulsations have not been detected from this source.