Relativistic Disk Reflection in the Neutron Star X-ray Binary XTE J1709-267 with NuSTAR

Abstract

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 L_(Edd) to ~0.06 L_(Edd) assuming a distance of 8.5 kpc. We have separately examined spectra from the low- and high-flux intervals, which are 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^(+3.0)_(-1.8) Rg (where R_g = GM/c^2) for the low-flux spectrum and 23.4^(+15.6)_(-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 (NS) or the magnetosphere. Based on the measured luminosity and the accretion efficiency for a disk around an NS, we estimate that the theoretically expected size for the boundary layer would be ~0.9-1.1 R_g from the NS'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 and M_(NS) = 1.4M⊙) of B ⩽ 0.75 – 3.70 x 10^9 G, though X-ray pulsations have not been detected from this source.