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 $\sim$0.04 L${}_{\mathrm{Edd}}$ to $\sim$0.06 L${}_{\mathrm{Edd}}$ 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 $13.8_{-1.8}^{+3.0}{R}_g$ (where $R_g=GM/c^2$) for the low flux spectrum and $23.4_{-5.4}^{+15.6}{R}_g$ 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 $\sim 0.9-1.1R_g$ 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 $a_{\ast }=0$ and $M_{NS}=1.4M_{\odot }$, of $B\le 0.75-3.70\times 10^9$ G, though X-ray pulsations have not been detected from this source.

arXiv commentAccepted for publication in ApJ, 5 pages, 4 figures, 1 table. arXiv admin note: text overlap with arXiv:1701.01774

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• AstroSat and NuSTAR observations of XTE J1739-285 during the 2019-2020 outburst
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