NuSTAR and XMM-Newton Observations of the Neutron Star X-ray Binary 1RXS J180408.9-34205

Abstract

We report on observations of the neutron star (NS) residing in the low-mass X-ray binary 1RXS J180408.9-34205 taken 2015 March by $\text{NuSTAR}$ and $\text{XMM-Newton}$ while the source was in the hard spectral state. We find multiple reflection features (Fe K${}_{\alpha }$ detected with $\text{NuSTAR}$; N VII, O VII, and O VIII detected in the RGS) from different ionization zones. Through joint fits using the self consistent relativistic reflection model {\sc relxill}, we determine the inner radius to be $\le 11.1R_g$. For a 1.4 M${}_{\odot }$ NS with a spin of $a_{\ast }=0$, this is an inner disk radius of $\le 22.2$ km. We find the inclination of the system to be between $18^{\circ }$-$29^{\circ }$. If the disk is truncated at a radius greater than the neutron star radius, it could be truncated by a boundary layer on the neutron star surface. It is also possible that the disk is truncated at the magnetospheric radius; conservative estimates would then imply $B\le (0.3-1.0)\times 10^9$ G at the magnetic poles, though coherent pulsations have not been detected and the source is not identified as a pulsar.

7 pages, 7 figures, 3 tables, Accepted to ApJ