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

TL;DR

This study analyzes NuSTAR and XMM-Newton observations of the neutron star binary 1RXS J180408.9-34205, revealing reflection features and constraining the inner disk radius and magnetic field, providing insights into accretion processes.

Contribution

First joint spectral analysis of this neutron star binary using relativistic reflection models to determine disk truncation and magnetic field estimates.

Findings

Inner disk radius ≤ 11.1 Rg or ≤ 22.2 km for a 1.4 M⊙ neutron star.

System inclination between 18° and 29°.

Magnetic field strength estimated at (0.3 - 1.0)×10^9 G.

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 $\emph{NuSTAR}$ and $\emph{XMM-Newton}$ while the source was in the hard spectral state. We find multiple reflection features (Fe K$_{\alpha}$ detected with $\emph{NuSTAR}$; 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 $\leq 11.1\ R_{g}$. For a 1.4 M$_{\odot}$ NS with a spin of $a_{*}=0$, this is an inner disk radius of $\leq22.2$ 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 $B\leq(0.3 -1.0)\times10^{9}$ G at the magnetic poles, though coherent pulsations have not been detected and the source is not identified as a pulsar.