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 NuSTAR and XMM-Newton while the source was in the hard spectral state. We find multiple reflection features (Fe K_ɑ detected with 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 relxill, we determine the inner radius to be ≤11.1 Rg . For a 1.4 M_☉ NS with a spin of a_* = 0, this is an inner disk radius of ⩽22.2 km. We find the inclination of the system to be between 18° and 29°. If the disk is truncated at a radius greater than the NS radius, it could be truncated by a boundary layer on the NS surface. It is also possible that the disk is truncated at the magnetospheric radius; conservative estimates would then imply B ⩽ (0.3-1.0) x 10^9 G at the magnetic poles, though coherent pulsations have not been detected and the source is not identified as a pulsar.