Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9-342058

Abstract

RXS J180408.9–342058 is a transient neutron star low-mass X-ray binary that exhibited a bright accretion outburst in 2015. We present NuSTAR , Swift , and Chandra observations obtained around the peak brightness of this outburst. The source was in a soft X-ray spectral state and displayed an X-ray luminosity of L X _~(2–3) × 10 37 ( D / 5.8kpc) 2 ergs − 1 (0.5– 10 keV). The NuSTAR data reveal a broad Fe–K emission line that we model as relativistically broadened reflection to constrain the accretion geometry. We found that the accretion disc is viewed at an inclination of i _~27 ◦ –35 ◦ and extended close to the neutron star, down to R in _~5–7.5 gravitational radii (_~11–17 km). This inner disc radius suggests that the neutron star magnetic field strength is B ~<2 × 10 8 G. We find a narrow absorption line in the Chandra /HEG data at an energy of _~7.64 keV with a significance of _~4.8 σ. This feature could correspond to blueshifted Fe XXVI and arise from an accretion disc wind, which would imply an outflow velocity of v out _~0.086 c (_~25 800kms − 1 ). However, this would be extreme for an X-ray binary and it is unclear if a disc wind should be visible at the low inclination angle that we infer from our reflection analysis. Finally, we discuss how the X-ray and optical properties of 1RXS J180408.9–342058 are consistent with a relatively small ( P orb ~<3h) binary orbit