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

1RXS 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 similar or equal to (2-3) x 10(37)(D/5.8 kpc)(2) erg s(-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 similar or equal to 27 degrees-35 degrees and extended close to the neutron star, down to R-in similar or equal to 5-7.5 gravitational radii (similar or equal to 11-17 km). This inner disc radius suggests that the neutron star magnetic field strength is B less than or similar to 2 x 10(8) G. We find a narrow absorption line in the Chandra/ HEG data at an energy of similar or equal to 7.64 keV with a significance of similar or equal to 4.8 sigma. This feature could correspond to blueshifted Fe XXVI and arise from an accretion disc wind, which would imply an outflow velocity of v(out) similar or equal to 0.086c (similar or equal to 25 800 km s(-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 less than or similar to 3 h) binary orbit.