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

1RXS J180408.9–342058 is a transient neutron star low-mass X-ray binary that exhibited a bright accretion outburst in 2015. We present $\textit{NuSTAR}$, $\textit{Swift}$, and $\textit{Chandra}$ 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 $L_\text{X}$ $\simeq$ (2–3) × 10$^{37}$($D$/5.8 kpc)$^2$ erg s$^{−1}$ (0.5–10 keV). The $\textit{NuSTAR}$ 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 $i$ $\simeq$ 27$^{\circ}$–35$^{\circ}$ and extended close to the neutron star, down to $R_\text{in}$ $\simeq$ 5–7.5 gravitational radii ($\simeq$11–17 km). This inner disc radius suggests that the neutron star magnetic field strength is $B$ $\lesssim$ 2 × 10$^8$ G. We find a narrow absorption line in the $\textit{Chandra}$/HEG data at an energy of $\simeq$7.64 keV with a significance of $\simeq$4.8$\sigma$. This feature could correspond to blueshifted Fe $\small \text{XXVI}$ and arise from an accretion disc wind, which would imply an outflow velocity of $v_\text{out}$ $\simeq$0.086$c$ ($\simeq$25 800 km s$^{−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_\text{orb}$ $\lesssim$ 3 h) binary orbit.