A persistent ultraviolet outflow from an accreting neutron star binary transient

TL;DR

This study presents the first evidence of a persistent, multi-phase outflow in a neutron star binary during a hard state, observed through UV and optical wind signatures, supporting models of continuous mass loss.

Contribution

It provides the first detection of UV wind-formed lines in a neutron star binary during a hard state, revealing a persistent, multi-phase outflow not previously observed in this state.

Findings

Detection of blue-shifted UV absorption lines in C IV, N V, He II during a hard state

Simultaneous optical blue-shifted absorption lines observed

Outflow is not connected to luminous flares, indicating persistent mass loss

Abstract

All disc-accreting astrophysical objects produce powerful outflows. In binaries containing neutron stars (NS) or black holes, accretion often takes place during violent outbursts. The main disc wind signatures during these eruptions are blue-shifted X-ray absorption lines, which are preferentially seen in disc-dominated "soft states". By contrast,optical wind-formed lines have recently been detected in "hard states", when a hot corona dominates the luminosity. The relationship between these signatures is unknown, and no erupting system has revealed wind-formed lines between the X-ray and optical bands yet, despite the many strong resonance transitions in this ultraviolet (UV) region. Here, we show that the transient NS binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption associated with C IV, N V and He II in time-resolved UV spectroscopy during a luminous hard state. This represents the first evidence for a warm, moderately ionized outflow component in this state. Simultaneously observed optical lines also display transient blue-shifted absorption. Decomposing the UV data into constant and variable components, the blue-shifted absorption is associated with the former. This implies that the outflow is not connect to the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or stratified outflow from the outer disc. This type of persistent mass loss across all accretion states has been predicted by radiation-hydrodynamic simulations and helps to explain the shorter-than-expected outbursts duration.