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All disc-accreting astrophysical objects produce powerful outflows. Inbinaries containing neutron stars (NS) or black holes, accretion often takesplace during violent outbursts. The main disc wind signatures during theseeruptions are blue-shifted X-ray absorption lines, which are preferentiallyseen in disc-dominated "soft states". By contrast,optical wind-formed lineshave recently been detected in "hard states", when a hot corona dominates theluminosity. The relationship between these signatures is unknown, and noerupting system has revealed wind-formed lines between the X-ray and opticalbands yet, despite the many strong resonance transitions in this ultraviolet(UV) region. Here, we show that the transient NS binary Swift J1858.6-0814exhibits wind-formed, blue-shifted absorption associated with C IV, N V and HeII in time-resolved UV spectroscopy during a luminous hard state. Thisrepresents the first evidence for a warm, moderately ionized outflow componentin this state. Simultaneously observed optical lines also display transientblue-shifted absorption. Decomposing the UV data into constant and variablecomponents, the blue-shifted absorption is associated with the former. Thisimplies that the outflow is not connect to the luminous flares in the data. Thejoint presence of UV and optical wind features reveals a multi-phase and/orstratified outflow from the outer disc. This type of persistent mass lossacross all accretion states has been predicted by radiation-hydrodynamicsimulations and helps to explain the shorter-than-expected outbursts duration.

A persistent ultraviolet outflow from an accreting neutron star binary transient