A persistent ultraviolet outflow from an accreting neutron star binary transient.

Area of Science:

• Astrophysics
• Accretion Physics
• Stellar Evolution

Background:

• Disc-accreting objects, like neutron stars and black holes, generate powerful disc winds.
• Wind signatures, such as X-ray absorption lines, are observed in soft states, while optical lines appear in hard states.
• The connection between these wind signatures across different accretion states and spectral bands remains unclear.

Purpose of the Study:

• To investigate the relationship between disc wind signatures in different spectral bands (X-ray, UV, optical) during accretion outbursts.
• To identify and characterize wind-formed lines in the ultraviolet region, bridging the gap between X-ray and optical observations.
• To understand the nature and behavior of disc outflows in transient neutron star binaries.

Main Methods:

• Time-resolved ultraviolet spectroscopy of the transient neutron star binary Swift J1858.6-0814 during a luminous hard state.
• Analysis of simultaneously observed optical spectral lines.
• Decomposition of UV spectral data into constant and variable components to isolate outflow characteristics.

Main Results:

• Detection of wind-formed, blue-shifted absorption lines of C IV, N V, and He II in the UV spectrum.
• Identification of transient blue-shifted absorption in simultaneously observed optical lines.
• Association of the blue-shifted UV absorption with a constant component, suggesting an outflow independent of luminous flares.

Conclusions:

• The simultaneous presence of UV and optical wind features indicates a multi-phase and/or spatially stratified evaporative outflow from the outer accretion disc.
• This outflow is likely a persistent feature across different accretion states, consistent with predictions from radiation-hydrodynamic simulations.
• The observed persistent mass loss may contribute to explaining the relatively short durations of accretion outbursts in such systems.