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A persistent ultraviolet outflow from an accreting neutron star binary transient

Abstract

All disc-accreting astrophysical objects produce powerful disc winds. In compact binaries containing neutron stars 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’1,2. By contrast, optical wind-formed lines have recently been detected in ‘hard states’, when a hot corona dominates the luminosity3. The relationship between these signatures is unknown, and no erupting system has as yet revealed wind-formed lines between the X-ray and optical bands, despite the many strong resonance transitions in this ultraviolet (UV) region4. Here we report that the transient neutron star binary Swift J1858.6-0814 exhibits wind-formed, blue-shifted absorption lines associated with C iv, N v and He ii in time-resolved UV spectroscopy during a luminous hard state, which we interpret as 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 associated with the luminous flares in the data. The joint presence of UV and optical wind features reveals a multi-phase and/or spatially stratified evaporative outflow from the outer disc5. This type of persistent mass loss across all accretion states has been predicted by radiation–hydrodynamic simulations6 and helps to explain the shorter-than-expected duration of outbursts7.

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Fig. 1: Overview light curves of the X-ray transient Swift J1858.6−0814.
Fig. 2: Apparently transient optical wind signatures.
Fig. 3: Average far-UV spectrum of Swift J1858.6−0814 during the luminous hard state.
Fig. 4: Spectral decomposition into a constant (blue) and flaring component (red).

Data availability

The data underlying this article are publicly available at https://archive.stsci.edu/hst/search.php program ID 15984 for HST/FUV data, http://archive.eso.org/cms.html program 190ID 2103.D-5052(A) for VLT/X-Shooter and https://gtc.sdc.cab.inta-csic.es/gtc/ program ID GTC23-19A for GTC/OSIRIS. X-ray data from NICER used all the OBSIDs starting with 120040, 220040, 320040 and 359201 accessible from HIESARC (https://heasarc.gsfc.nasa.gov/docs/nicer/nicer_archive.html). Source data are provided with this paper.

Code availability

Codes used for the analysis are available from the corresponding author upon reasonable request.

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Acknowledgements

N.C.S. and C.K. acknowledge support from the Science and Technology Facilities Council (STFC), and from STFC grant ST/M001326/1. Partial support for K.S.L.’s effort on the project was provided by NASA through grant numbers HST-GO-15984 and HST-GO-16066 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. N.C.S. thanks T. Royle for helping to coordinate the time-critical observations in this article. N.D.D. acknowledges support from a Vidi grant for the Netherlands Organization for Scientific Research (NWO). J.V.H.S. acknowledges support from STFC grant ST/R000824/1. M.A.P., J.C., F.J.-I. and T.M.D. acknowledge support from grants AYA2017-83216-P and PID2020-120323GB-I00. T.M.-D. also acknowledges RYC-2015-18148 and EUR2021-122010. T.M.-D. and M.A.P. acknowledge support from grants with references ProID2020010104 and ProID2021010132. J.M. acknowledges a Herchel Smith Fellowship at Cambridge. T.D.R. acknowledges a financial contribution from the agreement ASI-INAF n.2017-14-H.0. J.v.d.E. is supported by a Lee Hysan Junior Research Fellowship from St Hilda’s College, Oxford. G.V. acknowledges support by NASA grants 80NSSC20K1107, 80NSSC20K0803 and 80NSSC21K0213. M.Ö.A. acknowledges support from the Royal Society through the Newton International Fellowship program. J.A.C. acknowledges support from grants PICT-2017-2865 (ANPCyT), PID2019-105510GB-C32/AEI/10.13039/501100011033 and FQM-322, as well as FEDER funds.

Author information

Authors and Affiliations

Authors

Contributions

N.C.S. and C.K. wrote the original proposal, performed the data analysis and wrote the paper with significant feedback from K.S.L., D.A., F.M.V., S.d.P., J.M. and M. Middleton M.A.P., J.C., F.J.-I. and T.M.-D. provided the GTC data. J.V.H.S. reduced the X-Shooter data and assisted in designing the observations. D.J.K.B. and D.A. provided the X-ray data. D.A.H.B., J.A.C., V.A.C., N.D.D., S.d.P., M.D.T., R.F., C.G., J.V.H.S., M.P., M.Ö.A., L.R., T.D.R., J.v.d.E., F.M.V., M. Méndez, C.B., P.C., P.G., M.G., S.S., G.V.  and P.W. assisted in proposing and planning the multiwavelength observations. All authors contributed to the original proposal, discussed the results and commented on the manuscript.

Corresponding author

Correspondence to N. Castro Segura.

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Nature thanks Daniel Proga and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 The logarithmic far-UV flux distribution of J1858 during our observations.

The distribution is clearly bimodal, consistent with the visual impression from the light curve (Fig. 1b) of the variability being due to a flaring component that is superposed on a roughly constant component. The grey line is the optimal decomposition of the distribution into two Gaussians, as suggested by the KMM algorithm42. The blue and red lines correspond to the individual Gaussians. KMM rejects the null hypothesis of a single component with extremely high significance (p < 10−43).

Source data

Extended Data Fig. 2 The far-UV continuum and driving light curves.

The black histogram shows the light curve of Swift J1858.6−0814 constructed from three broad wavelength regions that exclude the three strongest emission lines (N v λ1240, Si iv λ1400 and He ii λ1640). The specific regions used were λλ1290 Å – 1390 Å, 1410 Å – 1630 Å, 1660 Å – 1850 Å. The light curve is shown normalized to an estimate of the underlying constant level (80 c s−1). The driving light curve used in the decomposition, D(t), was constructed from this and is shown as the red curve. It was obtained by subtracting the estimate of the constant level, setting any slightly negative values to zero, and using a 5-point, 2nd order Savitzky-Golay filter to produce a slightly smoother, higher S/N version of the light curve.

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Castro Segura, N., Knigge, C., Long, K.S. et al. A persistent ultraviolet outflow from an accreting neutron star binary transient. Nature 603, 52–57 (2022). https://doi.org/10.1038/s41586-021-04324-2

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