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Accretion disc winds in X-ray binaries

Teo Muñoz-Darias, María Díaz Trigo, Chris Done, Gabriele Ponti, Ryota Tomaru

Abstract

Despite early theoretical expectations that large-scale, massive outflows would be triggered by accretion onto black holes and neutron stars, their presence was not firmly established until the 2000s. Since then, they have been recognised as a common, perhaps ubiquitous, feature of accretion discs in X-ray binaries. Over the past two decades, our understanding of these outflows has expanded significantly, with their associated phenomenology now observed across the X-ray, ultraviolet, optical, and near-infrared regimes. In this review, we provide a comprehensive summary of the observational properties of both low- and high-ionisation winds, treating each separately as well as part of a broader phenomenon, and place these findings in the context of current theoretical modelling. We discuss their close connection with disc atmospheres, their impact on the accretion process, and their role within the broader framework that includes the radio jet and the different accretion flow configurations and states. We also address current challenges and outline some of the anticipated developments, particularly those linked to upcoming observational facilities.

Accretion disc winds in X-ray binaries

Abstract

Despite early theoretical expectations that large-scale, massive outflows would be triggered by accretion onto black holes and neutron stars, their presence was not firmly established until the 2000s. Since then, they have been recognised as a common, perhaps ubiquitous, feature of accretion discs in X-ray binaries. Over the past two decades, our understanding of these outflows has expanded significantly, with their associated phenomenology now observed across the X-ray, ultraviolet, optical, and near-infrared regimes. In this review, we provide a comprehensive summary of the observational properties of both low- and high-ionisation winds, treating each separately as well as part of a broader phenomenon, and place these findings in the context of current theoretical modelling. We discuss their close connection with disc atmospheres, their impact on the accretion process, and their role within the broader framework that includes the radio jet and the different accretion flow configurations and states. We also address current challenges and outline some of the anticipated developments, particularly those linked to upcoming observational facilities.
Paper Structure (51 sections, 9 equations, 13 figures, 4 tables)

This paper contains 51 sections, 9 equations, 13 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observational signatures
  3. P-Cygni line profiles
  4. Additional wind signatures
  5. Low ionisation winds
  6. Main spectral lines for low-ionised winds
  7. Lines tracing cold winds
  8. Lines tracing cool winds
  9. Observations of low-ionised winds
  10. Strong P-Cyg profile sources
  11. Sources with multiple detections across the accretion states
  12. Lower inclination sources
  13. Very high inclination sources with distinct detections
  14. Tentative detections
  15. Wind visibility across the states and ionisation effects
  16. ...and 36 more sections

Figures (13)

  • Figure 1: Diagram showing the spectral signatures from an expanding spherical wind. The outflow presents significant line opacity around a continuum source (a star in this case, but it can also be a disc), leading to the formation of P-Cygni profiles (sketched at the bottom in an intensity-versus-wavelength representation). These are characterised by an emission component superimposed on a blue-shifted absorption. Black arrows indicate the direction of the outflow, while blue arrows illustrate typical scattering interactions. Adapted from Matthews2016.
  • Figure 2: Simulated line profiles, distinct from P-Cyg profiles, resulting from the combination of (non-physical) absorption, emission, and double-peaked components expected to play a role in outflows arising in accretion discs. Adapted from MataSanchez2023.
  • Figure 5: Minute time-scale evolution of wind features in V404 Cyg. Top panel: Optical and X-ray light curves (normalised to the peak) during a 103 min GTC spectroscopic window. Bottom panel: Trailed spectrum (from 75 individual spectra) showing the evolution of H$\beta$ (bottom), as well as He ii$\lambda4686$ and the Bowen blend (approximately N iii + C iii at $\lambda \sim 4640$ Å). The H$\beta$ blue-shifted absorption (blue horizontal band) disappears during the main flare, when the higher ionisation features (i.e. He ii, N iii and C iii) become stronger. From data originally presented in Munoz-Darias2016.
  • Figure 6: XRISM spectrum (black) of GX 13+1 together with the fitted model (orange). The main transitions are labelled, with the Fe xxv and Fe xxvi lines indicated in cyan and orange, respectively. The data were taken during a luminous, likely super-Eddington, epoch. A slower, wind component ($\sim300$ km s$^{-1}$) and a faster ($\sim700$ km s$^{-1}$), highly ionised one are detected. Adapted from XRISM2025GX13.
  • Figure 7: XRISM view of Fe wind signatures in four NS LMXBs (from top to bottom: 4U 1916$-$054, 4U 1624$-$49, GX 13+1, and Cir X$-$1) and in the BH transient 4U 1630$-$47. Vertical dotted lines mark the rest-frame positions of transitions associated with different species of Fe, from neutral Fe (Fe-like) to Fe xxvi (H-like). Adapted from Tsujimoto2025.
  • ...and 8 more figures