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Accretion disc winds in X-ray binaries

This review comprehensively summarizes the observational properties of low- and high-ionization accretion disc winds in X-ray binaries across multiple wavelengths, contextualizes them within current theoretical models, and discusses their connections to disc atmospheres, accretion processes, and radio jets while outlining future challenges and developments.

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

Published 2026-04-08

📖 6 min read🧠 Deep dive

CC BY 4.0

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

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a cosmic dance floor where a tiny, invisible partner (a Black Hole or a Neutron Star) is spinning wildly with a larger, glowing partner (a normal star). The normal star is losing material, which swirls around the invisible partner in a giant, spinning whirlpool called an accretion disc.

For a long time, scientists thought this whirlpool just sucked everything in. But this paper reveals a shocking secret: The whirlpool is actually spitting things out.

Think of the accretion disc not just as a vacuum cleaner, but as a cosmic sprinkler system. As the material spins, it doesn't just fall in; it gets flung out into space in powerful, high-speed winds. This review paper is like a detective's case file, gathering all the clues from the last 20 years to prove these winds exist, understand how they work, and figure out why they matter.

Here is the breakdown of the story, using simple analogies:

1. The Two Types of "Cosmic Sneezes"

The paper explains that these winds come in two very different "flavors," depending on how hot and energetic they are. Think of it like the difference between a cold mist and superheated steam.

The "Cold" Winds (Low Ionization):
- What they are: These are made of cooler gas, mostly hydrogen and helium. They are like a thick, dusty fog.
- When we see them: We spot these mostly when the black hole is in a "grumpy" state (called the Hard State). During this time, the black hole is also shooting out a tight, focused beam of particles (a jet) like a laser pointer.
- How we find them: We see them in visible light and infrared (heat) from giant telescopes on Earth. They look like a "P-Cygni" signature—a specific pattern in the light that looks like a shadow moving toward us, proving the gas is rushing outward.
The "Hot" Winds (High Ionization):
- What they are: These are superheated, super-energetic plasma, stripped of their electrons. They are like a blast furnace.
- When we see them: We spot these when the black hole is in a "calm" state (called the Soft State). Interestingly, the "laser jet" usually disappears when these hot winds appear. It's like the system has to choose: either shoot a laser beam or blow a hot wind, but rarely both at the same time.
- How we find them: We can't see these with regular eyes; we need X-ray telescopes (like Chandra or the new XRISM satellite) to see the specific "fingerprints" of iron atoms being ripped apart by the heat.

2. The Geometry: A Flat Pancake vs. A Sphere

One of the biggest mysteries was: Where are these winds coming from?

The Theory: Imagine the accretion disc is a giant, flat pancake spinning on a table.
The Discovery: The winds seem to blow out mostly from the edges of the pancake (the equator), not from the top or bottom.
The Analogy: Think of a spinning pizza dough. If you spin it fast enough, the dough flies off the edges. If you are standing right next to the table (looking at the edge of the pizza), you see the dough flying at you. If you are looking from above, you might miss it. This explains why we only see these winds clearly in systems where we are looking at the disc from the side (high inclination).

3. The "Great Switch"

The paper describes a fascinating switch that happens during the black hole's outbursts:

Phase 1 (Hard State): The black hole is eating fast. It shoots out a Jet (a laser beam) and a Cold Wind (dusty fog).
Phase 2 (Soft State): The black hole settles down. The Jet turns off. Suddenly, a Hot Wind (superheated steam) turns on.
Why it matters: This suggests the wind and the jet are fighting for the same energy source. When the wind gets too strong, it might actually choke off the jet. It's like a garden hose: if you open the nozzle too wide to spray water everywhere (the wind), the focused stream (the jet) stops.

4. Why Should We Care? (The Impact)

You might ask, "So what? It's just gas flying away."
Actually, these winds are massive.

The Mass Problem: In some extreme cases, the wind is blowing away more mass than the black hole is actually swallowing! It's like a vacuum cleaner that accidentally sucks up the carpet, the rug, and the furniture, but only manages to keep a few crumbs.
The Consequence: Because the wind carries away so much stuff, it can actually starve the black hole. It can cut off the food supply, causing the black hole's "meal" (the outburst) to end abruptly.
The Angular Momentum: The wind also carries away "spin." Imagine a figure skater spinning. If they throw their arms out, they slow down. The wind acts like the skater's arms, slowing down the rotation of the disc and changing how the whole binary system evolves over millions of years.

5. The Future: A New Era of Seeing

The paper ends by looking forward. For a long time, we only had blurry pictures of these winds.

XRISM: A new satellite (XRISM) has just arrived with a "super-magnifying glass" for X-rays. It can see the wind's speed and shape with incredible detail.
James Webb Telescope: This will let us see the "cold winds" in infrared with amazing clarity.
The Goal: By combining these new tools, scientists hope to finally build a 3D movie of the wind. We want to know: Is the wind a smooth sheet of gas, or is it clumpy like a storm cloud? Is it one big layer, or are there different layers of hot and cold gas mixed together?

Summary

This paper tells us that Black Holes and Neutron Stars are not just cosmic vacuum cleaners; they are also cosmic sprinklers. They constantly blow massive winds that shape their environment, control their own growth, and interact with their companion stars. Understanding these winds is key to understanding how the universe recycles matter and energy.

1. Problem Statement

Despite early theoretical predictions (Shakura & Sunyaev, 1973) that accretion onto compact objects (Black Holes and Neutron Stars) should trigger massive outflows, the observational confirmation of these accretion disc winds was not firmly established until the 2000s. While jets (collimated outflows) were discovered in the 1970s, winds (non-collimated outflows carrying significant mass and angular momentum) remained elusive.

The core problem addressed in this review is the fragmented understanding of these winds across different spectral regimes. Historically, research has been siloed:

High-ionisation winds (X-ray domain, $\log \xi \gtrsim 1.5$ ) were primarily studied with Chandra and XMM-Newton.
Low-ionisation winds (Optical/NIR/UV, $\log \xi \lesssim 1.5$ ) were studied with ground-based telescopes.
There was a lack of a unified, multiwavelength framework connecting these phenomena, understanding their physical structure (stratified vs. multiphase), their launching mechanisms, and their impact on the accretion process and binary evolution.

2. Methodology

This paper is a comprehensive review synthesizing two decades of observational data and theoretical modeling. The methodology involves:

Multiwavelength Data Synthesis: Aggregating observational evidence from X-ray (0.1–10 keV), Ultraviolet, Optical, and Near-Infrared (NIR) regimes.
Classification by Ionisation: The authors categorize winds based on the ionisation parameter ( $\xi = L/nR^2$ $ξ = L / n R^{2}$ ):
- Low-ionisation (Cold/Cool): Detected via H and He I/II lines (Balmer, Paschen, Brackett series) and UV resonance lines (C IV, N V).
- High-ionisation (Warm/Hot): Detected via X-ray absorption lines of highly ionised metals (Fe XXV, Fe XXVI, O VIII, Ne X).
Comparative Analysis: Comparing properties between Black Hole (BH) and Neutron Star (NS) systems, and across different accretion states (Hard vs. Soft).
Theoretical Integration: Evaluating observational signatures against three primary launching mechanisms:
1. Radiative Driving: Super-Eddington radiation pressure and UV line driving.
2. Thermal Driving: X-ray heating of the disc atmosphere (Compton temperature).
3. Magnetic Driving: Magnetohydrodynamic (MHD) winds (e.g., Blandford-Payne mechanism).
New Instrumentation Context: Incorporating recent data from the XRISM mission (microcalorimeter Resolve), which offers unprecedented X-ray spectral resolution, and anticipating future capabilities from the James Webb Space Telescope (JWST) and IXPE (polarimetry).

3. Key Contributions

The paper provides the first unified multiwavelength summary of disc winds in X-ray binaries (XRBs). Its major contributions include:

Unified Classification: Establishing a clear taxonomy of winds based on ionisation state and spectral domain, bridging the gap between "X-ray winds" and "optical winds."
Accretion State Dependence: Defining a distinct dichotomy in wind detectability based on the accretion state:
- Hard State: Dominated by low-ionisation (cold/cool) winds, often observed simultaneously with radio jets.
- Soft State: Dominated by high-ionisation (hot) winds; low-ionisation signatures are often suppressed or absent in X-rays due to over-ionisation.
Geometry Constraints: Synthesizing evidence that winds are predominantly equatorial (confined to the disc plane), supported by the fact that X-ray wind detections are almost exclusively found in high-inclination systems ( $i \gtrsim 60^\circ$ ).
Mass and Angular Momentum: Quantifying that winds can carry mass loss rates ( $\dot{M}_{wind}$ ) comparable to or exceeding the accretion rate ( $\dot{M}_{acc}$ ), and play a critical role in angular momentum removal, potentially explaining rapid outburst decays.
Structure Models: Proposing that winds are likely multiphase and clumpy, with coexisting low-ionisation clumps embedded in a highly ionised flow, rather than purely stratified layers.

4. Key Results

A. Observational Signatures

P-Cygni Profiles: The most reliable indicator of outflows, consisting of blueshifted absorption and broad emission.
Low-Ionisation Winds:
- Detected in ~12 confident systems (e.g., V404 Cyg, GRS 1915+105, Swift J1357.2-0933).
- Velocities range from $1,000$ to $4,000$ km s $^{-1}$ .
- Best observed in the Hard state; signatures include P-Cygni profiles in H $\alpha$ , He I, and Brackett series.
- A "Nebular Phase" (intense, broad emission lines with high Balmer decrement) was identified in V404 Cyg, indicating a massive, expanding shell of ejected material.
High-Ionisation Winds:
- Detected in ~8 BH and ~13 NS systems (e.g., GX 13+1, 4U 1630-47).
- Characterized by blueshifted absorption in Fe XXV/XXVI lines.
- Velocities typically $200 - 2,000$ km s $^{-1}$ (though tentative detections suggest ultra-fast outflows up to $10^4$ km s $^{-1}$ ).
- Predominantly observed in the Soft state.

B. Physical Properties

Velocity vs. Inclination: High velocities are consistently measured in high-inclination systems. The lack of detections in low-inclination systems supports an equatorial geometry where the line of sight intersects the densest part of the wind.
Ionisation Effects: The visibility of wind features is heavily dependent on the ionisation state of the gas.
- In the Hard State, the spectrum is hard, leading to over-ionisation of the wind in the X-ray band (making X-ray lines invisible) but allowing low-ionisation lines to survive in the optical/NIR.
- In the Soft State, the softer spectrum allows for the existence of highly ionised plasma, making X-ray lines visible, while the intense UV/X-ray flux may over-ionise the optical lines.
Mass Outflow Rates: Estimates suggest $\dot{M}_{wind}$ can be $10-20$ times the accretion rate in extreme cases (e.g., V404 Cyg), capable of stripping the accretion disc and terminating outbursts prematurely.

C. Theoretical Implications

Launching Mechanisms:
- Thermal Winds: Best explain X-ray winds in Soft states (driven by Compton heating).
- MHD Winds: Likely required to explain the high velocities and angular momentum extraction, particularly in Hard states.
- Radiative Driving: Difficult to sustain in LMXBs due to X-ray over-ionisation, unless shielding (clumping) is present.
Wind Structure: Evidence points to a multiphase structure where dense, low-ionisation clumps coexist with a diffuse, high-ionisation medium, rather than a simple radial stratification.

5. Significance and Future Outlook

This review fundamentally shifts the paradigm of X-ray binary accretion by establishing winds as a ubiquitous feature, not an anomaly.

Regulation of Accretion: Winds are not just byproducts; they actively regulate the accretion flow, potentially quenching jets, stabilizing discs, and removing angular momentum.
Binary Evolution: The mass and angular momentum loss via winds significantly impact the long-term orbital evolution of the binary system.
AGN Connection: Understanding winds in stellar-mass systems provides a high signal-to-noise laboratory for understanding feedback mechanisms in Active Galactic Nuclei (AGN), where similar physics applies but at much larger scales.
Future Prospects: The paper highlights the transformative potential of XRISM (high-resolution X-ray spectroscopy) to resolve wind structure and JWST (NIR spectroscopy) to probe cold winds during soft states. It also emphasizes the role of IXPE in using polarimetry to map wind geometry.

In conclusion, the paper argues that accretion disc winds are a complex, multi-phase phenomenon that varies dynamically with the accretion state. Future research must integrate multiwavelength observations with advanced 3D radiative magnetohydrodynamic simulations to fully unravel the launching mechanisms and the global impact of these outflows.