🔭 astrophysics

An atypical X-ray variability component in the black hole candidate AT2019wey

This paper analyzes five years of NICER observations of the black hole candidate AT2019wey to identify and characterize a rare "imaginary QPO" in its cross spectrum, revealing its frequency and phase lag evolution across different spectral states and suggesting a tentative connection to type-C QPOs.

Original authors: Pengcheng Yang, Mariano Méndez, Sandeep K. Rout, Candela Bellavita, Federico García, Diego Altamirano, Ole König, Federico A. Fogantini

Published 2026-06-17

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Pengcheng Yang, Mariano Méndez, Sandeep K. Rout, Candela Bellavita, Federico García, Diego Altamirano, Ole König, Federico A. Fogantini

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 black hole not as a silent, dark vacuum, but as a cosmic lighthouse that flickers, pulses, and changes its rhythm as it eats material from a nearby companion star. This is the story of AT2019wey, a black hole system that astronomers have been watching for five years.

In this paper, the researchers act like cosmic detectives, listening to the "heartbeat" of this black hole to find a secret rhythm that was hiding in plain sight.

The Setup: A Cosmic Dance

Black holes in binary systems go through different "moods" or states as they consume matter. Sometimes they are calm and cool (the "Soft" state), and sometimes they are hot and chaotic (the "Hard" state). As they transition between these states, they emit X-rays that pulse with varying speeds.

Usually, when scientists look at these pulses, they use a tool called a Power Spectrum. Think of this like a musical equalizer that shows you how loud different notes are. If a black hole has a strong rhythm, you see a tall, sharp spike on this equalizer.

The Mystery: The "Ghost" Rhythm

For a long time, scientists thought they could see all the rhythms just by looking at the loudness (the Power Spectrum). But recently, a new technique revealed that some rhythms are "ghosts." They don't show up as loud spikes in the main equalizer, but they do show up in a hidden part of the data called the Cross Spectrum.

Imagine you are listening to a duet. If you only listen to the volume of the music, you might miss a subtle timing trick. But if you listen to how the two instruments are out of sync with each other (the phase), you hear a secret beat.

In AT2019wey, the researchers found exactly this. They discovered a rhythm, which they call an "Imaginary QPO" (Quasi-Periodic Oscillation).

Why "Imaginary"? It's not "fake." It's called this because it only appears in the mathematical "imaginary" part of the data analysis. It's a ghost in the machine that you can't see if you only look at the volume, but you can definitely feel it in the timing.

The Clues: The Cliff and the Dip

How did they know this ghost rhythm was real? They found two distinct "footprints" in the data:

The Cliff: Imagine walking along a flat path of time delays (phase lags). Suddenly, at the exact frequency of this ghost rhythm, the path drops off a cliff. The timing of the X-rays shifts dramatically.
The Dip: At the same frequency, the "coherence" (how well the X-rays in different energy bands march in step) suddenly drops. It's like two drummers who were perfectly in sync suddenly losing their rhythm for a split second.

These two features—a sudden drop in the path and a loss of sync—happened right where the "Imaginary QPO" was hiding.

The Evolution: A Slowing Heartbeat

The researchers watched this black hole over five years as it changed its mood from "Hard" to "Soft" and back.

The Slowdown: As the black hole got "harder" (hotter and more chaotic), this ghost rhythm slowed down. It started at a fast beat of about 5 beats per second and slowed down to 1 beat per second.
The Size Change: The researchers used the timing of these beats to guess the size of the "corona" (a cloud of super-hot gas surrounding the black hole). They found that as the rhythm slowed, the cloud of gas seemed to be expanding, growing from the size of a small city to the size of a large country.

The Shape of the Beat

They also looked at how the rhythm behaved across different colors of X-ray light (energies).

In the middle of the black hole's activity, the rhythm had a "U-shape" when plotted against energy. It was like a valley: the timing delay was high at low energies, dipped in the middle, and went back up at high energies.
As the black hole got even harder, this "U-shape" disappeared, and the rhythm became flat. This suggests the black hole's environment was changing, perhaps because the "corona" (the hot gas cloud) was getting cooler or changing shape.

The Verdict: What is this Ghost?

The team compared this "Imaginary QPO" to known types of black hole rhythms (called Type-A, B, and C QPOs).

It didn't fit the profile of the "Type-B" rhythm (which usually happens in a different state).
It matched the behavior of the Type-C QPO perfectly. These are the most common rhythms, usually found when the black hole is in a hard, active state.

The Conclusion: The "Imaginary QPO" isn't a new, weird type of black hole behavior. It's actually a standard Type-C rhythm that was just hiding in the data, invisible to old methods but clearly visible with this new "ghost-hunting" technique.

Why It Matters

This discovery is like finding a new instrument in an orchestra that you thought was silent. It proves that black holes have complex, layered rhythms. By using this new method, astronomers can now hear the "ghosts" in other black holes too, helping them understand the size and shape of the hot gas clouds that dance around these cosmic monsters.

In short: AT2019wey has a secret heartbeat. It's a Type-C rhythm that was hiding in the shadows, slowing down as the black hole got hotter, and revealing the size of the invisible gas cloud surrounding it.

Problem
Recent studies of black hole X-ray binaries (BHXBs) have identified a specific timing feature during the soft-to-hard transition at outburst decay: a narrow frequency range where phase lags between high- and low-energy X-ray light curves suddenly increase, accompanied by a drop in the coherence function. These features are associated with a quasi-periodic oscillation (QPO) that appears exclusively in the imaginary part of the cross spectrum (CS) and remains undetected in the power density spectrum (PDS), termed an "imaginary QPO." While such QPOs have been reported in sources like Cygnus X–1, MAXI J1820+070, and Swift J1727.8–1613, their properties and physical origins require further investigation across different sources. The paper addresses the presence and characteristics of this atypical variability component in the BHXB AT2019wey, a source that has been observed to spend most of its time in the low-hard state (LHS) and hard-intermediate state (HIMS).

Methodology
The authors analyzed five years of NICER (Neutron Star Interior Composition Explorer) observations of AT2019wey, covering the period from August 2020 to May 2025. The dataset included over 600 observations, from which more than 400 were selected after applying strict filtering criteria to minimize instrumental effects (e.g., light leaks, undershoot/overshoot rates).

The analysis followed a specific protocol:

Data Grouping: To enhance the signal-to-noise ratio (SNR) for detecting narrow timing features, the authors grouped 16.384-second segments based on their hardness ratio (HR). Ten distinct zones were defined along the hardness–intensity diagram (HID), separating the HIMS (HR < 0.21) from the LHS (HR > 0.21).
Spectral Analysis: For each zone, the authors computed the PDS and the real and imaginary parts of the CS for energy bands (0.3–2 keV and 2–12 keV).
Modeling: A simultaneous fitting technique was employed, modeling the PDS and the real/imaginary parts of the CS as a linear combination of Lorentzian functions. This approach, based on the work of Méndez et al. (2024), assumes that individual components are coherent across energy bands but mutually incoherent.
Energy-Dependent Analysis: The fractional root-mean-square (rms) amplitude, normalized covariance, and phase lags of the detected QPO were analyzed as a function of energy to characterize its spectral properties.

Key Contributions and Results

Detection of Imaginary QPO: The study confirms the presence of an imaginary QPO in AT2019wey, making it the fifth BHXB where such a component has been identified. The QPO is not significantly visible in the PDS but is clearly detected in the imaginary part of the CS, characterized by a quality factor ( $Q$ ) between $\sim1.5$ and $3.0$.
Frequency and State Evolution: As the source spectrum hardens (moving from HIMS to LHS), the centroid frequency of the imaginary QPO decreases from $\sim5$ Hz to $\sim1$ Hz.
Phase Lag and Coherence Features: The detection of the imaginary QPO is marked by a sharp increase in phase lags (a "cliff") and a simultaneous drop in the coherence function (a "dip") at the QPO frequency. The phase lag of the QPO increases from $\sim0.3$ rad to $\sim0.7$ rad as the source hardens.
Energy Dependence:
- RMS/Covariance: The rms spectrum of the imaginary QPO rises below $\sim2$ keV, peaks around $\sim2$ keV (shifting to lower energies as the source hardens), and decreases at higher energies. This "rising-falling" profile contrasts with the "rising-flat" profiles seen in other sources and suggests the presence of a relatively cool corona.
- Phase Lag Shape: During the HIMS, the phase-lag energy spectrum exhibits a typical U-shaped profile, with the minimum energy shifting to lower values as the source hardens. This U-shape disappears in the LHS, where the phase lags become monotonically increasing or flat.
Classification: The authors find a tentative connection between the imaginary QPO and type-C QPOs. This is supported by the anti-correlation between QPO frequency and broadband fractional rms, the frequency evolution with spectral state, and the fact that the imaginary QPO appears in the HIMS and LHS (states typical for type-C QPOs) rather than the soft-intermediate state (typical for type-B). In specific time intervals where Yao et al. (2021b) reported a type-C QPO, the authors' simultaneous fitting detected an imaginary QPO with the same frequency.

Significance
The paper establishes AT2019wey as a new host for imaginary QPOs, reinforcing the idea that these components are a common, albeit often hidden, feature in BHXB timing properties. The systematic analysis reveals a correlation between the phase lags of imaginary QPOs and their frequencies across multiple sources. The observed energy dependence of the rms and phase lags provides constraints on the geometry and temperature of the Comptonizing corona, suggesting a relatively cool corona in AT2019wey. While the authors note that the imaginary QPO shares phenomenological similarities with type-C QPOs, they maintain a modest stance, stating that while the evidence points toward a type-C association, the exact physical origin (e.g., whether linked to jet activity or specific coronal geometries) requires further investigation. The study does not definitively conclude whether the phase lags of imaginary QPOs exhibit the inclination dependence previously observed in standard type-C QPOs.