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The radial velocity curve for HeII emission cannot be used for component mass determination in SS433

Based on 150 measurements of the HeII 4686Å emission line obtained between 2020 and 2025, the study concludes that the line's formation region is too large and its radial velocity curve is decoupled from the orbital motion, rendering it unsuitable for determining the component masses in SS433.

Original authors: A. V. Dodin, K. A. Postnov, A. M. Cherepashchuk

Published 2026-02-17

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CC BY 4.0

Original authors: A. V. Dodin, K. A. Postnov, A. M. Cherepashchuk

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

The Big Picture: A Cosmic Detective Story

Imagine SS433 as a cosmic dance floor. It's a binary system where two stars are locked in a tight orbit: a massive, normal star (the "donor") and a mysterious, invisible partner (a black hole) that is eating the donor star's gas.

For decades, astronomers have been trying to figure out exactly how heavy the black hole is. To do this, they usually listen to the "music" of the system—specifically, the light emitted by a specific type of helium gas (called He II) swirling around the black hole. By measuring how fast this gas moves back and forth (its "radial velocity"), they can calculate the mass of the black hole.

The Problem: Different astronomers have been getting different answers. Some say the black hole is a neutron star (lightweight), others say it's a heavy black hole, and some say it's in between. It's like a group of people trying to guess the weight of a hidden elephant by listening to its footsteps, but everyone hears a different rhythm.

The New Investigation: A Long-Term Watch

The authors of this paper decided to solve the mystery by taking a much longer, more consistent look at the system.

The Old Way: Previous studies took a few snapshots over a short time.
The New Way: These astronomers watched SS433 for 6 years (2020–2025), taking measurements on 388 different nights. They used a telescope in the Caucasus Mountains to get a massive amount of data.

The Big Discovery: The "Ghost" in the Machine

After analyzing all this data, they found something shocking: The helium gas they were tracking isn't actually dancing with the black hole.

Here is the breakdown of their findings using simple analogies:

1. The "Stage" is Too Big

Imagine the black hole is a dancer on a small stage. Astronomers assumed the helium gas was a spotlight right on the dancer's chest. If the dancer moves, the spotlight moves.

However, the authors found that the helium gas isn't a spotlight; it's a giant, foggy cloud that surrounds the entire stage, the dancer, and the audience.

The Evidence: When the normal star passes in front of the black hole (an eclipse), it blocks the view of the black hole and the inner disk. If the helium gas were close to the black hole, its light should dim significantly during this eclipse.
The Result: The helium light didn't dim much at all. It was like trying to hide a giant stadium floodlight by holding up a small piece of paper; the light barely changed. This proves the gas cloud is huge and far away from the black hole.

2. The "Rhythm" is Wrong

If you track the movement of a dancer, their speed should follow a smooth, predictable sine wave (fast one way, slow down, fast the other way).

The Expectation: The helium gas should move in this smooth rhythm, matching the 13-day orbit of the stars.
The Reality: The helium gas moved erratically. Its speed didn't match the dance steps of the stars. Sometimes it seemed to speed up, sometimes slow down, but it didn't follow the rules of the binary orbit.
The Analogy: It's like trying to time a race by watching the crowd cheering in the stands rather than watching the runners. The crowd's noise (the gas) changes based on who is standing where, not based on how fast the runners are going.

3. The "Double-Decker" Confusion

The helium line often splits into two peaks (like a double-decker bus). Previous studies tried to measure the speed of the "bus." But the authors realized this "bus" is actually a swirling storm of gas that is being shaken by the binary system, not a solid object orbiting the black hole. The shape of the gas changes depending on the angle of the system, making the speed measurements unreliable.

The Conclusion: Start Over

The paper concludes that we cannot use the helium gas (He II) to weigh the black hole.

Why? Because that gas is too far away and too chaotic. It's not a faithful messenger of the black hole's motion.
The Consequence: All the previous calculations of the black hole's mass based on this specific gas line are likely wrong. The black hole might be much heavier (or lighter) than we thought.

The Takeaway

Think of SS433 as a complex machine. For years, scientists tried to measure the engine's speed by listening to the exhaust fumes (the helium gas). They realized too late that the exhaust fumes were swirling in a giant, turbulent cloud far away from the engine, influenced by the wind and the shape of the car, not just the engine's speed.

The lesson: To find the true weight of the black hole, we need to find a different "messenger"—a piece of gas or light that is actually close to the black hole and moving with it, not the giant cloud of helium that is just floating nearby.

In short: The helium line is a "fake news" reporter for the black hole's motion. We need to find a new reporter to get the real story.

1. Problem Statement

The Galactic microquasar SS433 is a massive eclipsing X-ray binary containing a black hole (or neutron star) with a supercritical accretion disk and relativistic jets. Determining the masses of its components is critical for understanding its evolution, yet it remains a subject of significant controversy.

The Conflict: Previous spectroscopic studies using the He II 4686 Å emission line to derive the radial velocity curve of the compact object have yielded contradictory mass estimates. Some studies suggest a neutron star ( $<2.5 M_\odot$ ), while others suggest a low-mass black hole ( $\sim4 M_\odot$ ) or a high-mass black hole ( $>8 M_\odot$ ).
The Assumption: These discrepancies arise from the assumption that the He II 4686 Å line originates from a region tightly bound to the compact object (e.g., the inner accretion disk or jet base), thereby reflecting its true orbital motion.
The Goal: To resolve these contradictions by analyzing a significantly larger, homogeneous dataset to determine if the He II line truly traces the orbital motion of the compact object or if its formation region is distinct and extended.

2. Methodology

The authors conducted a comprehensive observational campaign spanning 388 nights between 2020 and 2025.

Instrumentation:
- Spectroscopy: 2.5 m telescope at the Caucasian Mountain Observatory (CMO SAI MSU) using the Transient Double-beam Spectrograph (TDS). Resolution $\lambda/\text{FWHM} \approx 1000$ near He II 4686 Å.
- Photometry: 60 cm RC600 telescope (CMO SAI MSU) for BVRcIc bands.
Data Volume: Over 150 measurements of the He II line, representing an order of magnitude more data than previous studies.
Analysis Techniques:
- Phase Folding: Data was folded using updated ephemerides for the orbital period ( $P_{orb} \approx 13.08$ days) and precessional period ( $P_{prec} \approx 162.3$ days).
- Line Profile Fitting: Radial velocities were measured by fitting single Gaussians to the stationary He II component, excluding dates with moving lines or complex double-peaked profiles that could not be uniquely resolved.
- Comparative Analysis: The derived radial velocity curve was compared against the photometric light curve (eclipse positions) and theoretical models of orbital motion.
- Flux Analysis: The absolute flux and equivalent width (EW) of the line were analyzed to detect eclipses and orbital variability.

3. Key Contributions & Results

A. The Radial Velocity Curve is Not Sinusoidal

The constructed radial velocity curve for the He II 4686 Å line does not exhibit the expected sinusoidal shape of a binary orbit.

Inconsistency with Light Curve: When a formal fit is applied to the He II velocity data, it implies an orbital eccentricity of $e \approx 0.30$ . However, this fit predicts primary and secondary eclipse minima at orbital phases $\phi_{orb} \approx 0.11$ and $0.46$.
Observation: The actual photometric eclipses occur at $\phi_{orb} = 0.00$ and $0.52$. This phase mismatch proves that the He II line velocity does not track the center of mass of the compact object.

B. Independence from Precessional Phase

The radial velocity curve shows no systematic dependence on the precessional phase ( $\phi_{prec}$ ).

While the line profile occasionally splits into double peaks (indicating a circumbinary structure), the average radial velocity remains random and uncorrelated with the precession cycle. This contradicts models where the line formation region is strictly tied to the precessing accretion disk geometry.

C. Lack of Significant Eclipse (The "Non-Eclipsed" Region)

A critical test for the location of the emission source is whether it is eclipsed by the donor star.

Observation: During the primary and secondary eclipses (when the donor star blocks the accretion disk), the He II line flux and Equivalent Width (EW) increase relative to the continuum.
Interpretation: This anti-correlation indicates that the continuum (from the donor star and inner disk) is being blocked, while the He II emission remains constant.
Quantification: The authors estimate that the eclipsed fraction of the He II line flux is negligible ( $6\% \pm 7\%$ ), with an upper limit of 30%. This implies the He II emission region is significantly larger than the donor star and extends far beyond the accretion disk, likely forming a circumbinary shell or envelope.

D. Orbital Modulation of Absolute Flux

The absolute flux of the He II line varies sinusoidally with the orbital period, peaking at $\phi_{orb} = 0.5$ (when the disk is in front of the donor star).

This variability is attributed to the changing contrast between the constant emission line and the variable continuum, rather than the physical blocking of the emission source.

4. Significance and Conclusion

Primary Conclusion:
The He II 4686 Å emission line in SS433 does not reflect the orbital motion of the compact object. Instead, it originates from a large, extended circumbinary gas shell (likely a rotating envelope with spiral shock waves) that is perturbed by the binary system but is not gravitationally bound to the compact object in a way that traces its Keplerian orbit.

Implications:

Invalid Mass Estimates: All previous spectroscopic mass determinations for SS433 based on the He II 4686 Å radial velocity curve (yielding mass functions ranging from $\sim2 M_\odot$ to $\sim10 M_\odot$ ) are unrealistic and unreliable.
Nature of the Compact Object: The contradictions in the literature regarding whether SS433 hosts a neutron star or a black hole cannot be resolved using this specific spectral line. The authors' findings support independent evidence (orbital angular momentum balance and secular period increase) suggesting the compact object is a black hole with mass $m_X > 8 M_\odot$ .
Methodological Shift: Future mass determinations for SS433 must rely on other tracers (such as the moving emission lines associated with the jets or the eclipsing light curve geometry) rather than the stationary He II line.

In summary, this paper fundamentally challenges the standard interpretation of the He II line in SS433, demonstrating that its complex formation region renders it unsuitable for dynamical mass measurements of the compact object.