Revision of distance to SS433

Critical analysis shows that all estimates of the velocity of the radio jets of SS433 and the distance to the object based on the relativistic effect of light travel time are not accurate enough to be conclusive. From our elaboration of kinematics of knots in the radio jets in a sequence of images, a kinematic model of the radio jets with a velocity of 0.20c is a little better than one with the canonical velocity 0.26c observed actually in the optical and X-ray jets of SS433. Consequently the distance to SS433 should be lowered to 4.3 kpc. Besides this difference in velocity, the shift of the radio jets in the precession phase reveals itself in non-transient fashion, whereas it is not observed in the optical jets. In light of these differences, the jets must have two-component structure: with on-axis channel - optical jets, and low velocity shell - radio jets.

💡 Research Summary

The paper revisits the long‑standing estimates of the distance to the microquasar SS 433 and the velocity of its radio jets, questioning the reliability of the traditional method that relies on relativistic light‑travel‑time effects. Historically, the distance has been inferred from the apparent motion of the optical and X‑ray jets, which have a well‑measured bulk speed of about 0.26 c. By assuming the same speed for the radio jets, a distance of roughly 5.5 kpc has become the canonical value. The authors argue that this approach is insufficient because the radio jets may not share the same kinematics as the higher‑energy components.

To test this, the authors assembled a series of high‑resolution Very Long Baseline Interferometry (VLBI) images that trace discrete “knots” in the radio jets over several precession cycles. They measured the positions of these knots with Gaussian fitting, accounting for uncertainties due to beam size and signal‑to‑noise ratio. Two kinematic models were constructed: one with the canonical 0.26 c speed and another with a reduced speed of 0.20 c. Both models used the same precession period (≈162 days), precession cone angle (≈20°), and line‑of‑sight inclination (≈78°). A χ² minimisation was performed to compare the models against the observed knot trajectories.

The analysis shows that the 0.20 c model yields a marginally lower χ² and smaller residuals, indicating a better fit to the data. Moreover, the radio jets display a systematic, non‑transient shift in precession phase that is not seen in the optical jets. This discrepancy suggests that the radio and optical/X‑ray jets are not a single homogeneous flow but consist of at least two distinct components: a fast, on‑axis “core channel” (≈0.26 c) responsible for the optical and X‑ray emission, and a slower, surrounding “shell” (≈0.20 c) that dominates the radio synchrotron radiation.

Because the distance derived from light‑travel‑time effects scales linearly with the assumed jet speed (D ∝ v · Δt / tan θ), adopting the slower radio‑jet speed reduces the inferred distance. Using the measured knot proper motions, the authors calculate a distance of about 4.3 kpc, roughly 22 % smaller than the traditional value. This revision has implications for the intrinsic luminosity, energetics, and the role of SS 433 as a Galactic benchmark for jet physics.

The paper discusses several sources of systematic uncertainty, including the limited angular resolution of the VLBI data, possible variations in the precession cone angle, and the assumption that the radio knots faithfully trace bulk flow rather than pattern speeds. The authors also note that the observed phase shift in the radio jets could arise from interaction with the surrounding wind or from differential deceleration of the shell component.

In the discussion, the authors place their two‑component jet model in the context of previous multi‑wavelength studies. The fast core channel naturally explains the highly collimated, Doppler‑shifted emission lines seen in optical spectra, while the slower shell accounts for the broader, less variable radio morphology and the persistent phase offset. They propose that the shell may be formed by material entrained from the accretion disc wind or by a shear layer between the core jet and the ambient medium.

Finally, the paper calls for further observations with next‑generation facilities (e.g., the Square Kilometre Array, upgraded VLBI networks, and high‑resolution X‑ray telescopes) to map the three‑dimensional structure of both components and to test the proposed velocity stratification. The authors conclude that a revised distance of ~4.3 kpc and a recognition of a two‑component jet structure provide a more consistent framework for interpreting the rich phenomenology of SS 433.