[Paper Review] Revision of distance to SS433

A. A. Panferov|arXiv (Cornell University)|Jan 28, 2010

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper revises the distance to SS 433 from the widely accepted 5.5 kpc to 4.3 kpc by applying a kinematic model to time-resolved radio jet images. The model fits knot motions better with a jet velocity of 0.205c than the canonical 0.258c, and the discrepancy in precession phase between radio and optical jets suggests a two-component jet structure with high-speed core and low-velocity shell.

ABSTRACT

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.

Motivation & Objective

• Re-evaluate the distance to SS 433 using time-resolved radio jet images and kinematic modeling.
• Assess the validity of the canonical 5.5 kpc distance derived from Blundell & Bowler (2004) based on light travel time effects.
• Investigate inconsistencies between radio and optical jet kinematics, particularly in precession phase and velocity.
• Determine whether the radio jets exhibit acceleration or structural complexity inconsistent with a simple one-component model.
• Propose a two-component jet model to reconcile discrepancies between radio and optical jet observations.

Proposed method

• Apply a kinematic model of precessing jets with fixed ephemerides: precession angle χ = 98.2°, inclination i = 78.81°, cone half-angle θ₀ = 19.75°, precession period P₀ = 162.250 d, and orbital period Pb = 13.08211 d.
• Use the light travel effect formula μ = (vτ / (1 + vr/c)) / D to relate observed proper motion μ to jet velocity vj and distance D, accounting for relativistic projection effects.
• Fit model jet tracks to discrete knots in radio images from Stirling et al. (2002), optimizing vj and D independently via least-squares residuals.
• Test sensitivity of results to jet acceleration and phase shift, using residuals to assess model quality.
• Compare model fits using vj = 0.205c (best-fit) and vj = 0.258c (canonical) to determine which better matches observed knot positions.
• Analyze phase lags between radio and optical jets using precession phase shifts, confirming a persistent 8-day lead in radio jets.

Experimental results

Research questions

• RQ1 Does the kinematic model of radio jet knots support a distance to SS 433 different from the canonical 5.5 kpc?
• RQ2 Why do radio jet knots exhibit a systematic phase lag of ~8 days relative to the kinematic model prediction, while optical lines do not?
• RQ3 Can the observed residuals in knot positions be better explained by a lower jet velocity than the canonical 0.258c?
• RQ4 What does the discrepancy in precession phase and velocity between radio and optical jets imply about the jet structure?
• RQ5 Is the observed deviation from the kinematic model consistent with a two-component jet system (high-speed core + low-velocity shell) rather than simple acceleration?

Key findings

• The best-fit kinematic model yields a jet velocity of 0.205 ± 0.006c and a distance of 4.36 ± 0.12 kpc, significantly lower than the canonical 5.5 kpc.
• The model with vj = 0.205c produces residuals of 6.93 mas per knot, compared to 8.99 mas per knot for the canonical 0.258c model, indicating a 30% improvement in fit quality.
• The light travel effect predicts a residual difference of 5.73 mas per knot between the two models, consistent with the observed 2.06 mas per knot difference in residuals.
• Radio jets consistently lead the kinematic model prediction by ~8 days in precession phase, a feature not observed in optical Hα line shifts, indicating a fundamental kinematic difference.
• The persistent phase lag and velocity discrepancy suggest the radio jets are not a single-component flow but instead have a low-velocity shell structure, distinct from the high-speed optical jet core.
• Jet acceleration is not discernible in the data, but local deviations from the model are likely due to interactions between a high-speed core and a low-velocity shell in a two-component jet structure.