Structure and Magnetic Fields in the Precessing Jet System SS433 III. Evolution of the Intrinsic Brightness of the Jets from a Deep Multi-Epoch VLA Campaign

We present a sequence of five deep observations of SS433 made over the summer of 2007 using the VLA in the A configuration at 5 and 8 GHz. In this paper we study the brightness profiles of the jets and their time evolution. We also examine the spectral index distribution in the source. We find (as previously reported from the analysis of a single earlier image) that the profiles of the east and west jets are remarkably similar if projection and Doppler beaming are taken into account. The sequence of five images allows us to disentangle the evolution of brightness of individual pieces of jet from the variations of jet power originating at the core. We find that the brightness of each piece of the jet fades as an exponential function of age (or distance from the core), exp(-tau/tau’), where tau is the age at emission and tau’ = 55.9 +- 1.7 days. This evolutionary model describes both the east and west jets equally well. There is also significant variation (by a factor of at least five) in jet power with birth epoch, with the east and west jets varying in synchrony. The lack of deceleration between the scale of the optical Balmer line emission (10^15 cm) and that of the radio emission (10^17 cm) requires that the jet material is much denser than its surroundings. We find that the density ratio must exceed 300:1.

💡 Research Summary

The authors present a comprehensive study of the precessing microquasar SS 433 using five deep Very Large Array (VLA) observations obtained in the A‑configuration during the summer of 2007. Each epoch was observed simultaneously at 5 GHz (C‑band) and 8.5 GHz (X‑band), providing high‑resolution (≈350 mas) total‑intensity maps of the twin east‑ and west‑hand jets over a time span of roughly 80 days. The data were reduced with standard AIPS procedures, supplemented by a baseline‑calibration step required because the array was in a mixed VLA/EVLA state (“shared‑risk” mode). Imaging and self‑calibration were performed in DIFMAP, yielding rms noise levels of 20–60 µJy and peak flux densities that rose from ~0.23 Jy to ~0.47 Jy at 5 GHz over the campaign.

A key part of the analysis is the construction of spectral‑index maps. By convolving the 5 GHz images to a common 350 mas circular beam and comparing them pixel‑by‑pixel with the 8.5 GHz data, the authors derived α (where S ∝ ν⁻ᵅ) across the source. The spectral index is remarkably uniform, with values ranging from 0.68 to 0.79 and an overall mean of α = 0.74 ± 0.06. This uniformity indicates that synchrotron cooling is negligible on the observed scales (the electron radiative lifetime at 10 mG exceeds the jet age by orders of magnitude), and that the jet plasma maintains a similar energy distribution throughout its length.

The central scientific goal is to recover the intrinsic (co‑moving) brightness distribution of the jets, i.e., the power emitted per unit length as measured in the jet’s rest frame. Because SS 433’s jets are mildly relativistic (β≈0.26) and precess with a 163‑day period, both Doppler beaming and geometric projection must be corrected. The authors adopt the well‑established kinematic model (precession cone half‑angle ≈20°, inclination ≈80°, distance 5.5 kpc) to compute, for each point along the observed helix, the local velocity vector, line‑of‑sight angle θ, and the corresponding Doppler factor D =