A High-Resolution Radio Study of the W50–SS 433 System and the Surrounding Medium

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Abstract
We present VLA images at 327.5 and 1465 MHz of the nebula W50 surrounding the stellar source of relativistic jets SS 433. At 1.4 GHz, we have produced an image of W50 with an angular resolution of 55'' that contains all the spatial structures. The technique consists of a complex combination of 58 separate VLA pointings plus the addition of single-dish data into a single mosaic. For the first time, this high-quality radio image has revealed the puzzling structure of W50, which resembles a seashell. The present data confirm the connection between the subarcsecond relativistic jets from SS 433 and the extended nebula W50 over ~5 orders of magnitude in scale. The central component can be very accurately described by a circle 58' in diameter. The eastern wing exhibits a clear helical pattern that mirrors at larger scales the precession of the jets from SS 433. The western wing, smaller and brighter than its eastern counterpart, has a bright bowed arch at the edge, in the region where the structure appears to interact with a higher density medium. The spectral index of the radio continuum emission exhibits striking asymmetries: the central spherical component has a quite normal spectrum for shell-type supernova remnants, α ~ 0.5 (S ∝ ν-α), while it steepens toward the eastern wing (α ~ 0.8) and flattens toward the western one (α ~ 0.4). A "chimney," apparently radiating away from W50, is observed in the eastern wing. The atomic hydrogen emission was surveyed in an area of 4° × 4° around W50 using the NRAO Green Bank 140 foot (43 m) telescope. Based on these observations, unmistakable evidence of an interaction of W50 with the surrounding H I gas at the systemic radial velocity v = 42 km s-1 is found. Clear traces of the interaction of the jets of SS 433 with the surrounding gas are shown. Based on these observations a kinematic distance of 3 kpc is determined for the W50–SS 433 system. An elliptical shell of ~30,000 M⊙, expanding at about 76 km s-1, can be associated with the radio nebula. The kinetic energy transferred into the ambient medium is estimated (~2 × 1051 ergs), thus confirming that the relativistic jets from SS 433 represent an important contribution to the overall energy budget of W50.

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