| Abstract
| - Context. The Galactic microquasar SS 433 launches oppositely directed jets at speeds approximately a quarter of the speed of light. These appear to have punched through and beyond the supposed supernova remnant shell W 50. The problems with this interpretation are: (i) the precessing jets have somehow been collimated before reaching the shell; (ii) without deceleration, only recently launched jets would have reached no further; and (iii) certain features in the lobes are moving slowly or are stationary. Aims. Hydrodynamic computations have demonstrated that for at least one set of parameters describing the ambient medium, jets that diverge and precess are both decelerated and collimated; the conformation of W 50 could then have been sculpted by the jets of SS 433. However, the parameters adopted for density and pressure in these computations are not consistent with observations of jets at a few years old; nor do they represent conditions within a supernova remnant. Our aim is to investigate whether the computations already performed can be scaled to a realistic W 50. Methods. We find simple and physically based scaling relations. The distance to collimation varies inversely with the square root of the pressure of the ambient medium and the speed with which the head of a collimated jet propagates scales with the square root of the temperature. We extrapolate the results of the hydrodynamic computations to lower densities and pressures. Results. The jets of SS 433, launched into an ambient medium of pressure ~10 −9 erg cm −3 and temperature ~10 8 K, within a supernova remnant, could be responsible for the characteristics of W 50. The precessing jets are collimated within ~10 pc and the head of the resulting cylindrical jet propagates slowly. Conclusions. The problems of relating W 50 to SS 433 may now be solved.
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