Evolving morphology of the large-scale relativistic jets from XTE J1550-564

We present an in-depth study of the large scale, western jet of the microquasar XTE J1550-564, based on X-ray and radio observations performed in 2002-2003. The jet is spatially resolved in both observing windows. The X-ray jet is expanding in time along the axis of the jet's propagation: we observe the formation of a tail (∼2.25 arcsec), which appears to extend backwards with an apparent velocity ∼-0.10c. The origin of this feature is discussed in the framework of scenarios of energy dissipation. A single power law adequately describes the broad-band spectra, supporting a synchrotron origin of the X-ray emission. However, a spectral break at ≈10¹⁵ Hz is necessary in coincidence with a re-flare at 8.64 GHz in 2002 September. This finding may be indicative of emission from newly accelerated low-energy particles. The first detection of the jet is in 2001 February (F_{8.64 GHz} = 0.25 ± 0.09 mJy) in the flux rising phase. A phase of stable emission is followed by a rapid decay (t_decay = 167 ± 5 d). The decay at radio frequencies is significantly shorter than in X-rays (t_decay = 338 ± 14 d). We detected a high fraction (up to ∼9 per cent) of linearly polarized radiation at 4.8 and 8.6 GHz. The orientation of the electric vector is consistent with the picture of a shock-compressed magnetic field, and there are hints of variations on month-time-scales, possibly connected with the evolution of the jet structure.