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

Abstract

We present an in-depth study of the large scale, western jet of the microquasarXTEJ1550-564, 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 lowenergy particles. The first detection of the jet is in 2001 February (F8.64 GHz =0.25±0.09 mJy) in the flux rising phase.Aphase of stable emission is followed by a rapid decay (tdecay =167±5 d). The decay at radio frequencies is significantly shorter than in X-rays (tdecay = 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 shockcompressed magnetic field, and there are hints of variations on month-time-scales, possibly connected with the evolution of the jet structure.