  Powerful relativistic jets are among the most ubiquitous and energetic
observational consequences of accretion around supermassive black holes in
active galactic nuclei and neutron stars and stellar-mass black holes in x-ray
binary (XRB) systems. But despite more than three decades of study, the
structure and composition of these jets remain unknown. Here we present
spatially resolved x-ray spectroscopy of arc second-scale x-ray jets from the
XRB SS 433 analized with the Chandra advanced charge-coupled device imaging
spectrometer. These observations reveal evidence for a hot continuum and
Doppler-shifted iron emission lines from spatially resolved regions.
Apparently, in situ reheating of the baryonic component of the jets takes place
in a flow that moves with relativistic bulk velocity even more than 100 days
after launch from the binary core.
