Discovery of a Kiloparsec Scale X-ray/Radio Jet in the z=4.72 Quasar GB
  1428+4217
  We report the discovery of a one-sided 3.6" (24 kpc, projected) long jet in
the high-redshift, z=4.72, quasar GB 1428+4217 in new Chandra X-ray and VLA
radio observations. This is the highest redshift kiloparsec-scale X-ray/radio
jet known. Analysis of archival VLBI 2.3 and 8.6 GHz data reveal a faint
one-sided jet extending out to ~200 parsecs and aligned to within ~30 deg of
the Chandra/VLA emission. The 3.6" distant knot is not detected in an archival
HST image, and its broad-band spectral energy distribution is consistent with
an origin from inverse Compton scattering of cosmic microwave background
photons for the X-rays. Assuming also equipartition between the radiating
particles and magnetic field, the implied jet Lorentz factor is ~5. This is
similar to the other two known z ~ 4 kpc-scale X-ray jet cases and smaller than
typically inferred in lower-redshift cases. Although there are still but a few
such very high-redshift quasar X-ray jets known, for an inverse Compton origin,
the present data suggest that they are less relativistic on large-scales than
their lower-redshift counterparts.
