Fermi Rules Out the IC/CMB Model for the Large Scale Jet X-ray Emission
  of 3C 273
  The X-ray emission mechanism in large-scale jets of powerful radio quasars
has been a source of debate in recent years, with two competing
interpretations: either the X-rays are of synchrotron origin, arising from a
different electron energy distribution than that producing the radio to optical
synchrotron component, or they are due to inverse Compton scattering of cosmic
microwave background photons (IC/CMB) by relativistic electrons in a powerful
relativistic jet with bulk Lorentz factor Gamma ~ 10-20. These two models imply
radically different conditions in the large-scale jet in terms of jet speed,
kinetic power, and maximum energy of the particle acceleration mechanism, with
important implications for the impact of the jet on the large-scale
environment. A large part of the X-ray origin debate has centered on the
well-studied source 3C 273. Here we present new observations from Fermi which
put an upper limit on the gamma-ray flux from the large-scale jet of 3C 273
that violates at a confidence greater that 99.9% the flux expected from the
IC/CMB X-ray model found by extrapolation of the UV to X-ray spectrum of knot
A, thus ruling out the IC/CMB interpretation entirely for this source when
combined with previous work. Further, this upper limit from Fermi puts a limit
on the Doppler beaming factor of at least delta < 9, assuming equipartition
fields, and possibly as low as delta < 5, assuming no major deceleration of the
jet from knots A through D1.
