Fermi Rules Out the IC/CMB Model for the Large Scale Jet X-ray Emission of 3C 273
The source of the X-ray emission associated with the 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 larger-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 (from 3 - 10 GeV) of 4.9x10^-13 erg s^-1 cm^-2. This upper limit violates by almost a factor of ten 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. Further, the 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.