2006ApJ...648..910U

We have performed infrared imaging of the jet of the quasar 3C 273 at wavelengths of 3.6 and 5.8 µm with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. When combined with the radio, optical, and X-ray measurements, the IRAC photometry of the X-ray-bright jet knots clearly shows that the optical emission is dominated by the high-energy emission component of the jet, not by the radio synchrotron component, as had been assumed to date. The high-energy component, represented by a power law from the optical through X-ray, may be due to a second synchrotron component or to inverse Compton scattering of ambient photons. In the former case, we argue that the acceleration of protons exceeding energies of E_p∼10¹⁶ eV or possibly even to E_p∼10¹⁹ eV would be taking place in the jet knots of 3C 273, assuming that the acceleration time is proportional to the particle gyroradius. In contrast, the inverse Compton model, into which highly relativistic Doppler beaming has to be incorporated, requires very low energy electrons of E_e∼1 MeV in the jet knots. The present polarization data in the radio and optical would favor the former interpretation in the case of the 3C 273 jet. Sensitive and detailed measurements of optical polarization are important in order to establish the radiation mechanism responsible for the high-energy emission. The present study offers new clues as to the controversial origin of the X-ray emission seen in many quasar jets.