Fermi Rules Out the IC/CMB Model for the Large Scale Jet X-ray Emission of 3C 273

TL;DR

Fermi gamma-ray observations of 3C 273's large-scale jet disprove the inverse Compton scattering of CMB photons as the X-ray emission mechanism, favoring alternative models and constraining jet dynamics.

Contribution

This study provides the first definitive observational evidence ruling out the IC/CMB model for 3C 273's jet using Fermi data, refining jet physics understanding.

Findings

Fermi upper limits contradict IC/CMB predictions at >99.9% confidence.

The results constrain the jet's Doppler beaming factor to delta < 9.

The study limits jet deceleration assumptions, suggesting delta < 5.

Abstract

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.