Missing Gamma-Rays from kpc-scale AGN Jets: A Test of the IC/CMB Model

TL;DR

This study tests the IC/CMB model for X-ray emission in quasar jets by searching for gamma-ray signatures with Fermi/LAT, finding none, which suggests the jets are slower and the X-ray emission is likely synchrotron rather than inverse-Compton.

Contribution

The paper provides observational evidence against the IC/CMB model for kpc-scale quasar jets, challenging previous assumptions about jet speeds and emission mechanisms.

Findings

Gamma-ray emission predicted by IC/CMB model is not detected.

Jets are likely slower than previously assumed under the IC/CMB model.

X-ray emission probably originates from synchrotron processes, not inverse-Compton.

Abstract

The physical origin of the X-ray emission in powerful quasar jets has been a long-standing mystery. Though these jets start out on the sub-pc scale as highly relativistic flows, we do not have any direct measurement of their speeds on the kpc scale, where the vast distances from the core necessitate in situ particle acceleration. If the jets remain highly relativistic on kpc scales, then the X-rays could be due to inverse-Compton upscattering of CMB photons. However, the IC/CMB explanation predicts a high level of gamma-ray emission, which should be detectible by the Fermi/LAT. We have searched for and ruled out this emission at a high level of significance for the well-known sources 3C 273 and PKS 0637-752, suggesting the X-rays are synchrotron, though of unknown origin. These recent results with Fermi also suggest that the kpc-scale jets in powerful quasars are significantly slower than have been presumed under the IC/CMB model. I will discuss the surprising implications of these findings for the energetics and radiative output of powerful quasars as well as their impact on their environment.