The Origin of the X-ray Emission in Two Well-Aligned Extragalactic Jets: The Case for IC/CMB

Abstract: Over the past two decades, the most commonly adopted explanation for high and hard X-ray emission in resolved quasar jets has been inverse Compton upscattering of the Cosmic Microwave Background (IC/CMB), which requires jets which remain highly relativistic on 10-1000 kpc scales. In more recent years various lines of observational evidence, including gamma-ray upper limits, have disfavored this explanation in favor of a synchrotron origin. While the IC/CMB model generally predicts a high level of gamma-ray emission, it has never been detected. Here we report the detection of a low-state Fermi/LAT gamma-ray spectrum associated with two jetted AGN which is consistent with the predictions of the IC/CMB model for their X-ray emission. We have used archival multiwavelength observations to make precise predictions for the expected minimum flux in the GeV band, assuming that the X-ray emission from the kpc-scale jet is entirely due to the IC/CMB process. In both sources -- OJ 287 and PKS 1510-089 -- the minimum-detected gamma-ray flux level agrees with predictions. Both sources exhibit extreme superluminal proper motions relative to their jet power, which argues for the well-aligned jets required by the IC/CMB model. In the case of PKS~1510-089, it cannot be ruled out that the minimum gamma-ray flux level is due to a low state of the variable core which only matches the IC/CMB prediction by chance. Continued long-term monitoring with the Fermi/LAT could settle this issue by detecting a plateau signature in the recombined light-curve which would clearly signal the presence of a non-variable emission component.