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 that 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 active galactic nuclei that 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 kiloparsec-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 that 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 that would clearly signal the presence of a nonvariable emission component.