The Surprising Nature of kpc-scale Quasar Jets as Revealed by Gamma-ray Observations

Abstract

The physical origin of the X-ray emssion from radio knots in powerful quasar jets has been a long-standing mystery. Though we know that 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, then the X-rays could be due to inverse-Compton upscattering of CMB photons. However, the IC/CMB explanation also predicts a high level of gamma-ray emission. Our recent work shows that this emission is not seen to a very high significance (>99.9% confidence) in at least two sources, 3C 273 and PKS 0637-752. Further, our findings suggest that these jets are not highly relativistic and that the X-rays are synchrotron in origin. I will discuss some of the surprising implications of these findings for the energetics and radiative output of powerful quasars.