Radio/X-Ray Offsets of Large-Scale Jets Caused by Synchrotron Time Lags

In the internal shock scenario, we argue that electrons in most kiloparsec-scale (or even larger) jets can be accelerated to energies high enough to emit synchrotron X-rays, if shocks exist on these scales. These high-energy electrons emit synchrotron radiation at high frequencies and cool as they propagate downstream along the jet, emitting at progressively lower frequencies and resulting in time lags and hence radio/X-ray (and optical/X-ray if the optical knot is detectable) offsets at bright knots, with the centroids of X-ray knots being closer to the core. Taking into account the strong effects of jet expansion, the behavior of radio/X-ray and optical/X-ray offsets at bright knots in M87, Centaurus A, 3C 66B, 3C 31, 3C 273, and PKS 1127-145 is consistent with that of synchrotron time lags due to radiative losses. This suggests that the large-scale X-ray and optical jets in these sources are due to synchrotron emission.