Are the hotspots of radio galaxies the sites of in situ acceleration of relativistic particles?

Abstract

Using a large set of optically detected hotspots in powerful extragalactic double radio sources, we examine the basic question of whether the detection of optical synchrotron radiation requires in situ acceleration of relativistic electrons within the hotspots/lobes. For this, we take into account the jet's bulk relativistic motion, as well as its likely misalignment from the plane of the sky. Together, both these factors can drastically reduce the apparent range of the ultra-relativistic electrons ejected from the nucleus in the form of a jet. The conventionally adopted parameter space for the fundamental variables, namely, the hotspot magnetic field, radio source orientation angle relative to the line-of-sight and the bulk speed of the jet plasma, is considered. We find that the observed optical/near-IR synchrotron emission of the hotspots can be explained even if the radiating relativistic electrons were accelerated exclusively within the nuclear region, provided the energy losses incurred by the electrons during their transport down the jet are dominated by inverse Compton upscatterings of the cosmic microwave background photons. Under this circumstance, in situ acceleration of relativistic electrons inside the hotspots or lobes is not found to be mandated by their reported optical/near-infrared detections.