Rapid flux variations in compact radio sources: A shell model

Abstract

In this paper, we discuss the flux variations of a shell whose expansion and bulk-motion are both at relativistic speeds. It is shown that superluminal flux variations can be produced even if the Lorentz factors of expansion and bulk-motion take moderate values (2–3), and that this kind of shell model suggested here could be significant for interpreting very rapid flux variations at centimeter wavelengths (time scale of 1–10 days) in some radio variable sources.