Scintillation of extended, extragalactic IDV sources

Thomas Beckert (MPIfR)

Intraday Variability of compact, flat-spectrum quasars and BL Lac objects 
between 1 and 10 Ghz is nowadays interpreted as due to scintillation in the 
interstellar medium. We present detailed calculations of modulation index and 
structure function shape for IDV light curves based on a turbulent scattering 
model. Two different time-scales in the scintillation of extended sources are 
found. Variability is strongest at the transition from weak to strong 
scattering, where most asymptotic approximations fail. The consequences for 
turbulence in the ISM and the brightness temperature of IDV sources are 
briefly discussed.