Annual Modulation in Intraday Variability Timescales of Extragalactic Radio Sources

Abstract

Annual modulations in timescales of intraday variability (IDV here- after) are discussed for six extragalactic sources: 0716+714, 0917+624, 0954+65, 1749+70, 1803+78 and 2007+77. The timescales calculated from scintillation the- ory are compared with the observational data. It is emphasized that systematic observations are required to identify the phenomenon and to determine the motion of the interstellar medium with respect to the Local Standard of Rest. In addi- tion, significant deviations from the annual modulation could be due to intrinsic variations.