One-year modulation in the flux density time series of low frequency variables.

Low frequency variability of extragalactic radio sources is commonly interpreted in terms of refractive interstellar scintillation. ln order to study the effect of the Earth orbital motion through the scintillation pattern we have analysed 408 MHz variability data on 43 sources in a search for statistically significant one year periodicity which could be attributed to such a motion. This analysis has been carried out by calculating an average flux density structure function for all sources, as well as for subgroups based on galactic latitude, galactic longitude, ecliptic latitude. In all classes we find a clear characteristic time scale of about 0.5 years corresponding to a one year modulation in the time series. The statistical significance of this result is confirmed by Monte Carlo simulations of variable sources and calibrators. This result is also consistent with numerical simulations of both thin and thick screen scattering models based on the refractive interstellar scintillation theory. A significant constraint on the propagation velocity of the turbulent irregularities has been obtained (V_irr_ < 10 km/s), suggesting that the motion between the Earth and the scattering pattern is, on average, mainly determined by the motion of the Earth-Sun system with respect to the Local Standard of Rest and the Earth orbital motion around the Sun.