2006A&A...451...85G

The quasar B2005+403 located behind the Cygnus region in our Galaxy, is useful in studying the interplay between propagation effects, which are extrinsic to the source, and source intrinsic variability. On the basis of VLBI experiments performed at 1.6, 5, 8, 15, 22, and 43GHz during the years 1992-2003 and parallel multi-frequency monitoring of the total flux density (between 5-37GHz), we investigated the variability of total flux density and source structure. Below 8GHz, the point-like VLBI source is affected by scatter-broadening of the turbulent interstellar medium (ISM), which is located along the line of sight and likely associated with the Cygnus region. We present and discuss the measured frequency dependence of the source size, which shows a power-law with slope of -1.91±0.05. From the measured scattering angle at 1GHz of θ=77.1±4.0mas a scattering measure of SM=0.43±0.04m^–20/3kpc is derived, consistent with the general properties of the ISM in this direction. The decreasing effect of angular broadening towards higher frequencies allows us to study the internal structure of the source. Above 8 GHz new VLBI observations reveal a one-sided, slightly south-bending core-jet structure, with stationary and apparent superluminally-moving jet components. The observed velocities range from 6.3-16.8c. The jet components move on non-ballistic trajectories. In AGN, total flux density variations are often related to the emergence of new VLBI components. However, almost eleven years no new component was ejected in B2005+403. A striking feature in the flux density variability is a trough observed at 5-37 GHz and between 1996 and 2001. This trough is more pronounced at higher frequencies, where it shows a prominent flux density decrease of ∼50-60% on a time scale of 2-3yr. The trough can be explained as a blending effect of decreasing and increasing jet component fluxes. Densely time-sampled flux density monitoring observations with the 100 m Effelsberg telescope reveal an intra-day variability (IDV) at 1.6GHz with a modulation index of m=1.0%. At 5GHz less pronounced variations are seen (m=0.5%). This and a relatively short variability time scale of ∼0.1-days imply a second, less dense or turbulent scattering screen at nearby (few to hundred parsec) distance.