2009A&A...508..161G

Short timescale flux density variations in flat spectrum radio sources are often explained by the scattering of radio waves in the turbulent ionized interstellar medium of the Milky Way. One of the most convincing observational arguments in favor of this is the annual modulation of the variability timescale caused by the orbital motion of Earth around the Sun. J1128+5925 is a recently discovered IDV source with a possible annual modulation in its variability timescale. New observations suggest a change in its variability characteristics. We search for changes either in source structure or in the interstellar medium that can explain the variations in the IDV pattern of J1128+5925. Using Very Long Baseline Interferometry (VLBI), we study a possible relation between source orientation on the sky and anisotropy angle seen in the annual modulation. Additionally, obtaining a source size estimate from VLBI data enables us to calculate the distance to the screen causing the variations in J1128+5925. We observed the source in six consecutive epochs with the Very Long Baseline Array (VLBA) at three frequencies, 5GHz, 8GHz, and 15GHz in total intensity and polarization. This data are combined with our densely time-sampled flux-density monitoring performed with the radio telescopes at Effelsberg (Germany) and at Urumqi (China). The VLBA observations detected an east-west oriented core-jet structure with no significant motion in its jet. The expansion of the VLBI core leads to an estimate of mild relativistic speed (2.5c±1.4c). The position angle of the VLBI jet agrees with the angle of anisotropy derived from the annual modulation model. No significant long-term structural changes were observed with VLBI on mas-scales, although, the VLBI core-size expansion offers a possible explanation of the observed decrease in the strength of IDV. VLBI polarimetry measured significant changes in the electric vector position angle (EVPA) and rotation measure (RM) of the core and jet. Part of the observed RM variability could be attributed to a scattering screen (37pc distance) that covers the source (core and jet) and may be responsible for the IDV. Superposition of polarized subcomponents below the angular resolution limit may also affect the observed RM.