Very Long Baseline Array observations of the Intraday Variable source J 1128 + 592

Context. Short time-scale flux density variations of flat spectrum radio sources are often explained by the scattering of radio waves in the turbulent ionized Interstellar Matter of the Milky Way. One of the most convincing observational arguments in favor of this is the annual modulation of the variability time-scale caused by the Earth orbital motion around the Sun. J1128+5925 is a recently discovered IDV source with a possible annual modulation in its variability time-scale. New observations suggest a change in its variability characteristics. Aims. We search for changes in source-structure or in the interstellar medium to 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 enable us to calculate the distance to the screen causing the variations in J1128+5925. Methods. We observed the source in six consecutive epochs with the Very Long Baseline Array (VLBA) at three frequencies, 5 GHz, 8 GHz and 15 GHz 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). Results. The VLBA observations revealed an east-west oriented core-jet structure with no significant motion in the jet. The expansion of the VLBI core leads to an estimate of mild relativistic speed (2.5 c ± 1.4 c). 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, however, the VLBI core size expansion offers a possible explanation to the observed decrease of the strength of IDV. VLBI polarimetry revealed 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 (37 pc distance), which covers the source (core and jet) and which may be responsible for the IDV. Superposition of polarized sub-components below the angular resolution limit may affect the observed RM as well.