# High-Resolution Radio Observations of Five Optically Selected Type 2 [PERSON]
## 4 Data Analysis and Results  
### EVN Data  
The raw correlated VLBI visibility data that are now in public domain and can be obtained from the EVN Data Archive ([[https://old.jive.nl/archive-info?experiment=EP093A_150605](https://old.jive.nl/archive-info?experiment=EP093A_150605)]([https://old.jive.nl/archive-info?experiment=EP093A_150605](https://old.jive.nl/archive-info?experiment=EP093A_150605)) and [[https://old.jive](https://old.jive)]([https://old.jive](https://old.jive)). nl/archive-info?experiment=EP093B_150615) were loaded into AIPS for initial calibration. We followed standard procedures [34]. At the beginning, the data affected by known problems with antenna pointing and radio frequency interference were flagged. The visibility amplitudes were calibrated using antenna gains and system temperatures measured at the telescope sites. We used nominal system temperature values where measurements were unavailable. We corrected for the dispersive ionospheric delay using total electron content maps derived from global navigation satellite systems data, and also applied parallactic angle corrections. Using a 1-min scan on the strong fringe-finder source 0528+134, we performed an initial correction of instrumental phases and delays. We then performed fringe-fitting for all the five phase-reference calibrator sources and the fringe-finders (0528+134, 3C 138, and 4C 39.25) scheduled in the experiments. These calibrated visibility data were exported to the Difmap package [35] for hybrid mapping. This involved iterations of the CLEAN algorithm and phase-only self calibration, and finally phase and amplitude self-calibration. We used the brightness distribution models obtained for the calibrator and fringe-finder sources to derive overall antenna gain correction factors (up to 6% in case of some antennas). These were applied in AIPS to the target source data as well, to refine the initial amplitude calibration. As the next step, we repeated fringe-fitting for the five phase-reference calibrators in AIPS. This time the CLEAN component models of their brightness distributions were used as an input, to account for any residual phases due to their mas-scale structure. The solutions were interpolated and applied to the data of the respective target sources. The calibrated and phase-referenced visibility data of the targets (J0741+3020, J0956+5735, J1014+0244, J1026\(-\)0042, and J1447+0211) were transferred from AIPS to Difmap for imaging. We applied natural weighting with the visibility errors raised to the power \(-1\) (uvweight 0, -1 command in Difmap) in order to minimize the image noise. At both observing frequencies, 1.7 and 5 GHz, the dirty images of all target sources but J1026\(-\)0042 showed peaks with signal-to-noise ratios below 7, as seen in a rectangular field of view of 512 mas \(\times\) 512 mas centered on the brightest pixel. Therefore, we regard J0741+3020, J0956+5735, J1014+0244, and J1447+0211 as non-detections with the EVN. For J1026\(-\)0042, the calibrated visibility data were fitted directly with a circular Gaussian brightness distribution model in Difmap [35]. The images of J1026\(-\)0042 at both frequencies are displayed in Figure 1.  
Figure 1: VLBI images of SDSS J1026\(-\)0042 at 1.7 GHz (**left**) and 5 GHz (**right**). The red cross marks the _Gaia_ DR2 optical position [28; 29], the blue cross indicates the EVN 5 GHz position (see Section 5.1). The peak brightness values are 1.01 mJy beam\({}^{-1}\) at 1.7 GHz, and 0.20 mJy beam\({}^{-1}\) at 5 GHz. The contour levels are drawn at 3 \(\sigma\)\(\times\) (–1, 1, 2, 4), where the 3 \(\sigma\) image noise is 165 \(\mu\)Jy beam\({}^{-1}\) and 65 \(\mu\)Jy beam\({}^{-1}\) at 1.7 and 5 GHz, respectively. The size of the elliptical Gaussian restoring beam (HPBW) is shown as an ellipse in the lower-left corner of the images. The HPBW is 6.5 mas \(\times\) 4.5 mas with a major axis position angle \(PA=74.3^{\circ}\) (measured from north through east) at 1.7 GHz, and 1.83 mas \(\times\) 1.67 mas with \(PA=29.2^{\circ}\) at 5 GHz.