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2. VLA and VLBI observations

2.1. VLA observations

The source was observed with the VLA (Thompson et al. 1980) in the A configuration on 1990 May 17 at 8.4 and 15GHz (resolution tex2html_wrap_inline10500.30tex2html_wrap_inline1052 and tex2html_wrap_inline10500.17tex2html_wrap_inline1052 respectively) for about 15 minutes. Two IF channels each with a 50MHz bandwidth and separated by 50MHz were used at both frequencies. The data were calibrated using the standard VLA calibrators and the source imaged with the NRAO AIPS programs.

2.2. The arcsecond scale structure

The VLA image of 1422+202 at 8.4GHz shows that the source structure is mainly elongated north-south. It contains several blobs of emission labelled from a to e in Fig. 1 (click here). A faint extended region of emission (component f) is also seen off-axis near component e. This last component and component d are not detected at 15GHz (see Fig. 2 (click here)). Most of the emission from the remaining components is resolved out. Component e shows a ridge of emission along the major axis and the bright peak appears resolved in two components. Component b, unresolved at 8.4GHz shows here an extension in Position Angle (PA) tex2html_wrap_inline105040tex2html_wrap_inline1072. The brightness distribution can be fitted with a two Gaussian model. If the bright peak of emission in b is instead fitted with a single circular Gaussian model and then subtracted, the residual map shows 1-2mJy left south-west of the peak. So we believe that the extension is real.

Figure 1: 8.4GHz VLA. The beam is 0.30tex2html_wrap_inline10760.27tex2html_wrap_inline1052 in PA tex2html_wrap_inline1080. The noise is 0.06mJy tex2html_wrap_inline1082. Contours are at -0.2, 0.2, 0.4, 0.6, 0.8, 1, 2, 4, 8, 16, 32, 64, 128, 256mJy tex2html_wrap_inline1082. The peak flux density is 102.1mJy tex2html_wrap_inline1082. Component b is believed to be the core

Figure 2: 15GHz VLA map. The beam is 0.17tex2html_wrap_inline10760.15tex2html_wrap_inline1052 in PA 50tex2html_wrap_inline1072. The noise is 0.1mJy tex2html_wrap_inline1082. Contours are at -0.4, 0.4, 0.6, 0.8, 1, 2, 4, 8, 16, 32, 64mJy tex2html_wrap_inline1082. The peak flux density is 34.0mJy tex2html_wrap_inline1082. A cross marks the position of the optical counterpart

Previous observations of 1422+202 made with MERLIN at 408MHz and with the VLA at 5GHz can be found in Mantovani et al. (1992). The three VLA images of 1422+202 at 5, 8.4 and 15GHz were convolved with the same circular Gaussian beam (FWHM 0.5tex2html_wrap_inline1052). The spectral index distributions were obtained for two ranges, tex2html_wrap_inline1106 and tex2html_wrap_inline1108. The spectral index is everywhere much steeper than 0.4 (tex2html_wrap_inline1110). Some flattening of the slope is visible only for component b. We can say more about the spectral shape of the emission in b taking into account the MERLIN map at 408MHz. There the component b was not detected, and we can put an upper limit of tex2html_wrap_inline10505mJy to its emission. From the VLA convolved maps at 5, 8.4 and 15Hz we have 34, 26 and 15 mJy peak respectively, suggesting that 1422+202 has a Giga-Hz-peaked Spectrum core which peaks about tex2html_wrap_inline1114. This is confirmed if we also plot the flux density of 15.1mJy we got at 1.6GHz from the VLBI map (see Table 1 (click here)). Such a value fits with a curved spectral index peaking at tex2html_wrap_inline10504GHz. The component b is believed to be the core of 1422+202.

The overall structure of the source is thus rather asymmetric, with a long collimated wiggling jet pointing south, no evidence at the detection limit of our maps of a counter-jet, a weak nearby hot spot to north (component a) and a bright hot spot at the end of the jet on the opposite side (component e). The jet major axis changes in PA several time along its path. The core, for example shows an extension in PA tex2html_wrap_inline1118, quite different from the PA of the ridge of emission in component e which is tex2html_wrap_inline1120. Thus we suggest that the jet is the projected image of a helical precessing jet.

2.3. VLBI observations

The VLBI observations were made at 1.6GHz on 1987 March 1 with the EVN recording with the MarkIIIA terminal in Mode B and standard setup. The source 1422+202 was tracked for about 11 hours together with the calibration source OQ208, observed for three scans, 13 min long each, regularly spaced over the experiment. The data recorded at each station were correlated at the MarkIII correlator of the Max-Planck-Institut für Radioastronomie.

The raw data output from the correlator, were read with the MK3IN-program (Bååth & Mantovani 1991) and analysed with AIPS. Our aim was the detection and the imaging of the two components (the core and the south hot spot) separated by tex2html_wrap_inline10508tex2html_wrap_inline1052 detected during a VLBI pilot experiment with the short baseline Effelsberg-Westerbork. The wide field mapping technique described in Bååth (1991) could not be used directly for finding fringes. This technique requires a phase-cal signal in each independent IF-channel to allow the removal of the phase differences between the IF-channels in the postprocessing stage. Unfortunately, the phase-cal signal was not injected at all stations so we had to follows a different strategy. The fringes were searched with the task CALIB for each IF channel independently on the calibrator source OQ208. The solutions found for OQ208 were applied to 1422+202, which is 9.6tex2html_wrap_inline1072 away. This technique is equivalent to using a phase-cal signal, and allowed us to thereafter remove the single and multiband band delays on 1422+202. The multiband delays were fitted after averaging each IF over the frequency channels. In other words, the phase referencing technique, which usually requires to observe switching between the calibrator and the target source with a short duty cicle, was successfully applied even in this case where the calibration source was observed only three times.

The source 1422+202 was then imaged without obtaining any further fringe solution. It showed up with an absolute position which agrees with the VLA position. This will be discussed further in Sect. 3.2. The image had a well defined compact component coinciding with the expected position of the core. We then proceeded by fitting for station based phase offsets in order to further focus the image.

2.4. The milliarcsecond scale structure

The VLBI map is shown in Fig. 3 (click here). The map was obtained by restoring the field with a coarse beam of 0.15tex2html_wrap_inline10760.15tex2html_wrap_inline1052. All the extended structure has been resolved out. Only two components were detected in the imaged field, almost aligned north-south, separated by tex2html_wrap_inline10508tex2html_wrap_inline1052. The main component lies in the area were the core of the source is. The second component is weaker and slightly extended. Its position coincides with that of the south hot spot seen in the VLA maps.

Figure 3: 1.6GHz VLBI map. The beam is 0.15tex2html_wrap_inline10760.15tex2html_wrap_inline1052. The noise is 0.5mJy tex2html_wrap_inline1082. Contours are at -1.5, 1.5, 3, 5, 7, 9, 11, 13, 15, 17, 20mJy tex2html_wrap_inline1082. The peak flux density is 14.2mJy tex2html_wrap_inline1082. A cross marks the position of the optical counterpart

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