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Zhi-Qiang Shen, Lin-Lin Shang, Dong-Rong Jiang, Hong-Bing Cai, Xi Chen, Phase-Reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138, Publications of the Astronomical Society of Japan, Volume 58, Issue 6, 25 December 2006, Pages 1033–1037, https://doi.org/10.1093/pasj/58.6.1033
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Abstract
We describe a phase-reference VLBI observation that was conducted at 15.4 GHz by fast-switching VLBA antennas between the compact steep-spectrum radio source 3C 138 and the quasar PKS 0528|$+$|134, which are about |$4^\circ$| away on the sky. By comparing the phase-reference mapping with the conventional hybrid mapping, we demonstrate the feasibility of high-precision astrometric measurements for sources separated by |$4^\circ$|. VLBI phase-reference mapping preserves the relative phase information, and thus provides an accurate relative position between 3C 138 and PKS 0528|$+$|134 of |$\Delta\alpha =- {9{}^{\mathrm {m}}46\rlap {.}{}^{\mathrm {s}}531000} \pm {0\rlap {.}{}^{\mathrm {s}}000003}$| and |$\Delta\delta = {3{}^{\mathrm {\circ }}6{}^{\mathrm {\prime }}26\rlap {.}{}^{mathrm {\prime }}90311} \pm {0\rlap {.}{}^{\mathrm {\prime\prime}}00007}$| (J2000.0) in right ascension and declination, respectively. This gives an improved position of the nucleus (component A) of 3C 138 in J2000.0 to be |$\mathrm{RA} = {05{}^{\mathrm {h}}21{}^{\mathrm {m}}09\rlap {.}{}^{\mathrm {s}}885748}$| and |$\mathrm{Dec} = {16{}^{\mathrm {\circ }}38{}^{\mathrm {\prime }}22\rlap {.}{}^{\mathrm {\prime \prime }}05261}$| under the assumption that the position of calibrator PKS 0528|$+$|134 is correct. We further made a hybrid map by performing several iterations of CLEAN and self-calibration on the phase-referenced data with the phase-reference map as an input model for the first phase self-calibration. Compared with the hybrid map from the limited visibility data directly obtained from fringe fitting 3C 138 data, this map has a similar dynamic range, but a higher angular resolution. Therefore, the phase-reference technique is not only a means of phase connection, but also a means of increasing the phase coherence time, and thus allowing the self-calibration technique to be applied to much weaker sources.
