Phase-Reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138

Abstract

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.