Identification of Potential Weak Target Radio Quasars for ASTRO-G In-Beam Phase-Referencing

TL;DR

This paper presents a method to identify weak quasars suitable as in-beam phase-reference targets for the ASTRO-G space VLBI mission, enabling detailed radio structure studies of these faint sources.

Contribution

The study introduces an efficient selection technique combining optical and radio data to find potential VLBI targets near known calibrators for ASTRO-G.

Findings

Approximately 20 candidate quasars identified within 12" of known calibrators.

Most candidates have not been previously imaged with VLBI.

Method improves target selection independent of satellite maneuvering capabilities.

Abstract

We apply an efficient selection method to identify potential weak Very Long Baseline Interferometry (VLBI) target quasars simply using optical (SDSS) and low-resolution radio (FIRST) catalogue data. Our search is restricted to within 12" from known compact radio sources that are detectable as phase-reference calibrators for ASTRO-G at 8.4 GHz frequency. These calibrators have estimated correlated flux density >20 mJy on the longest ground-space VLBI baselines. The search radius corresponds to the primary beam size of the ASTRO-G antenna. We show that ~20 quasars with at least mJy-level expected flux density can be pre-selected as potential in-beam phase-reference targets for ASTRO-G at 8.4 GHz frequency. Most of them have never been imaged with VLBI. The sample of these dominantly weak sources offers a good opportunity to study their radio structures with unprecedented angular resolution provided by Space VLBI. The method of in-beam phase-referencing is independent from the ability of the orbiting radio telescope to do rapid position-switching manoeuvres between the calibrators and the nearby reference sources, and less sensitive to the satellite orbit determination uncertainties.