2026A&A...705A.105Z

Context. Our previous work identified a class of SDSS quasars with multiple Gaia detections, classifying them as candidates for various astrophysical systems, such as quasar-star pairs, dual quasars, and gravitationally lensed quasars. In this paper we present a pilot very long baseline interferometry (VLBI) study targeting a radio-bright subsample and report the first high-resolution imaging results.
Aims. By leveraging the milliarcsecond-scale resolution of VLBI and its precise astrometric coordination with Gaia, we aim to refine the classification of these multiple matched sources, search for potential dual active galactic nuclei (AGNs), and assess the efficacy of the combined Gaia-VLBI approach in resolving ambiguous quasar systems.
Methods. We cross-matched the Strange QUasar Candidates With ABnormal Astrometric Characteristics (SQUAB) quasar sample with the FIRST and NVSS catalogs, identifying 18 radio-emitting sources. The three brightest were selected for dual-frequency (1.6 and 4.9 GHz) VLBA observations. We performed VLBI imaging at both Gaia positions, constructed spectral index maps, and estimated brightness temperatures to characterize the radio morphology and physical properties.
Results. For the three target sources, our VLBI observations reveal compact radio structures consistent with single AGNs at the primary Gaia positions. No significant emission is detected at the secondary Gaia locations. These results support the interpretation of the sources as quasar-star pairs, in line with earlier studies.
Conclusions. This pilot study demonstrates the value of radio-VLBI high-resolution follow-ups on Gaia-selected quasar systems with multiple counterparts, showing how they can unambiguously reveal the true nature of these systems and help remove contaminants from dual AGN candidate samples.