LoTSS of dual AGN: enhanced detectability and prospects for a systematic search

Abstract

Context: Dual active galactic nuclei (DAGN) - systems of two accreting supermassive black holes at kpc-scale separations - trace galaxy mergers and are the progenitors of gravitational-wave sources targeted by pulsar timing arrays and the upcoming Laser Interferometer Space Antenna. However, their radio properties remain poorly constrained, particularly at high redshift. Aims: We demonstrate the detectability of high-redshift DAGN at 144 MHz through sub-arcsec imaging of the LOFAR Two-metre Sky Survey (LoTSS) and characterize their low-frequency radio properties. Methods: We cross-matched 92 spectroscopically confirmed DAGN (z > 0.3, projected separation r_p < 30 kpc) with the LoTSS DR2 catalog and reprocessed international LOFAR telescope (ILT) data for three systems (0.59 < z < 2.39), achieving ~0.3 arcsec resolution at 144 MHz. Multi-frequency analysis and brightness-temperature diagnostics were used to investigate the radio emission. Results: We obtained high-quality ILT images for all three systems. In one source at z = 1.49, both AGN are detected at >5 sigma, making it one of the highest-redshift and faintest radio-detected DAGN. Brightness temperatures confirm AGN-powered emission even in sub-mJy sources. In the other two systems only one AGN is detected; for one, the new data constrain the low-frequency spectrum of a compact steep-spectrum source, indicating that its morphology is driven by jet-ISM interaction rather than the companion galaxy. For the 92 objects sample, the LoTSS detection rate is ~24%, significantly higher than for matched single AGN (~9-10%), suggesting an enhanced detectability of DAGN in the LoTSS. Conclusions: Low-frequency sub-arcsec radio observations provide an efficient, dust-unbiased method to detect and characterize DAGN. This pilot study establishes a benchmark for the forthcoming all-sky ~0.3 arcsec iLoTSS survey and future SKA observations.