2024ApJ...965....9N

We present multiband follow-up observations of the supermassive binary black hole (BBH) candidate and misaligned double-double radio galaxy, J1328+2752. To investigate its parsec-scale structure, we have carried out observations with the Very Long Baseline Array (VLBA) and the European Very Long Baseline Interferometry (VLBI) Network. Additionally, we have obtained optical spectroscopic observations with the 3.6 m Devasthal Optical Telescope. Within 3.5 yr of our previous VLBI observations, the central parsec-scale radio structure of J1328+2752 has changed from a double component to a single central component and its flux density has increased by a factor of 3 in the 5 GHz VLBA observations. The new radio component is largely unresolved at 3, 5, and 8 GHz. The multifrequency radio data shows a convex-shaped spectrum for this unresolved component. The turnover frequency is at ∼3 GHz. While the total intensity image does not reveal any details, a super-resolved VLBA 5 and 8 GHz spectral index image does indicate the presence of two components at a separation of ∼4.2 pc with spectral indices -0.5 ± 0.3 and -0.9 ± 0.3. We have not observed a simultaneous change in the optical line profiles or intensities over the past few years. The radio structure, the variation of the flux density, and spectral shape can potentially be consistent both with the signature of a young radio source or a BBH at the center of J1328+2752.