The rise and fall of a binary AGN candidate: the story of PSO
  J334.2028+1.4075
  Apparently periodic optical variations of the luminous high-redshift (z=2.06)
quasar PSO J334.2028+1.4075 led Liu et al. (2015) to interpret the variability
as the orbital period of a binary supermassive black hole (SMBH) residing in a
single circumbinary accretion disk. The proposed orbital separation was around
0.006 pc, and the possible inspiral time about 7 yr in the rest frame of the
quasar. Such objects would be of high interest as the difficult-to-find end
products of binary SMBH evolution, and potential sources of low-frequency
gravitational waves. However, extending the time baseline of the variability
study, Liu et al. (2016) later found that the periodicity of PSO
J334.2028+1.4075 does not remain persistent. Foord et al. (2017) did not find
evidence for the binary active galactic nucleus scenario based on Chandra X-ray
observations. The object has also been studied in detail in the radio (Mooley
et al. 2018) with the Karl G. Jansky Very Large Array (VLA) and the Very Long
Baseline Array (VLBA), revealing a lobe-dominated quasar at kpc scales, and
possibly a precessing jet, which might retain PSO J334.2028+1.4075 as a binary
SMBH candidate. Here we report on our 1.7-GHz observation with the European
VLBI Network (EVN) which complements the VLBA data taken at higher frequencies,
and discuss the current knowledge about the nature of this interesting object.
