VLBA 24 and 43 GHz observations of massive binary black hole candidate
  PKS 1155+251
  PKS 1155+251 is a radio-loud quasar source at z=0.203. Observations using
very long baseline interferometry (VLBI) at ~2, 5, 8 and 15 GHz show that the
structure of the radio source is quite complicated on parsec scales and that
the outer hot spots are apparently undergoing a significant contraction.
Because these results cannot be fully explained based on the compact symmetric
object (CSO) scenario with a radio core located between the northern and
southern complexes, we made observations with the Very Long Baseline Array
(VLBA) at 24 and 43 GHz to search for compact substructures and alternative
interpretations. The results show that the radio core revealed in the previous
VLBI observations remains compact with a flat spectrum in our
sub-milli-arcsecond--resolution images; the northern lobe emission becomes
faint at 24 GHz and is mostly resolving out at 43 GHz; the southern complex is
more bright but has been resolved into the brightest southern-end (S1) and jet
or tail alike components westwards. Explaining the southern components aligned
westward with a standard CSO scenario alone remains a challenge. As for the
flatter spectral index of the southern-end component S1 between 24 and 43 GHz
in our observations and the significant 15 GHz VLBA flux variability of S1, an
alternative scenario is that the southern complex may be powered by a secondary
black hole residing at S1. But more sensitive and high-resolution VLBI
monitoring is required to discriminate the CSO and the binary black hole
scenarios.
