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PKS 1155+251 is a radio-loud quasar source at z=0.203. Observations usingvery long baseline interferometry (VLBI) at ~2, 5, 8 and 15 GHz show that thestructure of the radio source is quite complicated on parsec scales and thatthe outer hot spots are apparently undergoing a significant contraction.Because these results cannot be fully explained based on the compact symmetricobject (CSO) scenario with a radio core located between the northern andsouthern complexes, we made observations with the Very Long Baseline Array(VLBA) at 24 and 43 GHz to search for compact substructures and alternativeinterpretations. The results show that the radio core revealed in the previousVLBI observations remains compact with a flat spectrum in oursub-milli-arcsecond--resolution images; the northern lobe emission becomesfaint at 24 GHz and is mostly resolving out at 43 GHz; the southern complex ismore bright but has been resolved into the brightest southern-end (S1) and jetor tail alike components westwards. Explaining the southern components alignedwestward with a standard CSO scenario alone remains a challenge. As for theflatter spectral index of the southern-end component S1 between 24 and 43 GHzin our observations and the significant 15 GHz VLBA flux variability of S1, analternative scenario is that the southern complex may be powered by a secondaryblack hole residing at S1. But more sensitive and high-resolution VLBImonitoring is required to discriminate the CSO and the binary black holescenarios.

VLBA 24 and 43 GHz observations of massive binary black hole candidate PKS 1155+251