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

TL;DR

This study uses high-frequency VLBA observations to investigate the complex radio structure of quasar PKS 1155+251, exploring the possibility of a binary black hole system versus a standard CSO scenario.

Contribution

The paper presents 24 and 43 GHz VLBA observations revealing detailed core and jet structures, proposing a potential binary black hole scenario over the traditional CSO model.

Findings

The radio core remains compact with a flat spectrum.

The southern complex shows signs consistent with a secondary black hole.

High-resolution observations are needed to confirm the binary black hole hypothesis.

Abstract

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.