A self-lensing supermassive binary black hole at radio frequencies: the story of Spikey continues

TL;DR

This paper presents evidence for a binary supermassive black hole in quasar J1918+4937 through VLBI radio imaging and modeling, revealing a wiggled jet structure consistent with binary orbital dynamics and spin precession.

Contribution

It introduces a jet structural model for eccentric SMBH orbits and constrains the system's parameters, providing strong evidence for a binary SMBH with a jetted secondary.

Findings

Jet morphology aligns with binary SMBH scenario.

Radio flux decrease suggests jet inclination change due to spin-orbit precession.

Secondary SMBH likely the jetted component.

Abstract

The quasar J1918+4937 was recently suggested to harbour a milliparsec-separation binary supermassive black hole (SMBH), based upon modeling the narrow spike in its high-cadence Kepler optical light curve. Known binary SMBHs are extremely rare, and the tight constraints on the physical and geometric parameters of this object are unique. The high-resolution radio images of J1918+4937 obtained with very long baseline interferometry (VLBI) indicate a rich one-sided jet structure extending to 80 milliarcseconds. Here we analyse simultaneously-made sensitive 1.7- and 5-GHz archive VLBI images as well as snapshot 8.4/8.7-GHz VLBI images of J1918+4937, and show that the appearance of the wiggled jet is consistent with the binary scenario. We develop a jet structural model that handles eccentric orbits. By applying this model to the measured VLBI component positions, we constrain the inclination of the radio jet, as well as the spin angle of the jet emitter SMBH. We find the jet morphological model is consistent with the optical and radio data, and that the secondary SMBH is most likely the jetted one in the system. Furthermore, the decade-long 15-GHz radio flux density monitoring data available for J1918+4937 are compatible with a gradual overall decrease in the the total flux density caused by a slow secular change of the jet inclination due to the spin-orbit precession. J1918+4937 could be an efficient high-energy neutrino source if the horizon of the secondary SMBH is rapidly rotating.