Abstract

We apply the binary black hole model to explain the wiggles in the milliarcsec radio jet of the superluminal quasar 1928+738 (4C73.18) observed with VLBI at 1.3 cm wavelength by Hummel et al. (1992). The period and amplitude of the wiggles can be explained as due to the orbital motion of a binary black hole with mass of order 108 solar masses, mass ratio larger than 0.1 and orbital radius ∼ 1016 cm. The jet's inclination to the line of sight should be small, confirming the standard interpretation of superluminal motion and one-sidedness as due to relativistic motion in a direction close to the line of sight. The small orbital radius suggests that the binary has been losing a significant amount of orbital energy during the last 107 years, possibly by interaction with the matter which is flowing through the active galactic nucleus.

Introduction

Galaxy mergers must have been a common phenomenon especially during the collapse and virialisation of rich groups and clusters of galaxies. These mergers lead to the formation of massive binary black holes in galactic nuclei if black holes of 107−9M⊙ are formed in the nuclei of most bright galaxies at redshifts of about 2. A massive binary black hole (MBBH) may manifest itself by Lens-Thirring precession of a jet emitted along the spin axis of one of the holes (Begelman et al. 1980, hereafter BBR).