Radio Monitoring of OJ 287 and Binary Black Hole Models for Periodic Outbursts

Abstract

The BL LacÈtype active galaxy OJ 287 exhibits a 12 year periodicity with a double-peaked maxima in its optical Ñux variations. Several models sought to explain this periodicity, the Ðrst one Ðrmly estab-lished in any active galactic nucleus (AGN), as a result of the orbital motion of a pair of supermassive black holes. In one class of models the orientation of the jets changes in a regular manner, and the optical Ñaring is due to a consequent increase in the Doppler boosting factor. In another class of models the optical Ñaring reÑects a true increase in luminosity, either due to an enhanced accretion during the pericenter passage or due to a collision between the secondary black hole and the accretion disk of the primary black hole. However, these models have been based solely on the optical data. Here we consider the full radio Ñux density monitoring data between 8 and 90 GHz from the Michigan, and MetsaŽ hovi, Swedish-ESO Submillimeter Telescope AGN monitoring programs. We Ðnd that the radio Ñux density and polarization data, as well as the optical polarization data, indicate that the Ðrst of the two optical peaks is a thermal Ñare occurring in the vicinity of the black hole and the accretion disk, while the second one is a synchrotron Ñare originating in a shocked region down the jet. None of the proposed binary black hole models for OJ 287 o ers satisfactory explanations for these observations. We suggest a new scenario, in which a secondary black hole penetrates the accretion disk of the primary during the pericenter passage, causing a thermal Ñare visible only in the optical regime. The pericenter passage enhances accretion into the primary black hole, leading to increased jet Ñow and formation of shocks down the jet. These become visible as standard radio and optical synchrotron Ñares roughly a year after the pericenter passage and are identiÐed with the second optical peaks. In addition to explaining the radio and the optical data, our model eliminates the need for a strong precession of the binary and for an ultramassive (º1010 primary black hole. If our interpretation is correct, the next periodic M _) optical Ñare, a thermal one, should occur around 2006 September 25. Nonthermal, simultaneous optical and radio Ñares should follow about a year later.