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

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Abstract

The BL Lac-type active galaxy OJ 287 exhibits a 12 year periodicity with a double-peaked maxima in its optical flux variations. Several models sought to explain this periodicity, the first one firmly established 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 flaring is due to a consequent increase in the Doppler boosting factor. In another class of models the optical flaring reflects 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 flux density monitoring data between 8 and 90 GHz from the Michigan, Metsähovi, and Swedish-ESO Submillimeter Telescope AGN monitoring programs. We find that the radio flux density and polarization data, as well as the optical polarization data, indicate that the first of the two optical peaks is a thermal flare occurring in the vicinity of the black hole and the accretion disk, while the second one is a synchrotron flare originating in a shocked region down the jet. None of the proposed binary black hole models for OJ 287 offers 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 flare visible only in the optical regime. The pericenter passage enhances accretion into the primary black hole, leading to increased jet flow and formation of shocks down the jet. These become visible as standard radio and optical synchrotron flares roughly a year after the pericenter passage and are identified 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 (≥1010 M☉) primary black hole. If our interpretation is correct, the next periodic optical flare, a thermal one, should occur around 2006 September 25. Nonthermal, simultaneous optical and radio flares should follow about a year later.

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