Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
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6.6. Periodicity of OJ 287

In one exceptional blazar, OJ 287, there is evidence for periodic flaring on a time scale of ~ 12 years (Takalo 1994). Only three cycles have been seen during the epoch of dense monitoring, but the third of these was predicted to within six weeks from the light curve of the preceding 100 years (Sillanpaa et al 1988, Babadzhanyants et al 1992, Kidger et al 1992). Including the most recent data, the Fourier transform of the optical light curve shows six peaks, the two strongest of which correspond to 12.13 years, and its first harmonic, 6.07 years (Sillanpaa et al 1996). The period, if real, has a range of at least 1%; also, OJ 287 exhibits outbursts outside of the periodic oscillations, including some of comparable amplitude. A better assessment of the statistical significance of this period remains to be done.

If the light curve of OJ 287 does indeed contain a 12-year periodic component, its explanation in terms of precession of orbiting black holes is not straightforward: It requires two black holes with very high masses, 108 and 1.7 × 1010 Msun, and high eccentricity, epsilon = 0.7, with a very short lifetime of 104 years (Lehto & Valtonen 1996).

In no other blazar is there convincing evidence for either short- or long-term periodicities, so this is an extremely important precedent. Other periodicities - rotation of the position angle of the optical polarization with an apparent period of 27-35 days (Efimov & Shakhovskoy 1996), smaller fluctuations in intensity with periods of 10-20 min (Carrasco et al 1985, Komesaroff et al 1988, De Diego & Kidger 1990) - have also been reported for OJ 287 but have been (at best) of a transitory nature. We note that the observed rotation of the polarization position angle, even if not strictly periodic, is a remarkable phenomenon, which may be related to a lighthouse effect within a magnetized relativistic jet (Camenzind & Krockenberger 1992).

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