Astron. Astrophys. 355, 915-921 (2000)

1. Introduction

OJ287 is a BL Lac object which presents violent optical variability. Its light curve, going back to almost 100 years, shows three different types of variability. The strongest flares, of more than three magnitudes in the V band have a duration of a few months, almost constant color indices and very similar temporal behavior (Sillanpää et al. 1985). They occur with a periodicity of about 11.6 years and a total of 8 consecutive maxima were already detected (Sillanpää et al. 1988, 1996). Variations of several months were observed in the recurrence period, and of two magnitudes in the flare intensity. Another type of variability is represented by non-periodic flares of smaller amplitude and duration of a few days. Their spectral evolution resemble synchrotron flares from expanding, initialy compact and optically thick regions (Kidger et al. 1991). Finally, low intensity flickering is continuously present, sometimes with very short time periodicity (Carrasco et al. 1985; González-Pérez et al. 1996). At radio frequencies, good correlation is found between variability at high frequencies and the occurrence of optical synchrotron flares, with very little or no delay between them. However, the very strong periodic optical flares do not seem to have counterparts in the radio region (Valtaoja 1996). A detailed study of variability at radio frequencies was made by Hughes et al. (1998) using wavelet analysis. They found a periodicity of 1.6 years in the complete data series and of 1.1 years close to the epoch of the 1983 optical flare.

OJ287 is highly polarized, both at optical and radio wavelengths. The degree of linear polarization and its position angle change with time scales of hours to years. The optical polarization appears to increase with decreasing flux densities, reaching values as high as 40% (Hagen-Thorn 1980; Smith et al. 1987; Sillanpää 1991). At radio frequencies the polarization of the total flux was never larger than 15% but individual VLBI features presented values of more than 64% (Roberts et al. 1987; Aller et al. 1991), imposing serious constraints to the beaming models (Cawthorne & Wardle 1988). A systematic change in the mean polarization angle was observed between 1971 and 1991, both at radio and at optical wavelengths, the dispersion around the mean value being larger at optical than at radio wavelengths (Sillanpää 1991).

OJ287 has also been detected at high energies. The X-ray flux densities varied by a factor of three between measurements spaced by several years. Upper limits obtained by HEAO 1 and ROSAT instruments increase this factor to 20 or more (Worral et al. 1982; Comastri et al. 1995; Urry et al. 1996). The -ray emission was below the detection limit of the CGRO/EGRET instrument, except for a marginal detection at the epoch of the 1994-1995 optical flare (Schrader et al. 1996).

At parsec scales OJ287 has a very compact structure and it is frequently used as a VLBI phase reference. As many other BL Lac objects, it presents a one-sided jet, even though extending for only 3 mas when observed at 5 and 8.4 GHz, with a few knots expanding at moderate superluminal velocities (Gabuzda et al. 1989; Gabuzda & Cawthorne 1996; Vicente et al. 1996). However, closely spaced 43 GHz VLBI observations at the epoch of the 1994 optical flare showed a large number of features, extending up to distances of 2 mas from the core, expanding with large superluminal velocities (Marscher & Marchenko 1997). Similar results were obtained in the analysis of Geodetic 8.3 GHz VLBI maps at epochs 1990-1996 (Tateyama et al. 1999). VLA images show radio structure at a distance of 25 arc sec from the core ( kpc). Very high dynamical maps reveal an underlying curved tail joining both sources (Kollgaard et al. 1992; Perlman & Stocke 1994). This extended structure was studied at optical wavelengths, both with ground based telescopes (Silanpää et al. 1999; Heidt et al. 1999) and the HST (Jannuzi et al. 1997). The more reliable ground base observations (NOT and VLT) show a compact host galaxy which has a small offset to the south with respect to the active nucleus. Several models involving a binary pair of black holes have been proposed to explain the observed optical periodicity. Models in which the optical variability period coincides with the orbital period were proposed by Sillanpää et al. (1988) and Lehto & Valtonen (1996). The optical flares would be produced by the interaction between the secondary, in a very eccentric orbit, and the accretion disk surrounding the primary black hole. Katz (1997) suggested an alternative model, also based in a pair of black holes, the flares would be due to the increase in the beaming factor when a precessing jet passes very near the line of sight. The precession would be driven by the gravitational torque of the companion on the accretion disk of the primary black hole. A similar model was used by Abraham & Carrara (1998) and Abraham & Romero (1999) to explain the variation of the superluminal velocities and position angles of the VLBI features in the jets of 3C279 and 3C273.

In this paper the model is applied to OJ287. Although the parsec scale structure and the kinematics of the superluminal features do not define the jet as well as in 3C273 or 3C279, the reliably determined optical period and the good temporal coverage of the light curve during the last strong flares compensate this deficiency. It was possible to obtain the geometrical parameters of the precessing jet and its Lorentz factor.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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