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Astron. Astrophys. 345, 760-768 (1999)

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1. Introduction

The variability of active galactic nuclei has provided means of studying the central engine, its nature and properties. The monitoring campaigns and programs of interesting extragalactic objects have provided extensive light curves to study.

Blazar OJ 287 ([FORMULA]) has been observed for over 100 years, partly accidentally (Takalo 1994). Since 1993 it has been observed extensively in the OJ-94 project (Takalo 1996) and has been included in many other monitoring campaigns. Similar to other blazars, it shows large and rapid variations in all observed wavelengths and polarization. Usually blazars are considered to be the bright centers of elliptical galaxies, however OJ 287 appears as a point source in optical and radio observations, although there are some indications of an underlying galaxy (Benítez at al. 1996).

The optical light curve of OJ 287 has many features. The most clear ones in the historic light curve from 1890 to 1998 are the quasi-periodic outbursts at about 12 year intervals (Sillanpää et al. 1988, 1996a). More recent observations have revealed a double-peak structure in the outbursts (Sillanpää et al. 1996b, see also Fig. 1). The peaks are not always of the same size in the outbursts, and there is a hint of [FORMULA] yr period in the size of the peaks.

[FIGURE] Fig. 1. The upper 3 panels show optical observations of OJ 287 from Fall 1993 to Spring 1998 in the VRI bands. The lowest panel shows the historical V band light curve and the 1989 fade points from Fall 1984 to Spring 1989.

There are also numerous other interesting features in the light curve; there are times of fast variations and times of relatively quiescent periods. In winter-spring 1989 the radio flux began a slow descent, and in May the optical flux suddenly dropped to all-time low values (Takalo et al. 1990). The flux returned to normal soon after the deep minimum. This event was different from the reported double-minima (Sillanpää et al. 1988).

There are many models explaining the behavior of OJ 287 (see Sillanpää 1996b for a review). Most of the models involve a supermassive binary black hole in a configuration that produces the 12-year periodic outbursts and the double-peak structure of each major outburst. These models can be divided roughly into two categories: "changes in the alignment of the jet" models and "changes to the accretion rate" models. The former include, e.g., the beaming model by Villata et al. (1998), and the latter, e.g., the tidal interaction model by Sillanpää et al. (1988).

There are few theories explaining the sudden fade of 1989 (Takalo et al. 1990; Kidger et al. 1991). The three suggested mechanisms in Takalo et al. (1990) are: (1) an obscuring dust cloud between us and the central source, (2) a "switching-off" of the central power source, and (3) a temporary misalignment of the jet. All of the mechanisms could produce another fade, and some could even be predicted with a refined model.

Sillanpää et al. (1988) noticed that the light curve of OJ 287 has two different (quasi-) periodic structures: a maximum outburst structure with a period of 11.65 yr and a double minimum structure with a period of 11.05 yr. The interval between the minima was found to be [FORMULA] yr. The observed maxima were not equally spaced, with up to 0.75 yr difference to the expected time. The aforementioned periodicities led to a model explaining the maxima with a binary system with a 11.65 yr period. The model includes a supermassive black hole binary, where (at least) the more massive component (the primary) has an accretion disk. The smaller component (the companion) moves in the plane of the disk and tidally perturbs the disk during its pericenter passages, causing enhanced inflow to the primary. In this model, the binary orbit has an eccentricity of 0.7. The minima are interpreted as eclipses of part of the disk of the more massive black hole by the accretion disk of the less massive component. The double minimum structure suggests that there are two bright emission regions that are eclipsed in succession. Timescale analyses led to black hole masses of [FORMULA] [FORMULA] and [FORMULA] [FORMULA].

After the 1994 monitoring campaign, it could be seen from the light curve that the outburst structure is actually double-peaked (Sillanpää et al. 1996b). The black hole binary model was unable to produce double peaked outbursts, and Lehto & Valtonen (1996) revised the model to produce two peaks at every pericenter passage. Actually, the new model produces three peaks. The double peaks are the result of the crossing of the accretion disk of the primary by the companion black hole, and the original tidal interaction produces a longer lasting rise of the flux seen as an increase in the base level of the flux. The companion now moves perpendicular to the plane of the accretion disk of the primary. The crossings remove a bubble of matter, which rises from the disk and expands until it becomes too optically thin to be seen in the optical regime. The same tidal action mechanism of the original model still works, when the companion black hole is close to the pericenter. All three emission events at every period take place after a delay; the crossings do not produce radiation until the bubble has emerged from the disk and expanded enough, and the pericenter passing produces tidal instability in the accretion disk, which drives matter into the primary black hole. The crossing delays have been modeled in detail in Lehto & Valtonen (1996). On the basis of their binary black hole model Lehto & Valtonen (1996) suggested that the 1989 fade could be the signature of an eclipse and predicted that the next one would occur at the beginning of 1998.

Pietilä (1998) has studied the possibilities of the Lehto & Valtonen model and other delay options, such as no delay, constant delay, and delay proportional to the infall time. The delay in Lehto & Valtonen depends on the properties of the accretion disk, which is modeled after the Sakimoto & Coroniti (1981) disk model. The crucial parameters of the disk are the viscosity coefficient [FORMULA] and the mass accretion rate [FORMULA]. Pietilä lists limits for the parameters of the binary and the accretion disk, and the future and past events (flares, pericenter passages, and eclipses). The limit for the 1998 fade - interpreted as an eclipse - is [FORMULA].

New hydrodynamical simulations imply that the crossing of an accretion disk by a black hole produces a fountain on both sides of the disk (Ivanov et al. 1998). This partly solves the asymmetry problem of the crossings in the Lehto & Valtonen model, as the observed outbursts are nearly symmetric (Sillanpää et al. 1996b).

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999