OJ 287: The Rosetta stone of blazars

Vistas in Astronomy, Vol. 38, pp. 77-109, 1994

©1994 Elsevier ScienceLtd Printed in Great Britain. All rights reserved. 0083-6656/94 $26.00

Pergamon 0083-6656(94)00004-2

OJ 287: THE ROSETTA STONE OF BLAZARS L. O. Takalo Tuorla Observatory, Tuorla, 21500 Piikld6, Finland

ABSTRACT We have collected all the available observations of blazar OJ 287 published by the end of 1992. OJ 287 has all the characteristics of blazars, but it also exibits some unusual behaviour that has not been observed in other blazars, for example the reported periodic variability, with periods ranging from tens of minutes to tens of years. In this article the observations of OJ 287 are described and are discussed in terms of current blazar models.

I. Introduction

Blazars are among the most violent objects known in the Universe, showing large and rapid variability in all observed wavelengths with strong and highly variable polarization. They show variations of several magnitudes in time scales of weeks (e.g. OJ 287; Takalo et al. 1990) and small amplitude variations in time scales down to minutes (Wolstencroft et al. 1982). Blazars are considered to he the bright centers of elliptical galaxies. These underlying galaxies have been observed in some nearby blazars (e.g. Markarian 421; Kikuchi and Mikami 1987). No definite detection of this underlying galaxy has been observed on OJ 287. It is a point source both in the optical and radio bands. Quite often a blazar center emits as much energy as a whole galaxy from an area about the size of the solar system. The only known mechanism able to create this much energy in so small an area is an accreting supermasssive black hole. Among the first ones to connect black holes with blazars were Elliot and Shapiro (1974). They proposed a model, with an accreting black hole in the center of a blazar, in order to explain the rapid variability. Blanford and Rees (1978) proposed a model for blazar AO 0235+164, in which a jet is coming from the central black hole. They explained the large variations in AO 0235+164 by shocks occuring in this jet. This model was presented in the first ever blazar conference: "The Pittsburg conference of BL Lac objects". Later Begelman, Blanford and Rees (1984) expanded and modified this model. This later article forms the basis for most of the current models trying to explain the observed phenomena in blazars. Since then the model has been expanded and modified (e.g. Marcher et al. 1992 and references therein). Now the basic model consists of a central black hole, with or without an accretion disk, and a

77

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L.O. Takalo

jet coming from the center with shocks occuring in this jet. This model explains quite well the observed radio behaviour in blazars (e.g. Valtaoja et al. 1992b). In the optical regions however the model has some problems (e.g. Sillap~i~ et al. 1991). The rapid polarimetric variations are especially difficult to explain with the standard model. Valtaoja et al. (1991) proposed a model, in which the optical polarization behaviour also could be explained with the shocks in the jet, like the radio variability. Several good review articles have been published on blazars. These articles have discussed optical and infrared polarization properties (Stein, O'Dell and Strittmatter 1976; Kinman 1978; Stein 1978; Angel and Stockman 1980), radio polarizations (Saikia et al. 1988), infrared emission (Rieke and Lebofsky 1979), continuum emission mechanisms (Bregman 1990) and variability of the central engine (Wallinger et al. 1992). During the last few years blazars have been also discussed in several international conferences; The Como conference (BL Lac Objects, 1989) and the Turku conference (Variability of Blazars, 1992) being the ones that were totally committed to blazar research. Blazars have also been discussed in many conferences and meetings on Active Galactic Nuclei. Some of the relevant meetings are referenced below, when discussing the observations of OJ 287. OJ 287 is one of the best observed blazars. It has been included in almost all optical and radio monitoring campaings conducted on blazars (references are listed below). It is also one of the few blazars of which there is a lot of historical data available (e.g. Kidger et al. 1992 and references therein). In this paper we have collected all the observations of OJ 287 available to us by the end of 1992. We will discuss the observations in different wavebands and do simple comparisons between these observations at different wavelength regions. We will try to discuss the proposed models in the context with the observations. Of course I cannot be sure to have found all articles that list or discuss observations of OJ 287. Omission of any article is my mistake and I would be grateful of any comments on missing articles.

2. Discovery of OJ 287 OJ 287 was discovered in radio observations by Dickel et al. (1967) at the Vermillion River Observatory; their source VRO 20.08.1. It was extensively observed during the Ohio University radio survey (Kraus et al. 1968; Kraus and Andrew 1970; Jauncey et al. 1970). These observations indicated that OJ 287 had an unusual and possibly variable spectrum in the radio bands (see also Blake 1970). OJ 287 was tentatively identified optically by Thompson et al. (1968), from Palomar Sky Survey plates, as a flat spectrum stellar source. Unfortunately their radio position was not accurate enough for an unambiguous identification. Blake (1970), having a more accurate radio position, was able to identify OJ 287 with a stellar object with both blue and red magnitude of 14.5. Kinman and Conklin (1971) confirmed this identification and observed OJ 287 to show large optical variability. It was observed to show only continuum emission (Adam et al. 1972), and high linear polarization (Kinman and Conklin 1971; Nordsieck 1972). On the sky survey plates it appeared as a point source (Kinman and Conklin 1971). This was confirmed by Strittmatter et al. (1972). All these observations indicated that OJ 287 was similar to BL Lac, the prototype of blazars. Note, however, that Kinman (1975) saw a faint extension towards NNW in plates taken with the 4-m Mayall telescope, when OJ 287 was in a faint state. Hutchings et al. (1984) found that OJ 287 was unresolved, except marginally at the lowest luminosities. No definite detections of the host galaxy for OJ 287 have been reported. Nor has there been any reports of any cluster of galaxies close to OJ 287, like is the case with some other blazars (e.g. AO 0235+164).

OJ 287: The Rosetta Stone of Blazars

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2.1 Spectral lines and redshift The question of spectral lines and redshift have been considered by a number of researchers. The first spectroscopic observations of OJ 287 by Lynds (see Kinman and Conklin 1971) and Strittmatter et al. (1972) revealed a continuum spectrum with no absorption or emission lines. Nishida and Jugaku (1976) did not detect any spectral lines in their observations taken during a faint phase in 1974. Spectroscopic observations of OJ 287 by Miller et al. (1978) showed a weak spectral feature at 6538A, which they identified as the [O III] line with a redshift of 0.306. They calculated the optical spectral index for OJ 287 to be -1.37. Later spectroscopic observations by Sitko and Junkkarinen (1985) and Stickel et al. (1989) have confirmed this redshift. These observations showed the [O III] lines and also weak H-beta and H-alpha lines in the spectra. 3. The Observations

3.1. X-rays The first reported X-ray observations of OJ 287 were the EINSTEIN IPC measurements by Owen et al. (1981). They reported X-ray (1 KeV) and 90 GHz observations, finding that there was a clear correlation between the X-ray and mm observations. The spectral index between these bands was -0.86; between optical and X-ray bands the spectral index was -1.24. In total OJ 287 was observed 13 times with the EINSTEIN Observatory ,using the IPC detector, mostly during May 1980 (Madejski and Schwartz 1983, 1988). These observations showed variability, by at least a factor of three on time scales of months or more but not on time scales of days. Madejski and Schwartz (1983) found a spectral index a, in the 0.2-4keV band, of -0.91 (+0.40, -0.38). They conclude that the SSC model cannot explain the X-ray observations unless one invokes relativistic bulk motion with a F -2. Other X-ray observations were made with the EXOSAT satellite between 1983 and 1985 (Pollock et al. 1985; Giommi et al. 1990). OJ 287 was observed I1 times. Some of these observations were done during the large outburst seen in OJ 287 in 1983/84. Large amplitude variations were seen in time scales of months. No variations were seen in time scales of days. Pollock et al. (1985) combined data from other wavebands with the X-ray observations. These observations were not taken truly simultaneously. His analysis showed that all the wavebands, except radio, varied in phase with the X-rays but with different amplitudes. The variability amplitude in X-rays was much greater than in the optical or infrared. After the large outburst OJ 287 was observed in a low state with constant X-ray flux (Giommi et al. 1990). 3.2. UV In the UV-bands OJ 287 has been observed with the IUE satellite over twenty times (Edelson et al. 1992), mostly during the 1983/84 outburst. This outburst is clearly seen in the UV light curve, with small amplitude daily variations. These variations are clearly seen in Figure 1, where we have collected the IUE observations from Marachi et al. (1986), Hanson and Coe (1985). See also Figure 3c in Edelson et al. (1992). As can be seen from figure 1, the 1983/84 outburst was much more prominent in the 150(O region than at the 2500,~ region. Marachi et al. (1986) detected variations in time scales of days during their monitoring observations. They and also Hanson and Coe (1985) detected spectral variations in the IUE observations on similar time scales. However, there was no correlation between the spectral and intensity variability. The only exception to this were the observations taken during the outburst of 1983/84 (Hanson and Coe 1985). Then the spectrum became harder as the object became brighter. JPVA 38:1-F

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The UV-spectral index shows small variability in time scales of months (Edelson et ai. 1992). The average spectral index was about -1.3 in the short wavelength IUE region and -1.8 in the long wavelength region (Table 3; Edelson et al.). The combined spectral index was -1.5 (Edelson et al.) i

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3.3. Optical The historical light curve of OJ 287 can be studied starting from 1894 (Visvanathan and Elliot 1973). The first observations were on photographic plates taken for the Harvard plate collection. In these plates OJ 287 was serendipitously observed and can be seen during its outbursts. Between 1920 and 1960 OJ 287 was observed similarly serendipitously in several observatories around the world (Gaida and R0ser, 1982; Craine and Warner 1973; Takalo, 1982; Visvanathan and Elliot 1973). All these photographic observations were made with blue sensitive plates. The large outburst seen in OJ 287 in 1972/73 created a lot of interest for this object. Since then it has been included in all large long term monitoring programs (Webb et al. 1988; Sillanp~i et al. 1991; Takalo et al. 1992c; Valtaoja et al. 1991; Xie et al 1992 and references listed in these articles).

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A historical B-band light curve of OJ 287 is shown in Figure 2. The data for this light curve was collected from the articles listed in Table 1. Table 1. References to the historical B-band light curve of OJ 287, shown in Figure 2. Babadzhanyants et al. (1976) Barbieri et al. (1979) Bertraud and Pollas (1972) Beskin et al. (1985) Bortle (1983a, b) Carini et al. (1992) Corso et al. (1984a,b; 1986, 1988a, b) Craine and Warner (1973) Doroshenko et al. (1985) Folsom et al. (1971) Frochlich et al. (1974) Gaida and RSser (1982) Goldsmith and Weistrop (1973) Haarala et al. (1983) Hagen-Thorn (1980) Hagen-Thorn et al. (1977, 1978, 1979, 1983, 1987, 1991) Holmes et al. (1984) Kidger (1988) Kikuchi (1989) Kikuchi et al. (1976, 1988) Kurochkin (1971 a,b) Landolt (1972) Li et al. (1986) Locher (1971) Lloyd (9184) Lyuty (1976) McGimsey et al. (1975) Mead et al. (1990) Metik and Prokof'eva (1981) Miller 1988 Miller et al. (1976) Moles et al. (1984, 1985) Monella (1987a,b) O'Dell et al. (1978a,b) Petrov (1972) Pica et al. (1983) Pollock (1975) Pollock et al. (1979) Pushell and Stein (1980) Schaefer (I 980) Selmes et al. (1975) Sillanpiili (1987, 1989) SiHanpiiil et al. (1985,1988,1989, 1991a,b,1992a,b,c) Sitko and Junkkarinen (1985) Sitko and Sitko (1991) Sitko et al. (1983,1985) Smith et al. (1975, 1985) Strittmatter et al. (1972) Stull (1972) Takalo (1982,1990,1991) Takalo et al. (1990,1992a,b) Tsessevich (1972) Usher (1979) Valtaoja et al. (1987, 1991) Verdenet (1984,1985,1987) Veron and Veron (1975) Visvanathan (1973) Visvanathan and Elliot (1973) Webb et al. (1988) Weistrop et al. (1985) Wing (1973) Xie et al. (1987,1988a,b, 1989, 1990,1991a,b, 1992).

In the light curve several outbursts can clearly be seen, the largest ones seen during 1972/73 and 1983/84. Deep minima can be seen to occur during 1976, 1985, 1987-88 and 1989 (Takalo et al. 1990; Kidger et al. 1991). There are no observations of the possible earlier minima, but this is mainly due to the limiting magnitudes of the earlier observations. The average magnitude in all colour bands has been fainter after the 1983/84 outburst than during any other observations of OJ 287. In the B-band this difference is about one magnitude. This light curve has been analysed by Babadzhanyants et al. (1992), Kidger et al. (1992c) and Sillanp~ et al. (1988a). All these studies have found periodic variations, with a period of 11.6 years. Sillanp~ii et al. (1988a) proposed a model for OJ 287, based on the analysis of this light curve. The model consists of two massive black holes (masses l0 s and 106 M O) orbiting each other in an eccentric

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orbit with the 11.65 year period, The outbursts are generated by tidal interaction in the accretions disks around the two black holes that are in the center of OJ 287. This tidal interaction enchances the accretion rate into the larger hole, causing the outburst. This model can also be used to explain the observed minima during 1976-77 and 1987-88. These are eclipses, with a 11.05 year period (Sillanp~l an Valtonen 1989b). During these times the smaller black hole with its accretion disk comes in front of the larger black hole, thus blocking some of the light from it. The lifetime of this system is 105 years, after that time the black holes will coalesce due to gravitational radiation (Sillanp/i~ et al. 1988a). See also Corso et al. (1984a), Valtaoja et al. (1989) and Valtonen et al. (1988a,b) for comments on this model. This model predicts that the next outburst in OJ 287 should happen during the fall of 1994. In the other optical bands there are fewer observations, especially before 1972. In all the bands the overall behaviour of the light curves is very similar as can be seen from Figure 3. The outburst structure was very similar during the 1972/73 and 1983/84 outbursts (Sillanp~i et al. 1985). Both outbursts show a rapid rise to the maximum with a "shoulder" and a slower decline back to the base intensity. Also during both outburst maxima there is a secondary maximum about one year after the primary maximum. This is shown in Figure 4. The behaviour after the outburst is also quite similar with small amplitude variations superimposed on top of the overall decline. 3.3.1 Rapid variations. In addition to this long-term variability OJ 287 has been observed to show variations in time scales from weeks to one day or less. Such variations were first reported by Folsom et al. (1971). Other reports on rapid optical variability have been made by Epstein et al. (1972), Smith et al. (1975), McGimsey et al. (1975), Veron & Veron (1975), Sillanpli~i et al. (1991, 1992c), Carini et al. (1992). During these observations variations in time scales from tens of minutes to days have been observed. Most of these variations are seen to be random individual events in the object. The most likely reason for these variations is the injection of relativistic electrons in to the jet and the subsequent decay of the electron energy (e.g. Brown et al. 1990). There are also several reports on periodic variations in OJ 287 in time scales from tens of minutes to days. It is the only blazar in which this kind of periodic variations have been reported. Kinman (1975) reported on optical variations with a period of eight days. Tsessevich (1972) reported on periodic optical variations with an amplitude of one magnitude and period of 26 days. Neither of these variations has been conclusively confirmed by other observations. This could be partly due to the lack of long enough continous monitoring observations. Also these periodic variations can be "lost" under the larger, long term variations. Sillanp~i~i (1991a) found periodic variations in the optical light curve after the 1983/84 outburst, during the declining phase, with a period of 9.3 days. OJ 287 is one of the blazars, in which fast periodic variability has been reported, both in the optical and radio wavelengths (see Kintzel et al. 1988; Kidger et al 1992a, and references therein). The fastest reported periodic variations are the ones by Despande et al. (1991), who reported on a periodic 6 minute variation in the optical white light polarization. Reports on periodic optical variability include Visvanathan and Elliot (1973), who detected a 39.2 minute periodicity. This was confirmed by Frolich (1973) and Frolich et al. (1974), but Kiplinger (1974) could not find this periodicity. Carrasco et al. (1985) detected periods of 23.0 and 40.0 minutes in the optical bands. At the same time Valtaoja et al. (1985) detected a 15.7 minute period in the radio region. Dreher et al. (1986) detected rapid, non-periodic, variations at VLA observations at 5, 15 and 22GHz taken during early 1983. Some of these observations (5GHz) were taken coincident with the Valtaoja et al. (1985) observations. Kinzel et al. (1988) detected a 35 minute period at 7 mm wavelength during February 1986 but could not see the period one year later. De Diego and Kidger (1990) reported on a 19 minute period in the optical B,V,R and I bands, with different amplitudes in the different bands.

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Kiplinger (1974) and Miller et al. (1976) made concerted efforts to find variations in time scales less than a day but did not find any such variations. Carini (1991) did not detect any periodic variations in his 24 nights of observations of OJ 287. He did see some microvariability events though. All these observations on detections and non-detections of periodicities in OJ 287 indicate that these periodicities are, at best, sporadically seen in this object. On the other hand OJ 287 is the only blazar, in which such periodicities have been observed. Kidger et al. (1992a) noticed that the observed periods of these fast variations fall into two separate bins; at roughly 20 and 40 minutes. They consider the possibility that this is caused by the orbital periods in the accretion disks of the two black holes in the center of OJ 287. 3.3.2 Optical spectrum variations. Kinman (1976) was the first one to notice that the colour of OJ 287 is also strongly variable. Takalo and Sillanp~i/i (1989) found long term variability in the optical spectral index and colour of OJ 287 since the large outburst in 1972. The B-V colour index changed from 0.3 (1971) to 0.7 (1988) at the same time as the spectral index changed from --0.8 to -2.0 (see also Kikuchi 1989). There is also a correlation between the B-V colour index and the spectral index with the V-band magnitude. When OJ 287 is fainter B-V is larger and the spectrum is steeper (see Gear et al. 1986; Kidger et al. 1992d for similar correlation in the infrared). 3.3.3 Optical polarization The first polarimetric observations by Kinman and Conklin (1971) and Nordsieck (1972) indicated large and variable polarization for OJ 287. Polarimetric observations of OJ 287 during the outburst of 1972/73 (Hagen-Thorn 1980) showed large variations in both the polarization level and position angle. Angel et al. (1978) reported white light polarimetric observations, that showed polarization varying between 13 and 22% over four months. No night to night variations were detected. Position angle was between 63 and 86 degrees. A 6 degree rotation in one day was seen in the position angle. Later OJ 287 has been included in all major polarimetric monitoring programs (Hagen-Thorn 1980; Impey et al. 1982, 1984; Kikuchi 1992; Mead et al. 1990; Smith et al. 1987; Takalo et al. 1990, 1992c; Valtaoja et al. 1991). These monitoring observations have clearly shown large and variable polarization in OJ 287. The range of measured polarizations is from 0 to 40 per cent (e.g. Hagen-Thorn 1980; Smith at al. 1985). The position angle has also shown variations from 0 to 180 degrees. Sillanp~i~i (1991b) reported on simultaneous optical and radio position angle rotation based between the 1973 and 1984 outbursts. A preferred position angle in the optical can be seen when averaging the historical polarization observations at 100 degrees. There is a tendency that when there are large variations in the position angle, the polarization level is low (see Sillanp~i~i et al. 1992c and references therein). Table 2. References to the polarization light curves shown in Figure 5.

Efimov and Shakhovskoy (1972) Holmes et al. (1984a) Mead et al. (1990) Sitko et al. (1985) Takalo (1990) Valtaoja et al. (1991).

Hagen-Thorn (1980) Kikuchi (1992) Sillanp~iii et al. (1991) Smith et al. (1987) Takalo et al. (1992c)

89

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Figure 5 shows the optical polarization and position angle behaviour in OJ 287 since 1971. The data has been collected from the articles listed in Table 2. As can be seen from the figure, the polarization level shows large, random variations, in time scales from days to months. The position angle shows also fast random variations in time scales similar to those of the polarization (Figure 5). The average position angle is around 100 degrees. The "envelope" of the position angle variations seems to follow a sinewave in Figure 5. Faster variations have also been observed in polarization and position angle. For example Sillanpiiii et al. (1991, 1992a,c,d) observed polarization variations in time scales of hours. They also detected the fastest ever position angle rotation, with 50 degree rotation in five hours. They proposed a model with flares orbiting in the accretion disk around the central black hole. The two component

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model, with shocks in the jet can not explain the polarization level and brightness behaviour during this rotation. Kikuchi et al. (1988) detected similar rotation in the position angle simultaneously in radio and optical bands in time scales of days. Kikuchi modelled these rotations by a helical magnetic field in the jet in OJ 287. Holmes et al. (1984a) also observed rotations in the polarization position angle in time scales of days, explaining their observations by a two component model. 3.3.4 Frequency dependent polarization and/or position angle. Multicolour polarimetric observations have been done of OJ 287 by the following groups: Brindle et al. (1986), Holmes et al. (1984a), Impey et al. (1982, 1984), Mead et al. (1990), PushbeU and Stein (1980), Sillanp/iii et al. (1991), Sitko et al. (1985), Smith et al. (1987), Takalo (1990), Takalo et al. (1992), and Valtaoja et al. (1991). The observations by Mead et al., Sillanp~i~i et al. Takalo, Takalo et al. and Valtaoja et ai. were truly simultaneous in five (UBVRI) bands, the other observations in the list were only quasisimultaneous in the observed bands. During some of these polarimetric observations OJ 287 is seen to show frequency dependent polarization (FDP) and/or position angle (FDPA). Mostly the FDP has the polarization decreasing towards the red. The FDP is more commonly seen than the FDPA (Ballard et al. 1990, Nilsson et al. 1992). There is no correlation between the occurence of FDP and/or FDPA; one can occur with or without the other one being present. No correlation is seen with the occurence of FDP or FDPA with the polarization level or the position angle or with the brightness of OJ 287. Sillanp[i[i et al. (1991) observed both the FDP and FDPA to vary in time scales of hours during a five night monitoring campaign on January 1990. These observations clearly show that in order to study the FDP and/or FDPA one needs truly simultaneous observations in all available colour bands. 3.4. Infrared OJ 287 has been extensively observed in the infrared wavelengths since 1976. The long term infrared light curves are shown in Figure 6. The data shown in this figure were collected from the articles listed in Table 3. The 1983/84 outburst is clearly seen in Figure 6. The outburst seems to be very "narrow", being similar to the optical outburst. The outbursts shows also a double maxima, similar to that in the optical bands. After the outburst the infrared light curve has shown no large variations. During 1985 there seems to be a minimum also in the infrared light curves, at the same time as in the optical.

Table 3. References to the historical infrared light curve of OJ 287, shown in Figure 6.

Allen et al. (1982) Brindle et al. (1986) Holmes et al. (1984a,b) Lorenzetti et al. (1989) Pushell (1980) Sitko et al. (1983, 1985, 1991)

Bersanelli et al. (1992) Gear et a1.(1986) Impey et al. (1982, 1984, 1988) Mead et al. (1990) Rieke et al. (1977) Takalo et al. (1992a).

Small amplitude variations in time scales from minutes to days are seen occasionally in the light curves (Wolstencroft et al. 1982; Takalo et al. 1992a; Kidger et al. 1992d). Kidger et al. (1992d) detected infrared spectral variations in time scales of tens of minutes during 5 nights of monitoring in

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April 1991. Gear et al. (1986) found an excellent correlation between the J-band infrared flux and spectral index; as the source gets fainter, the spectrum gets steeper. They used observations taken during three years during the decline from the 1983/84 outburst. Later Kidger et al. (1992d) found similar correlation in observations taken during five nights. The correlation is also seen in observations taken within a single night. This correlation can be explained by the injection and reacceleration of relativistic electrons with a constant energy index which steepens as the electrons are affected by radiation losses. In the far infrared region the first observations of OJ 287 are the ones by Harvey et al. (1984). They detected OJ 287 in 50 and 100~t during the beginning of the 1983/84 outburst. IRAS observations of OJ 287 have been reported by Landau et al. (1986). 3.4.1 Polarization Infrared polarimetric observations have been done with the UKIRT by Brindle et al. (1986), Holmes et ai. (1984a), Impey et al (1982,1984), Mead et al. (1990). These observations have shown that the polarization behaviour is similar to the one at the optical bands, with large random variations. The average polarization in the infrared is somewhat larger than the one at optical bands. The position angle at the infrared is similar to the one observed at the optical bands. Holmes et al. (1984a) observed large position angle rotations in their optical and infrared observations taken with the UKIRT. They modelled this rotation with a two component model, in which a shock occured in the stable jet. 3.5. Millimeter Millimetric observations by Fogarty et al. (1971) at 90 GHz and by Kinman and Conklin (1971) at 85.3 Ghz showed large variations in the measured flux in time scales of weeks. On the other hand Epstein et al. (1980) did not see any variability in their 3.3mm observations during one week in July 1979. Epstein et al. (1982) found rapid variability at 3.3mm (90GHz) during their 9 years of monitoring of OJ 287. They found the peak flux at this band to occur at 1972.17 (0.01), during the 1972/73 outburst. Owen and Mufson (1977) observed at 90GHz. They found the spectral index from 5 to 90 GHz to be -0.7. Ennis et al. (1982) reported on observations made at lmm wavelength between 1975 and 1981. They observed strong variability in time scales of months. This variability correlated very well with the 2cm observations by Aller et al. (1981), taken quasisimultaneously. Edelson (1987) and Steppe et al. (1987, 1992) observed at 90, 150 and 230 GHz during 1986 to 1990. At 90 GHz the light curve (Steppe et al. 1990) shows a clear outburst at the begining of 1986 and a minimum at 1989. Besides these major events the light curve shows small amplitude variations. At the other frequencies the behaviour is not so clear. Edelson (1987) calculated the spectral index at the mm band to be +0.16. 3.6. Radio Harvey et al. (1972) detected large variations at 2.8 cm observations in time scales of days in OJ 287, confirming the results obtained by Andrew et al. (1971). Dent and Hobbs (1973) observed at 31.4GHz, seeing clearly the 1972/73 outburst in their data. Dent and Kapinsky (1976) monitored several quasars between 1969 and 1975, including OJ 287 at 7.96GHz. In their observations the 1972 outburst showed a double peak structure, with the maximum fluxes occuring at 1972.2 and 1973.03. There were also small amplitude variations occuring during the observations. Webber et al. (1976) observed at 18 cm between 1973 and 1975. Their observations showed small amplitude variability.

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(0.09). Fiedler et al. (1987) reported on daily monitoring at 2695 and 8085 MHz. They saw strong variations in both bands. Most of these variations were correlated. The 1983 outburst was more clearly seen at 8085 MHz, and also at other times the variability was more pronounced at this band. During 1985 there was a large flare at this band, that was not seen at 2695MHz. The flare lasted a couple months. This flare was also observed by Aller et al. (1992a, b) at 8 and 14 GHz, and by Ter~isranta et al. (1992) at 22 and 37 GHZ. No corresponding flare is seen in the optical data (see Figure 7). A structure function analysis based on the observations by Fiedler et al. (1987) shows that the maximum variability time scale seen in the data is of the order of a few hundred days. Pustil'nik and Aliakberov (1992) reported on rapid variability on time scales of days seen in OJ 287 in December 1982 at 8.2 cm radio flux at the RATAN-600 telescope. No correlations were seen between the simultaneous optical and radio observations. Systematic radio monitoring has been conducted at the Michigan observatory at 4.8, 8 and 14 GHz since 1968 (Aller et al. 1981, 1985, 1991, 1992a,b), at the Crimean observatory at 22 and 37 GHz ~ f a m o v et al. 1979, 1980, 1981, 1983, 1984) and at Mets/ihovi radio station at 22 and 37 GHz since 1980 (Salonen et al. 1983, 1987; Te~sranta et al. 1987, 1992). The Michigan monitoring includes also polarization measurements. Radio lights curves based on the above mentioned articles are shown in Figures 7. The outburst of 1983/84 is clearly seen in the light curves. Also seen are the minima at 1985 and 1989, seen also in the other bands. The radio monitoring shows large variations in all time scales from days to years (Lalnela and Valtaoja 1992). 3.6.1. Polarization The first radio polarization observations at 1602MHz by Berge et al. (1972) showed some small polarization. Radio polarization observations at 6cm in milliarcsecond resolution by Roberts et al. (1984) showed a weakly polarized core and strongly polarized jet components. The position angle showed large changes, indicating changes in the optical depth and/or in the orientation in the magnetic field. Radio polarization monitoring has been done at the Michigan Radio Observatory since 1970 at 4,8, 8 and 14 GHz (Aller et al. 1981,1985, 1991, 1992a,b). Figures 8 show the observed light curves. As can be seen the polarization level is somewhat smaller than that observed at optical regions (Figure 5). There seems to be a trend that the average polarization is lower now than what it was in the 1970's. This is best seen at 8GHz, since there the scatter is smaller than at the other bands. Otherwise the polarization level shows large random variations. Weak correlations can be seen with both the flux and the position angle during the decline from the 1983 outburst. The polarization level and flux show a minimum while the position angle shows large variations. Sillanpii,~ (1991b) found a simultaneous optical and radio polarization position angle rotation occuring between the 1972/73 and 1983/84 outbursts. The rotation rate was 2-2.5 degrees per year. A similar trend was also seen after the 1983 outburst.

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3.6.2 VLBI Cohen et al. (1971) found OJ 287 to be unresolved in their 3.8 cm observations. Lawrence et al. (1985) measured the angular size of OJ 287, at 22GHz, to be (0.21_+0.10)mas. On the other hand Antonucci and Ulvestad (1985) found OJ 287 to be completely unresolved in their 20cm VLA observations. Gabuzda et al. (1989) detected superluminal motion, with 13h = 3, in OJ 287 at their 6 cm VLBI observations. Roberts et al. (1987) and Cawthorpe and Wardle (1988) did VLBI polarimetric observation, detecting superluminally moving knots in OJ 287. 3.7. Multifrequency campaigns Epstein et al. (1972) conducted a multifrequency campaign from optical to radio bands, during the 1972/73 outburst. They detected large variability in time scales of hours in different bands, with no correlations between the bands. At 2.21.tin they saw a 25% increase in the flux in one hour. The following day they detected a 40% change in the 3.5mm flux. During the 1972 outburst Kinman et al. (1974) conducted coordinated observations of OJ 287 at radio and optical wavelengths, including some polarization measurements. They observed significant changes in the 4.5cm and shorter wavelengths in time scales of the order of a day. These rapid fluctuations at short centimeter wavelengths were in phase with each other. No correlation was seen between radio and optical variations.. The polarization in both optical and radio wavelengths varied in similar time scales as the total flux in these bands. Jones et al. (1981) reported on multifrequency radio observations of OJ 287 taken during 1978-79 at bands from lmm to 11 cm. They obtained spectra between these frequencies that showed small variations. Owen et al. (1978, 1980) did a similar study, also with some optical observations, during 1977. Their results confirmed the ones by Jones et al. (1981) and also the ones by O'Dell et al. (1977, 1978a,b). O'Dell et al. obtained observations from optical to radio frequencies during 1975-77. They found strong correlations between the infrared and optical variations. The mean observed infrared to optical spectral index was 0.9. Rudnick et al. (1978) did coordinated polarimetric observations from centimeter to visual wavelengths during 1977. They noticed that the polarization position angle was the same at all observed wavelengths.The spectrum at this epoch peaked between 9 and 3ram. At this wavelength interval also the polarization degree increased significantly. All the observations indicated that there exists a strong relationship between the emitting regions at these different wavelength bands. Worral et al. (1982) had (quasi)simultaneous observations fom UV to the radio wavelengths, and combined these with non-simultaneous X-ray observations (see also Chisellini et al. 1986). They observed a decrease by a factor of 0.7 in the UV flux, with no corresponding decrease in the centimeter wavelength emission. Another multifrequency set of observations of OJ 287 was made by Landau et al. (1983, 1986), who observed from 20 cm to the UV. They observed quite large spectral changes between different observing epochs. No evidence for a spectral break was seen during these observations. Madejski and Schwartz (1988) combined their X-ray observations with observations at other wavelengths obtaining an overall spectrum from radio to X-ray bands. They concluded that the spectrum can be modelled with two spatial components; one characterized by the 3 month radio variability time scale and a smaller component responsible for the near infrared, optical and UV emission, characterized with the variability time scale of one day in these regions. Based on this model they calculated the magnetic field strengths in these regions to be 0.032G and 2.6x104 G, respectively. Gear et al. (1985,1986) reported on multifrequency observations between one micron and 2 mm taken over two years, during the 1983/84 outburst. They saw large spectral changes during these observations, especially after the large infrared flare observed by Holmes et al. (1985). The spectral

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turnover frequency was estimated to be at 2x1012 Hz. This behaviour can be explained by shock waves travelling in an adiabatically expanding relativistic jet. Brown et al. (1989a,b) presented multifrequency, from X-rays to radio bands, quasisimultaneous observations of OJ 287 taken between 1984 and 1986. They noticed that the infrared and optical variations are well correlated and that there are significant correlations also between the centimeter variations and those at higher frequencies. Also they detected evidence of a lag between the low-frequency and high-frequency variations. Based on these observations they were able to estimate the magnetic field strength at the flaring region(s) in OJ 287 to be 0.5 Gauss.

4. Variability correlations. Pompreys et al. (1976) found that OJ 287 showed significant correlations between optical and 1.8/2.8 cm variations with a time delay of--0.875 years. Usher (1979) compared optical and 0.3-18 cm radio data taken mostly during the 1972/73 outburst or during the decline from this outburst. He found that most of the optical outbursts are synchronous with their radio counterparts. In particular Usher found that the large August 1972/73 optical outburst was followed by a radio event in 1972.0, while the highest 2.8cm flux was observed in 1973.0 to occur simultaneously with a smaller optical burst. Balonek (1982) analysed the optical and radio observations between 1970 and 1976 finding that the emissions in these bands are correlated with a time delay of -0.9 or 0.0 years between the bands. He concluded that the zero delay was more likely to be real. Valtaoja et al. (1987), using a larger data set, found that the optical events precede the radio ones by less than one year. The time delay was found to be proportional to the wavelength, at least at lower frequencies. Above 10GHz this time delay may be constant at about two months. Hufnagel and Bregman (1992) analysed the optical B-band and the radio flux density (4.8, 8 and 14 GHz) observations by cross-correlating the light curves. They found that the higher frequences lead the lower ones both in amplitude and time: The 14.8 GHZ variations precede those at 8 GHz by 30 days. And the optical variations lead the 8 GHz radio variations by 400 days.

5. Conclusions We have presented a detailed description of all the data available of the blazar OJ 287. This data shows clearly that OJ 287 is an unique object in terms of the amount of data of it at different frequencies. And also in terms of the variability characteristics observed in it. For example in the Mets~ihovi radio monitoring OJ 287 is seen to be variable down to the shortest measureable time scales, being the only blazar with such behaviour. Microvariability is also observed in the optical and infrared bands, with time scales down to few minutes. The most surprising observed characteristic of OJ 287 is the reported periodic variations, both in time scales from tens of minutes to years. It is the only blazar in which such phenomenon has been reported. One problem here is the fact that these rapid periodic variations seem to be present only occasionally. One possible explanation for these variations is that sometimes there is matter rotating either in the jet or in the accretion disk. The yearly variations (the 11.65 year period) has existed at least 100 years. These variations were modelled by the binary black hole model. This periodicity can be tested now, since the predicted new outburst should occur during 1994. Also there has been no conclusive detection of any underlying galaxy in OJ 287, even though one should be detectable at the redshift of 0.3. There has been no reports of any galaxy cluster close to OJ 287. In OJ 287 we have observed all the characteristics of blazars together with the unique behaviour discussed above. It could be that all blazars have similar behaviour than OJ 287, we just do not have enough data on them. On the other hand OJ 287 could be unique, especially concerning its possible binary black hole nature.

OJ 287: The Rosetta Stone of Blazars

101

The observational data base of OJ 287 makes it the best candidate for a very detailed investigation and for testing different blazar models. Understanding all the characteristics of OJ 287 will give us important information for understanding other blazars and the blazar phenomenon.

Acknowledgements:

I would like to thank Dr. A. Sillanp/i~i for useful comments and for critical reading of the article.

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