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

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3. Results

We have good coverage at the R and V bands and fair at I band. There are some observations in the B band and only one in the U band. The radio observations cover the time-span pretty well except for the critical February 1998. Fig. 1 shows all the optical observations in the V, R, and I bands since Fall 1993; in the V band figure we have included the published data from the OJ-94 project (Sillanpää et al. 1996a, 1996b). The lower part of the figure shows the historical light curve before the 1989 fade in the V band. Fig. 2 has radio observations from the same epochs as Fig. 1 in 22 and 37 GHz. The areas around the 1989 and the 1998 minima have been expanded to Fig. 3 and Fig. 4. The polarization data have been listed in Table 2.

[FIGURE] Fig. 2. The upper part shows the radio observations of OJ 287 from Fall 1993 to Spring 1998 and the lower part shows the radio observations from Fall 1984 to Spring 1989 in the 22 and 37 GHz bands.

[FIGURE] Fig. 3. Optical observations of OJ 287 during the 1989 fade (Takalo et al. 1990) and from campaign in the UBRVI bands.

[FIGURE] Fig. 4. Radio observations of OJ 287 during the 1989 fade (Takalo et al. 1990) and from campaign in 22 and 37 GHz.


Table 2. The polarization observations made at the NOT. P is the polarization degree and [FORMULA] is the polarization position angle.

There have been two major outbursts in the optical every decade for the last 30 yr. The last six outbursts have peaked at 1971.87 (37 mJy, 12.5 mag), 1973.00 (21 mJy, 13.1 mag), 1983.02 (22 mJy, 13.1 mag), 1984.18 (11 mJy, 13.8 mag), 1994.85 (10 mJy, 13.9 mag), and 1995.98 (11 mJy, 13.8 mag) with the peak flux and magnitude in the V band in parentheses (Sillanpää et al. 1996a, 1996b). The two peaks in the eighties are before the beginning of the lowest panel in Fig. 1. The height of the peaks has decreased in these outbursts, but looking at the historic light curve since 1890 gives an impression of an oscillating feature in the power of the outbursts; there is hint of a [FORMULA] yr period (Sillanpää et al. 1996b). The two latest outbursts can be seen in Fig 1. They are somewhat symmetric about the point 1995.4 (Sillanpää et al. 1996b). The colors of OJ 287 are usually very stable even during the outbursts, which can be seen in comparisons of the simultaneous V,R, and I data in Fig. 1. There is lot of smaller [FORMULA] mag variation around the base level, which stays quite high after the second outburst and decreases slowly until the end of 1997.

In the later half of the eighties there were no major outbursts. In 1985 the brightness rose up to 14.5 mag, but after that it stayed around 15.5 mag with [FORMULA] mag variations. In the end of 1988 the variations increased and the brightness went up to [FORMULA] mag. There were two sharp peaks before it started to fade.

The radio data from the two epochs are completely different (see Fig. 1). Since 1993 there was only one outburst; this was almost simultaneous with the optical outburst peaking at [FORMULA]. The maximum flux level was [FORMULA] Jy in both wavelengths. After that outburst OJ 287 has been extremely quiet. The flux has steadily been increasing from [FORMULA] to [FORMULA] Jy, with almost no short-term fluctuations. In the late eighties, the flux level was [FORMULA] Jy, but the variations were much more violent as the flux varied from [FORMULA] to [FORMULA] Jy, as seen in Fig. 2.

If we look more closely at the minimum of early 1998 in Fig. 3, we see that the brightness had started to decline in December 1997. The decline is fairly linear and steady, and can be seen in all optical bands that have enough data. In the beginning of February 1998, the magnitude reached [FORMULA] mag, when it suddenly rises by [FORMULA] mag and then comes down to its previous level in about a week. This rise can be seen in the V, R, and I plots, but the other bands lack data from the same period. The sharp peak before the minimum is simultaneous in all available bands. The V and R data show that the brightness could have dropped even more after the sharp rise; the minimum values have large errorbars compared to other points. The time of the deep minimum of R=[FORMULA] and V=[FORMULA] mag was [FORMULA] in both bands.

The 1989 data shows a fade that is much deeper than the fade in 1998 (see Fig. 3). The magnitudes of the deepest minima were R=16.63 mag and V=17.40 mag, [FORMULA] mag less than in 1998. The minima were not exactly simultaneous in all bands. The V minimum was at 1989.38. There is some similarity in the two fades. Although the time coverage of the data is not as good as ours, the B,V, and R band data seem to show that there is also a sharp [FORMULA] mag peak before the minima. These peaks are also exactly simultaneous. The I band data is different from the other bands, but the minimum might have been misclassified and interpreted to precede the other minima by a week. If one identifies the I band minimum with the local minimum before the peak, then they are simultaneous.

The degree of polarization of OJ 287 at optical wavelengths has increased from [FORMULA] in 1993 to [FORMULA] in 1997 with highs over [FORMULA] (Efimov & Shakhovskoy 1997). At the same time, the position angle was seen to rotate counter-clockwise [FORMULA]day in 1993-1996 (Efimov & Shakhovskoy 1996), which changed to nonlinear variability between [FORMULA] and [FORMULA] in 1996-1997 (Efimov & Shakhovskoy 1997). The latest values in spring 1997 were [FORMULA] and [FORMULA]. Our observations are listed in Table 2. The degree of polarization is lower than the trend in 1993-1997 would have predicted, but there have been quite large variations before and [FORMULA] is a very typical value for OJ 287. The polarization angle changed about [FORMULA] in the 16 days, which is somewhat slower than the linear trend of change of polarization angle in 1993-1996, and the values are similar to the ones in 1996-1997.

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© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999