3. Data reduction

The method of data reduction had been detailed in the previous paper (Guibin et al. 1995). The comparison stars 4, 10 and 11 are intercalibrated in the I band each night. The magnitudes outside the atmosphere of the intercalibrated comparison stars are slightly different from the values given by Smith et al. (1985) in some nights. The differences reach about and for star 4, 10 and 11 respectively. The uncertainties in the magnitudes of intercalibrated comparison stars are , and for star 4, 10 and 11 respectively. In the BVR bands, the magnitudes and errors given by Smith et al. (1995) are used. The weighted average of the extinction values measured on stars 4, 10 and 11 is used to calibrate the object. The calibrated magnitudes of OJ 287 are tabulated in Table 5 (available electronically). The uncertainties in the table include the errors on the measured magnitudes of the object and the comparison stars and the uncertainties in the magnitudes of the comparison stars. The colour index was calculated from two consecutive observations in different bands. The average of colour indices and magnitudes in BVRI are listed in Table 3 (click here) for each night.

 

UT Date <B> <V> <R> <I> <B-V> <V-R> <R-I>

4/11/94 14.775 (0.005)

11/7/94 13.90 13.69 13.178 0.21 0.54

(0.03) (0.04) (0.003) (0.05 ) (0.04)

11/8/94 14.50 14.09 13.73 13.217 0.41 0.36 0.50

(0.05) (0.03) (0.04) (0.002) (0.06) (0.05) (0.04 )

2/8/95 16.05 15.57 15.15 14.550 0.48 0.42 0.65

(0.05) (0.03) (0.04) (0.002) (0.06) (0.05) (0.04)

2/22/95 16.11 15.69 15.30 14.728 0.50 0.31 0.54

(0.07) (0.02) (0.04) (0.002) (0.08) (0.06) (0.04 )

2/23/95 16.23 15.71 15.32 14.724 0.52 0.39 0.57

(.08) (0.04) (0.04) (0.002) (0.09 ) (0.06) (0.04 )

 

In order to search for rapid variability, the measured magnitude of the blazar, star 4 and star 10 are directly compared, the differential magnitudes Dm1 between the blazar and star 4 and Dm2 between star 10 and star 4, are shown in Figs. 1-6, The uncertainty of each data point in the figures is calculated on the basis of the errors in the measurement of the differential magnitude of the two mutually-compared stars. Comparing the Dm1 and Dm2 light curves one can determine whether the blazar is variable or not. Also, we calculate the flux ratio based on the differential magnitude, R₁ and R₂, being the flux ratios of OJ 287 to star 4 and of star 10 to star 4 respectively. The flux ratio has the same variance terms as the differential magnitude. The averages of relative errors of R₁ and R₂ and the rms values of R₁ and R₂ relative to its average are also calculated. The rms value could represent a measure of the amplitude of variations of flux ratio during the sampling interval in the night. We compare the proportion of the rms value to the average of the flux ratio with the mean of relative errors of flux ratio. If the former is definitely greater than the later, the rapid variability of the flux ratio during the sampling interval is observed; if the former is about the same as the latter, the flux ratio is not variable, it means that the are all equal to zero in the two objects. If R₁ is variable and R₂ has no variability, the conclusion of variations of the blazar during sampling interval can be obtained. Furthermore, the (relative to the weighted mean of the data) test was carried out, thus the significance of variability can be seen.