The multi-star CCD photometric monitoring was carried out for 21 nights during February/March 1994 using two 1-metre class telescopes located at Naini Tal and Kavalur in India, at a separation of 2500 km. Due to the high declination of the blazar the airmass was generally larger than 1.5, but the object could be monitored typically for about 5 hours in a night. The positional offsets, the calibrated BV Johnson and RI Cousins magnitudes and instrumental colours of the three comparison stars used for differential photometry of the blazar are listed in Table 1. The photometric measurements for stars 1 and/or 2 have been carried out by Takalo et al. (1994); Ghisellini et al. (1997) and Villata et al. (1998) in the different passbands. Within the errors, the magnitudes derived by them for a star agree fairly well. For stars 1 and 2, the BVR data are taken from Villata et al. (1998) as they are the most accurate measurements; while the I magnitudes are adopted from Ghisellini et al. (1997). For star 3, these values are determined from stars 1 and 2, using the differential instrumental magnitudes measured in our campaign. A log of observations for the entire campaign is given in Table 2. Further details of the observations are summarized below.
|Figure 1: DLCs in the B, V, R, I passbands deduced from the UPSO observations on 19 nights (excepting for February 28 on which I-band data could not be obtained, whereas for the last 3 nights B-band observations could not be made). The symbols crosses, squares, filled circles and triangles refer to B, V, R and I passbands respectively. In order to plot the DLCs with clarity in the B, V, R, I sequence from bottom to top, we have applied fixed offsets of 0.25 mag in B, 0.0 mag in V and -0.1 mag in R and I for the DLCs involving the blazar and 0.1 mag in B, 0.0 mag in V and -0.05 mag in R and -0.1 mag in I for the DLCs involving the two comparison stars 1 and 2|
|Figure 2: DLCs in the R and I passbands obtained at VBO for 5 consecutive nights. The temporal overlap with the UPSO data on the first 3 nights (Fig. 1) shows good consistency between the two datasets. Note that the comparison star 1 is common to the two datasets. For clarity in plot, the offsets applied are -0.1 mag in both R and I for the DLCs involving the blazar, and -0.1 mag in I and 0.0 mag in R for the DLCs involving the two comparison stars 1 and 3|
The data were reduced at the VBO VAX 11/780 system using DAOPHOT software package, as described in Gopal-Krishna et al. (1993; 1995). In order to maximise the signal/noise ratio, we performed the aperture-growth analysis described by Howell (1989). Accordingly, a circular aperture of 10 pixels (=7) diameter was adopted for the photometry of the blazar as well as the comparison stars. The DAOPHOT algorithm also provided estimates of the statistical errors of the computed relative magnitudes, which were usually < 1% for all the passbands. Using the computed instrumental magnitudes, differential light curves (DLCs) were generated. These represent the relative instrumental magnitudes of the blazar relative to the two comparison stars, and the same for the pair of comparison stars (Figs. 1 and 2). Note that the comparisons used are stars 1 and 2 for the UPSO DLCs and stars 1 and 3 for the VBO DLCs. It is seen that the average instrumental colours of the blazar are not much different from those of the three comparison stars (Table 1).
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