3 Observational results

The observation have been done in four B, V, R, I optical bands. The light curves complement each other, since in some cases we have observations in one band, but not on the others. In the following description for the variability behaviour of each individual object, we will combine the data in all four bands. Since the B band light curves are often poor in time resolution and accuracy, the data in the B band are paid less attention than the data in the other three bands.

3.1 H 0323+022

H 0323+022 is a bright X-ray-selected BL Lac object. Variability on time scale of hours have been observed in the X-ray regime (Margon & Jacoby 1984). During the three years monitoring (1987-1990), its range of variation is 16.13 - 17.07 mag in the V band (Jannuzi et al. 1993).

  

Figure 1: Light curve of H 0323+022 on 08 November 1996. Star A and B are of the sequence of Smith et al. (1991)

  

Figure 2: Light curve of H 0323+022 on 11 November 1996

  

Figure 3: Light curve of H 0323+022 on 13 December 1996

  

Figure 4: Light curve of H 0323+022 on 15 December 1996

  

Figure 5: Light curve of H 0323+022 on 05 January 1997

The light curves of our observations are displayed in Figs. 1-5. The top panels display the differential magnitudes between H 0323+022 and the comparison star A, O-A. Star A and B are of the sequence of Smith et al. (1991). The bottom panels display the deviations of differential magnitudes from the mean differential magnitude between comparison star B and A, $\bigtriangleup(B-A)$, which provide an indication of the stability of the standard stars used for calibration and in the case that O has a comparable magnitude with B (like BL Lac and OJ 287, there are more than two standard stars in its field for selection), provide a reliable measure of the observational uncertainty. The open circles ($\circ$) are for I band data, filled circles ($\bullet$) are for R, ($\ast$) for V and ($\oplus$) for B. All subsequent figures will be constructed in the same fashion. Our observations obtained on 08 November 1996 show that the source exhibited no convincing microvariation (Fig. 1). Observations obtained on 10 November indicate a gradual decline in the source's brightness of $\bigtriangleup I=0.23$ mag (Fig. 2). The source was next observed on 13 december 1996, and showed the same brightness level as that in November. No convincing microvariation in the source was recorded during this night (Fig. 3). Figure 4 shows the observation obtained on 15 December 1996. The source first declined by $\bigtriangleup I=$ 0.14 mag in 15 minutes (JD = 2450433.192 to 2450433.202), then began to brighten continuously. From the subsequent R band observations, we can see that the source continued this rise in brightness till JD = 2450433.215, lasting 18 minutes, then began to fade till JD = 2450433.232 and began another rising. The amplitude of this flare is at least 0.30 mag in the I band (take R-I=16.31-15.50). The observations obtained on 05 January 1997 are displayed in Fig. 5. From JD = 2450450.137 to 2450450.141 the source brightened by $\bigtriangleup I=0.15$ mag in about five minutes. In the subsequent V, B band observation it continued this rise till JD = 2450454.176, then faded by $\bigtriangleup B=$ 0.22 mag. On JD = 2450451.181 the source brightened again by $\bigtriangleup V=0.18$ mag in five minutes then $\bigtriangleup R=$ 0.38 mag in 8 minutes, then quickly faded by $\bigtriangleup R=$ 0.42 mag in 12 minutes, $\bigtriangleup I=0.21$ mag in about 2 minutes, and again began to brighten. The amplitude of this flicker is about $\bigtriangleup R=0.63$ mag (take R-I=16.33-15.47).

3.2 H 0548-322

H 0548-322 is a X-ray-selected BL Lac object. During their 3-year monitoring, Jannuzi et al. (1993) found this object to vary by more than 0.5 mag. H 0548-322 has been included in our monitoring program since 1992, and a flare of $\bigtriangleup V=0.58$ mag in 67 minutes was detected on 28 February 1992 (Xie et al. 1995).

  

Figure 6: Light curve of H 0528-322 on 13 December 1996. Star A and B are of the sequence of Smith et al. (1991)

  

Figure 7: Light curve of H 0528-322 on 15 december 1996

  

Figure 8: Light curve of H 0528-322 on 01 January 1997

  

Figure 9: Light curve of H 0528-322 on 03 January 1997

  

Figure 10: Light curve of H 0528-322 on 05 January 1997

Figure 6 displays our new observations obtained on 13 December 1996. The light curve indicates that the source was stable on this night. On the next night the source was observed brightening 0.08 mag in the I band within about 11 minutes (from JD = 2450433.247 to 2450433.255). At the end of the observation another flicker was detected. Within 13 minutes (JD = 2450433.288 to 2450433.297) the source brightened by 0.21 mag in the R band, then faded $\sim$0.08 mag to the end of our observation (see Fig. 7). H 0548-322 was next observed on the night of 01 January 1997. On JD = 2450450.155 the source began to oscillate in brightness, and reached local maxima on JD = 2450450.172, 2450450.192 and 2450450.228, with an amplitude being about 0.1 mag in the R band (Fig. 8). The observations obtained on 3 January 1997 are displayed in Fig. 9. From JD = 2450452.120 to 2450452.137 the source faded $\bigtriangleup I=0.1$ mag, then brightened by $\bigtriangleup I=$ 0.08 mag (to JD = 2450452.157). From JD = 2450452.174 to 2450452.208 the source brightened by $\bigtriangleup R=0.08$ mag (take V-R=15.92-15.38), then began to fade to the end of our observation. On 5 January 1997, the source first gradually faded $\bigtriangleup I\sim$ 0.19 mag (take V-I=15.92-15.38) within about 36 minutes (from JD = 2450433.233 to 2450454.258), then gradually brightened by $\bigtriangleup V\sim$ 0.28 mag (take V-R=15.88-15.47) in about 38 minutes (from JD = 2450433.258 to 2450454.287, see Fig. 10). In general, the source exhibited no long term variation from December 1996 to January 1997 (see Figs. 6-10). It kept at a brightness level of V $\approx$ 15.9 mag, $R\approx 15.3$ mag and $I\approx$ 14.7 mag.

3.3 PKS 0735+178

  

Figure 11: Light curve of PKS 0735+178 on 13 December 1996. Star C and D are of the sequence of Smith et al. (1985)

  

Figure 12: Light curve of PKS 0735+178 on 02 January 1997

  

Figure 13: Light curve of PKS 0735+178 on 03 January 1997

This is a radio-selected BL Lac object. Historical light curves indicate that its range of variability in the B band is 13.90-17.72 mag (Pollock et al. 1979; Xie 1987, 1988b, 1990, 1991, 1994). Figure 11 shows the new observations obtained on 13 December 1996. During the observation of 75 minutes, the source faded steadily at least by $\bigtriangleup I=$ 0.22 mag (take R-I=16.13-15.63, V-R=16.49-16.21). The observations obtained on 02 January 1997 are displayed in Fig. 12. During the first 19 minutes I band observation, the source was stable, then on JD = 2450451.171 it began to brighten and on JD = 2450451.179 reached a local maximum of R=16.19 mag, then began to fade. On JD = 2450451.190 it reached a local minimum of R=16.29 mag, and again began to brighten slowly by about 0.52 mag (take V-R=16.69-16.26, B-V=16.53-17.13) to the end of the observation. However, the standard stars are not very stable (see the bottom panel in Fig. 12). On 3 January 1997, we recorded a rapid flare for this source. From JD = 2450452.228 to 2450452.267 (in 56 minutes) it brightened by about 0.65 mag (take B-I=17.16-15.71, B-V=16.92-16.63), then fade sharply about 0.41 mag (take V-R=16.64-16.26, see Fig. 13). Figures 11-13 show no long term variation trends, the source kept at a faint level of B $\approx$ 17.0 mag, V $\approx$ 16.6 mag.

3.4 PKS 0754+101

PKS 0754+101 is a radio selected BL Lac object. The near-IR observations by Falomo et al. (1992) indicated that this source was not very active from 1987 to 1989. Massaro (1995) found that the range of variability was about one magnitude during their near-IR observation from 1986 to 1992.

  

Figure 14: Light curve of PKS 0754+101 on 13 December 1996. Star A and B are of the sequence of Smith et al. (1985)

  

Figure 15: Light curve of PKS 0754+101 on 14 December 1997

  

Figure 16: Light curve of PKS 0754+101 on 02 January 1997

Our observations are displayed in Figs. 14-16. On the two successive nights of 13 and 14 December 1996, the source exhibited no intranight variation and no microvariations each night (Figs. 14 and 15). Two weeks later, on 2 Jan. 1997 the source's brightness was 0.4 mag fainter in the V band. First the source brightened by 0.17 mag in the I band within about 11 minutes (from JD = 2450451.257 to 2450451.265). At the end of the observation on this night, the source exhibited a quick decrease in its brightness of $\bigtriangleup B=0.47$ mag in 22 minutes (Fig. 17).

3.5 OJ 287

OJ 287 is a bright and classical BL Lac object. There are a lot of information on it in the literature. It has been included in almost all optical and radio monitoring campaigns conducted on blazars (Takalo 1994). The historical light curves have been compiled by Craine & Warner (1973); Visvanathan & Elliot (1973); Miller et al. (1976); Gadia & Roser (1982); Webb et al. (1988); Sillanp$\rm\ddot{a}\ddot{a}$ et al. (1988); Kidger et al. (1992); Takalo (1994). Takalo (1994) has collected all the available observations for this source published by the end of 1992, and found that the total range of variability observed for this object was 18.0 - 12.5 mag in the B band, more than 5 mag. Based on the historical light curves dating back to as early 1894, Sillanp$\rm\ddot{a}\ddot{a}$ et al. (1988, 1996a) and Kidger et al. (1992) obtained a variability period of 11.65 years and proposed that OJ 287 is a binary pair of supermassive black holes with an orbital period of 9 years in the rest frame of OJ 287. The observations during 1993-1996 confirmed this discovery (Kidger et al. 1995; Sillanp$\rm\ddot{a}\ddot{a}$ et al. 1996a,b). Observations also indicate that the constructed light curves seem to show indication at some shorter periods. Kinman et al. (1974) found an 8-day period for its variability, and Xie et al. (1994) confirmed this periodicity. In addition to long-term variations, this objects has also been observed to exhibit variations with time scales of a day or less.

  

Figure 17: Light curve of OJ 287 on 02 January 1997. Star 10 and 11 are of the sequence of Smith et al. (1985)

OJ 287 has been included in our monitoring program since the early 1980's. However during this observing season, OJ 287 was only observed on one night, 2 January 1997. The observations are displayed in Fig. 17. From JD = 2450451.326 to 2450451.340, the source underwent a continuous decline in 22 minutes, then began to brighten. The brightness in the B band on this night was about B=15.5 mag, one magnitude brighter than the mean level of 16.5 mag since 1985 (Takalo 1994), and 1.1 mag fainter than the maximum of B=14.4 mag in November 1994, which shows that OJ 287 was still in its high state since the outburst in 1993/94.

3.6 H 2154-304 (PKS 2155-304)

X-ray-selected, H 2154-304 is a well observed BL Lac object. The earliest optical monitoring for it by Miller & McAllster (1983) revealed that the source was not very violent in the optical regime. Pica et al. (1988) have reported that the range of variation of H 2154-304 was 12.93-14.00 mag in the B band during five years (1979-1986). Carini & Miller (1992a) have presented their long-term photometric monitoring results (1979-1990) with a range of variation of 12.33-13.86 in the V band. Zhang & Xie (1996) have constructed an historical light curve for H 2154-304 in which the variation ranges were 12.82- 14.20 mag in the B band. H 2154-304 is well known for its short time scales of variability from optical to X-ray wavelengths (Brindle 1986). Large amplitude X-ray flux variations on a time scale of a few hours was found during the EXOSAT observations in 1983 and 1985 (Giommi et al. 1990). Optical variation of the order of one day have been observed by Smith et al. (1992). A world wide campaign to monitor H 2154-304 had been undertaken throughout 1991 November, in which H 2154-304 were simultaneously observed in X-ray, ultraviolet, optical, infrared and radio regime, and dramatic, large-amplitude rapid oscillations of about one day have been observed (Urry 1993; Brinkman 1994; Courvoisier 1995).

H 2154-304 has been included in our monitoring program since 1993, and the results show that the object was found with a 0.5 mag flare (Xie et al. 1995). We carried out new observations during 09-11 November 1996. The observations indicate that the source was stable and faint at the level of V=13.26 mag, 0.6 mag brighter than the faintest magnitude of 13.86.

  

Figure 18: Light curve of H 2154-304 on 09 November 1996. Star 2 and 3 are of the sequence of Smith et al. (1991)

  

Figure 19: Light curve of H 2154-304 on 10 November 1996

  

Figure 20: Light curve of H 2154-304 on 11 November 1996

The observations obtained on 9 November 1996 are displayed in Fig. 18. The source was stable during the first 82 minutes. At the end of our observation the source exhibited a slow decrease in its brightness of $\bigtriangleup V=$ 0.09 mag (see Fig. 18). On the next night, 10 November the source was about 0.1 mag brighter, and also showed a slow decrease (Fig. 19). On 11 November 1996 the source was stable, but was about 0.1 mag brighter than it was on the night before (Fig. 20).

3.7 BL Lac

BL Lac is the prototype of the BL Lac object class of AGN and one of the best studied blazars. Shen & Usher (1970) have investigated its historical light curves, and found a range of variation of 4.2 magnitudes in the V band and a strong outburst in which BL Lac varied over nearly its entire range ($12.4\leq B\leq 16.7$ mag) during 400 days. The most recent investigations on its long term variability has been done by Webb et al. (1988) and Carini et al. (1992b) which shows that the faintest magnitude is B=17.99 mag and V=16.73 mag.

  

Figure 21: Light curve of BL Lac on 07 November 1996. Star B and C are of the sequence of Smith et al. (1985)

  

Figure 22: Light curve of BL Lac on 08 November 1996

  

Figure 23: Light curve of BL Lac on 11 November 1996

  

Figure 24: Light curve of BL Lac on 12 December 1996

  

Figure 25: Light curve of BL Lac on 14 December 1996

BL Lac has been monitored since the beginning of our monitoring program in 1980. A burst of 0.56 mag/40 min in the B band was found in 1985 and confirmed in 1988 (Xie et al. 1988, 1990). A burst of 0.48 mag/2 hr in the B band was observed on 1 October 1991, and another burst of 0.37 mag/20 min in the B band was observed on 3 November 1991 (Xie et al. 1994). Figure 21 shows the new observations obtained on 07 November 1996. The light curve shows that BL Lac brightened $\bigtriangleup I=0.25$ mag in 30 minutes, then declined by 0.21 mag in 31 minutes. However, the shape of the light curve for standard stars is similar with that of BL Lac, with a amplitude of 0.14 mag (see the bottom panel of Fig. 21). Thus, the microvariation in BL Lac is less convincing. The observations obtained on 08 November 1996 indicate that during the first hour the source was observed, it exhibited no significant variations. However, over the last 38 minutes the source underwent a rapid flare of $\bigtriangleup R=0.11$ mag (See Fig. 22). On 10 November, the source was 0.21 mag fainter (in the R band) than it was at the end of the observation on 08 November. The source was at the same brightness level of the night before and exhibited no convincing microvariation during the observation on the night of 11 November (Fig. 23). Figure 24 presents the observation obtained on the night of 12 December 1996. Compared with the last observation obtained on 11 November, the source brightened about 0.5 mag in the V band, 0.47 mag in the R band and 0.39 mag in the I band. During the observation, the source exhibited no significant rapid variations on this night. The observations obtained on 14 December are displayed in Fig. 25. The light curve shows no significant variations, but compared with the observations obtained on the night before, the source faded by about 0.3 mag in the I band to the same brightness level with that in November 1996.

In general, the source exhibited no obvious long-term variation trends over the last two months of 1996. It kept at a faint level on which some irregular rapid variations were superimposed (see Figs. 21-25 and Table 2). Over the length of these observations, BL Lac had a total variability amplitude of $\bigtriangleup V=0.5$ mag, $\bigtriangleup R=0.47$ mag, $\bigtriangleup I=$ 0.39 mag. On JD = 2450398.090 it declined to a minimum B=16.62 mag, only 0.6 mag brighter than the faintest level B=17.2 mag registered by Webb et al. (1988), and on JD = 2450399.068 it declined to V= 15.58 mag, which was 1.15 mag brighter than the faintest level V=16.73 mag registered by Carini et al. (1992b).