The average calibrated B,V,R and I magnitudes of the blazar during present observations are 14.7, 14.2, 13.8 and 13.3 respectively. A comparison of these numbers and the present light curve (Fig. 4) with optical light curves given by Ghisellini et al. (1997) indicates that during the entire period (30 days) of our campaign, the blazar was in a relatively low state without any rapid large amplitude ( 1 mag) brightening or fading as the exhibited maximum variations are only 0.2 mag in all four passbands.
From the DLCs of all 4 passbands, it appears that the blazar outbursts are superposed on a constant base level emission. This behaviour differs from those observed during the extended optical monitoring campaigns of Wagner et al. (1996) and Ghisellini et al. (1997) when the base levels were found to either drop from a "high'' to a "low'' state or vice-versa by almost a factor of 2 or so within about a day (see also Quirrenbach et al. 1991). The quasi-periodicity of the optical light curve with a characteristic time scale of 1 day, which was then found to persist for a week is not clearly evident during our observations, only some indication of it is present during the first week (Fig. 3). A rigorous comparison of the present data with the earlier dataset would be difficult, because the location of this blazar restricted the monitoring from India during the month of March to just 4-6 hours per night, at most, thereby leaving large gaps in the light curves. It is clear, nonetheless, that in all four passbands the blazar exhibited variations of up to 0.2 mag between two consecutive nights, as measured relative to both comparison stars. Furthermore, the light curves in the four passbands are tightly correlated, indicating that the spectrum of the flaring component is not much different from that of the "base'' emission. Recall that a close similarity of the light curves at R, V and B during the quasi-periodic variations was also noticed during the 1990 campaign (Quirrenbach et al. 1991; Wagner et al. 1996). However, the campaign by Ghisellini et al. (1997) indicates that in the "low'' optical state of the blazar, the (B-R) spectral index responds to fast optical variations occurring on day-like time scales, the spectrum becoming bluer with increasing brightness in R.
|Figure 4: The light curve in the R passband (lower panel) is compared with the corresponding optical spectral index (upper panel)|
An important issue in the variability studies concerns the shape(s) of the profiles of the flares. For some of the densely monitored blazars, the large individual optical flares seen on the time-scale of 1 day are characterized by exponential growth and decay profiles, as, e.g., in the cases of the intra-day variable blazars PKS 2155-304 (Urry et al. 1993) and S5 0954+658 (Wagner et al. 1993). In contrast, the individual flares on day-like time scale recorded in 1990 and 1994-95 optical monitoring of the present blazar S5 0716+71 are well described by linear profiles (cf. Wagner et al. 1996; Ghisellini et al. 1997). Since any dichotomy of this nature can signify fundamentally different emission processes, we have made an attempt to determine the shapes of the individual optical flares of S5 0716+71 recorded during our campaign.
As seen from Figs. 1 and 2, the blazar showed on 3 nights prominent optical flares with rates per hour, sustained over a minimum of two hours (see also, Fig. 5). Note that the positive bump peaking around UT on the DLCs of March 4 is clearly related to the variation of the comparison star 1 (Fig. 1) and hence we have ignored the affected portion of the blazar DLC from subsequent analysis. Since all three flares indicate a linear temporal variation on the magnitude scale, we have fitted least-square regression lines to these flaring segments of the DLCs. These best-fit lines are shown in Fig. 5 and their slopes are given in Table 4, together with the regression coefficients. It is interesting that for each flare the slopes are essentially the same for the different passbands. The difference in the slopes is always . It is therefore not statistically significant. Moreover, the regression coefficients for the linear fits are 90% in all cases, excepting the I-band profile on March 18 which is noisy, as evident from the DLC of the comparison stars (Fig. 1). Thus, at least over the few hours spanned by these observations, all the three prominent events of intra-night variability can be described by linear trends on magnitude scale (which, therefore, corresponds to an exponential intensity variation). Similar linear intra-night magnitude variations in B and R bands have been noticed by Ghisellini et al. (1997) on two occasions. Unfortunately, none of the flaring segments of the present DLCs encompass the intensity turnover point (which is not unexpected considering the modest durations of these DLCs). Therefore, the question of linear vis-a-vis exponential variation cannot yet be settled conclusively, though an exponential intensity variation seems to be consistent with all the prominent intra-night flares recorded in the present as well as Ghisellini et al. (1997) optical observations of S5 0716+71. It would be of great interest to enquire if the shapes (linear/exponential) of the outbursts of a given blazar depend on the time-scale of the outburst and whether they vary from one occasion to another.
|Figure 5: DLCs relative to the comparison star 1 in the R and I passbands for those three nights when the blazar intensity varied by per hour for a minimum of 2 hours. The data for the remaining 2 passbands have not been plotted as the temporal coverage was not so dense as in R and I, due to lower sensitivity of the CCD. Note that the positive bump seen in the profiles of March 4 near 19 UT is clearly due to a variation of the comparison star (see Fig. 1) and therefore has been ignored in the computation of the best fit-line|
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