Multiband optical monitoring of the blazars S5 0716+714 and BL Lacertae

Abstract

We report the results of multiband optical photometric monitoring of two well-known blazars, S5 0716+714 and BL Lacertae, carried out during 1996 and 2000–01 with an aim to study optical variations on time-scales ranging from minutes to hours and longer. The light curves were derived relative to comparison stars present on the CCD frames. Night-to-night intensity variations of ≥ 0.1 mag were observed in S5 0716+714 during a campaign of about 2 weeks in 1996. A good correlation between the light curves in different optical bands was found for both inter-night and intra-night observations. In all, two prominent events of intra-night optical variability were detected in S5 0716+714. Each of these rapidly varying segments of the light curves can be well fitted with an exponential intensity profile whose rate of variation is essentially the same in both the cases. Our long-term monitoring data of S5 0716+714 showed a distinct flare around JD 245 1875, which can be identified in the BVRI bands. This flare coincides with the brightest phase recorded during 1994–2001 in the long-term light curves reported by Raiteri et al. No evidence for the object to become bluer when brighter was noted on either inter-night or intra-night time-scales. On the other hand, our essentially simultaneous multiband optical observations of BL Lacertae in 2001 October showed flux variations that were not achromatic. This blazar was definitely found to become bluer when brighter on intra-night time-scales, and there is a less significant trend of the same type on inter-night time-scales. Based on five nights of observations during a week, BL Lacertae showed a peak night-to- night variability of ∼0.6 B mag. Thus, we found that the present optical observations of the two prominent blazars, made with similarly high sensitivity, reveal a contrasting behaviour in terms of the dependence of spectral hardening with increasing brightness, at least on intra-night, and possibly also on inter-night, time-scales.