Multi-colour optical monitoring of the intra-day variable blazar S5 0716+71

Abstract

We report multi-colour optical monitoring of the intra-day variable blazar S5 0716+71 with a dense temporal coverage over a total duration of 4 weeks in February/March 1994. Four lters (BV Johnson and RI Cousins) were used on a majority of the nights. The light curves of the blazar were derived relative to two compar- ison stars present on the CCD frames. Intensity varia- tions of 0.05 mag from night-to-night were observed in half the cases and a good correlation between the light curves in the dierent passbands was noticed (as also for the observed intra-night variations). There seems to be no correlation between flux and optical spectral index of the blazar. In all, three prominent events of intra-night variability were detected, each with a rate of 0.1 mag per hour, lasting for at least 3 hours. Each of these flaring segments of the blazar light curves can be well tted with an exponential intensity prole whose rate of variation is essentially the same for all the passbands. No clear event of ultra-rapid fluctuations of the blazar on time scale 1 hour was detected in these multi-colour data. Present ob- servations have been carried out during a low brightness level of the blazar and they have no large amplitude vari- able events.