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

¹ U.P. State Observatory, Manora Peak, Naini Tal, India
² Indian Institute of Astrophysics, Bangalore 560034, India
³ N.C.R.A.-T.I.F.R., Pune University Campus, Pune-411007, India
⁴ Landessternwarte Heidelberg-Koenigstuhl, 69117 Heidelberg, Germany

Send offprint request to: R. Sagar, Naini Tal

Received: 28 July 1998
Accepted: 31 August 1998

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 filters (BV Johnson and RI Cousins) were used on a majority of the nights. The light curves of the blazar were derived relative to two comparison stars present on the CCD frames. Intensity variations of ≥ 0.05 mag from night-to-night were observed in half the cases and a good correlation between the light curves in the different 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 fitted with an exponential intensity profile 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 hour was detected in these multi-colour data. Present observations have been carried out during a low brightness level of the blazar and they have no large amplitude variable events.