The intra-night optical variability of the bright BL Lacertae object S5 0716+714

¹ Stazione Astronomica di Vallinfreda, via del Tramonto, 00020 Vallinfreda, Italy e-mail: enrico.massaro@uniroma1.it
² Dipartimento di Fisica, Università La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy

Received: 21 July 2005
Accepted: 14 December 2005

Abstract

Aims.We address the topic of the intra-night optical variability of the BL Lac object S5 0716+714.

Methods.To this purpose a long-term observational campaign was carried out, from 1996 to 2003, which allowed the collection of a very large data set, containing 10 675 photometric measurements obtained in 102 nights.

Results.The source brightness varied in a range of about 2 mag, although the majority of the observations were performed when it was in the range . Variability time scales were estimated from the rates of magnitude variation, which were found to have a distribution function well fitted by an exponential law with a mean value of 0.027 mag/h, corresponding to an e-folding time scale of the flux  h. The highest rates of magnitude variation were around 0.10–0.12 mag/h and lasted less than 2 h. These rates were observed only when the source had an R magnitude <13.4, but this finding cannot be considered significant because of the low statistical occurrence. The distribution of has a well-defined modal value at 19 h. Assuming the recent estimate of the beaming factor , we derived a typical size of the emitting region of about  cm. The possibility of searching for a possible correlation between the mean magnitude variation rate and the long-term changes in the velocity of the superluminal components in the jet is discussed.