Violent intra-night optical variability of the blazar S4 0954+65 during its unprecedented 2015 February outburst

In this letter, we present results from intranight monitoring in three colours (BRI) of the blazar S4 0954+65 during its recent (2015 February) unprecedented high state. We find violent variations on very short time-scales, reaching magnitude change levels of 0.1-0.2 mag h(-1). On some occasions, changes of similar to 0.1mag are observed even within similar to 10 min. During the night of 2015 February 14, an exponential drop of similar to 0.7 mag is detected for about 5 h. Cross-correlation between the light curves does not reveal any detectable wavelength-dependent time delays, larger than similar to 5 min. Colour changes 'bluer-when-brighter' are observed on longer time-scales. Possible variability mechanisms to explain the observations are discussed and a preference to the geometrical one is given.