Violent intranight 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 ∼0.1 mag are observed even within ∼10 min. During the night of 2015 February 14, an exponential drop of ∼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 ∼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.