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

In this letter we present results from intra-night monitoring in three colors (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. On some occasions, changes of ~0.1 mag are observed even within ~10 min. During the night of 14.02.2015 an exponential drop of ~0.7 magnitudes is detected for about 5 hours. Cross-correlation between the light curves does not reveal any detectable wavelength-dependent time delays, larger than ~5 min. Color 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.