Multi-band optical variability of the Blazar S5 0716+714 in outburst
  state during 2014-2015
  We analyzed the multi-band optical behaviour of the BL Lacertae object, S5
0716+714, during its outburst state from 2014 November - 2015 March. We took
data on 23 nights at three observatories, one in India and two in Bulgaria,
making quasi-simultaneous observations in B, V, R, and I bands. We measured
multi-band optical fluxes, colour and spectral variations for this blazar on
intraday and short timescales. The source was in a flaring state during the
period analyzed and displayed intense variability in all wavelengths. R band
magnitude of 11.6 was attained by the target on 18 Jan 2015, which is the
brightest value ever recorded for S5 0716+714. The discrete correlation
function method yielded good correlation between the bands with no measurable
time lags, implying that radiation in these bands originate from the same
region and by the same mechanism. We also used the structure function technique
to look for characteristic timescales in the light curves. During the times of
rapid variability, no evidence for the source to display spectral changes with
magnitude was found on either of the timescales. The amplitude of variations
tends to increase with increasing frequency with a maximum of $\sim$ 22% seen
during flaring states in B band. A mild trend of larger variability amplitude
as the source brightens was also found. We found the duty cycle of our source
during the analyzed period to be $\sim$ 90%. We also investigated the optical
spectral energy distribution of S5 0716+714 using B, V, R, and I data points
for 21 nights. We briefly discuss physical mechanisms most likely responsible
for its flux and spectral variations.
