Optical variability in IBL S5 0716+714 during the 2013-2015 outburst
  With an aim to explore optical variability at diverse timescales in BL Lac
source S5 0716+714, it was observed for 46 nights during 2013 January 14 to
2015 June 01 when it underwent two major outbursts. The observations were made
using the 1.2-m Mount Abu InfraRed Observatory telescope mounted with a CCD
camera. On 29 nights, the source was monitored for more than two hours,
resulting in 6256 data points in R-band, to check for the intra-night
variability. Observations in B, V and I bands with 159, 214, and 177 data
points, respectively, along with daily averaged R-band data are used to address
inter-night and long-term variability and the color behavior of S5 0716+71. The
study suggests that the source shows significant intra-night variability with a
duty cycle of more than 31% and night-to-night variations. The average
brightness magnitudes in B, V, R & I bands were found to be 14.42(0.02),
14.02(0.01), 13.22(0.01) & 13.02(0.03), respectively, while S5 0716+714 was
historically brightest with R = 11.68 mag on 2015 January 18, indicating that
source was in relatively high state during this period. A mild bluer when
brighter behavior, typical of BL Lacs, supports the shock-in-jet model. We
notice larger amplitudes of variation when the source was relatively brighter.
Based on the shortest time scale of variability and causality argument, upper
bound on the size of the emission region is estimated to be 9:32x 10^{14} cm
and the mass of the black hole to be 5.6 x10^8Ms.
