Optical Variability in IBL S5 0716+714 during the 2013-2015 Outbursts

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 2 hr, resulting in 6256 data points in the R band, to check for the intra-night variability (INV). Observations in the 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 INV with a duty cycle of more than 31% and night-to-night variations. The average brightness magnitudes in the B, V, R, and I bands were found to be 14.42(0.02), 14.02(0.01), 13.22(0.01), and 13.02(0.03), respectively, while S5 0716+714 was historically brightest with R = 11.68 mag on 2015 January 18, indicating that the source was in a relatively high state during this period. A mild bluer-when-brighter behavior, typical of BL Lacs, supports the shock-in-jet model. We noticed larger amplitudes of variation when the source was relatively brighter. Based on the shortest timescale of variability and causality argument, the upper bound on the size of the emission region is estimated to be 9.32 ×10¹⁴ cm, and the mass of the black hole is estimated to be 5.6 ×10⁸ M <inf>o</inf>.