Optical variability in IBL S5 0716+714 during the 2013-2015 outburst

TL;DR

This study presents extensive optical monitoring of BL Lac S5 0716+714 over two years, revealing significant intra-night and inter-night variability, color behavior, and estimates of emission region size and black hole mass.

Contribution

It provides detailed multi-band optical variability analysis during a major outburst, including the first long-term intra-night variability study for this source.

Findings

Significant intra-night variability with >31% duty cycle.

Source was in a high brightness state during the 2015 outburst.

Estimated emission region size is approximately 9.32 x 10^{14} cm.

Abstract

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.