Multiband optical variability of the blazar S5 0716+714 in outburst state during 2014–2015

Abstract

We analyzed the multi-band optical behaviour of the BL Lacertae object, S5\n0716+714, during its outburst state from 2014 November - 2015 March. We took\ndata on 23 nights at three observatories, one in India and two in Bulgaria,\nmaking quasi-simultaneous observations in B, V, R, and I bands. We measured\nmulti-band optical fluxes, colour and spectral variations for this blazar on\nintraday and short timescales. The source was in a flaring state during the\nperiod analyzed and displayed intense variability in all wavelengths. R band\nmagnitude of 11.6 was attained by the target on 18 Jan 2015, which is the\nbrightest value ever recorded for S5 0716+714. The discrete correlation\nfunction method yielded good correlation between the bands with no measurable\ntime lags, implying that radiation in these bands originate from the same\nregion and by the same mechanism. We also used the structure function technique\nto look for characteristic timescales in the light curves. During the times of\nrapid variability, no evidence for the source to display spectral changes with\nmagnitude was found on either of the timescales. The amplitude of variations\ntends to increase with increasing frequency with a maximum of $\\sim$ 22% seen\nduring flaring states in B band. A mild trend of larger variability amplitude\nas the source brightens was also found. We found the duty cycle of our source\nduring the analyzed period to be $\\sim$ 90%. We also investigated the optical\nspectral energy distribution of S5 0716+714 using B, V, R, and I data points\nfor 21 nights. We briefly discuss physical mechanisms most likely responsible\nfor its flux and spectral variations.\n