Multiband optical–NIR variability of blazars on diverse time-scales

Abstract

To search for optical variability on a wide range of time-scales, we have carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in <it>B</it>, <it>V</it>, <it>R</it>, and <it>I</it> passbands were determined for ∼7000 new optical observations from 114 nights made during 2011–2014, with an average length of ∼4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse time-scales. Discrete correlation functions were computed among <it>B</it>, <it>V</it>, <it>R</it>, and <it>I</it> observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using <it>B</it>, <it>V</it>, <it>R</it>, <it>I</it>, <it>J</it>, and <it>K</it> passband data. We found that the sources almost always follow a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.