Abstract:
To search for optical variability on awide 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 B, V, R, and I 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 B, V, R, and I 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 B, V, R, I, J,
and K 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.
