Multi-band optical variability of 3C 279 on diverse timescales
  We have monitored the flat spectrum radio quasar, 3C 279, in the optical $B$,
$V$, $R$ and $I$ passbands from 2018 February to 2018 July for 24 nights, with
a total of 716 frames, to study flux, colour and spectral variability on
diverse timescales. 3C\,279 was observed using seven different telescopes: two
in India, two in Argentina, two in Bulgaria and one in Turkey to understand the
nature of the source in optical regime. The source was found to be active
during the whole monitoring period and displayed significant flux variations in
$B$, $V$, $R$, and $I$ passbands. Variability amplitudes on intraday basis
varied from 5.20\% to 17.9\%. A close inspection of variability patterns during
our observation cycle reveals simultaneity among optical emissions from all
passbands. During the complete monitoring period, progressive increase in the
amplitude of variability with frequency was detected for our target. The
amplitudes of variability in $B$, $V$, $R$ and $I$ passbands have been
estimated to be 177\%, 172\%, 171\% and 158\%, respectively. Using the
structure function technique, we found intraday timescales ranging from $\sim
23$ minutes to about 115 minutes. We also studied colour-magnitude relationship
and found indications of mild bluer-when-brighter trend on shorter timescales.
Spectral indices ranged from 2.3 to 3.0 with no clear trend on long term basis.
We have also generated spectral energy distributions for 3C\,279 in optical
$B$, $V$, $R$ and $I$ passbands for 17 nights. Finally, possible emission
mechanisms causing variability in blazars are discussed briefly.
