Optical intra-day variability in 3C 66A: 10 years of observations
  We present results based on the observations of the blazar 3C 66A from 2005
November 06 to 2016 February 14 in the BVR and I broadbands using 1.2m
telescope of the Mt. Abu InfraRed Observatory (MIRO). The source was observed
on 160 nights out of which on 89 nights it was monitored for more than 1 hr to
check for the presence of any intra-day variability (IDV). The blazar 3C 66A
exhibited significant variations in the optical flux on short and long term
time scales. However, unlike highly variable S5 0716+71, it showed IDV duty
cycle of about 8% only. Our statistical studies suggest the IDV time scales
ranging from $\sim$ 37 min to about 3.12 hours and, in one case, a possibility
of the quasi-periodic variations with characteristic timescale of $\sim$ 1.4
hrs. The IDV amplitudes in R$-$band were found to vary from 0.03 mag to as much
as 0.6 mag, with larger amplitude of variation when the source was relatively
fainter. The typical rate of the flux variation was estimated to be $\sim$0.07
mag hr$^{-1}$ in both, the rising and the falling phases. However, the rates of
the brightness variation as high as 1.38 mag/hr were also detected. The
shortest timescale of the variation, 37 min, sets an upper limit of $6.92
\times 10^{14}$ cm on the size of the emission region and about $3.7 \times
10^8\ \mathrm{M}_{\odot}$ as an estimate of the mass of the black hole,
assuming the origin of the rapid optical variability is in close vicinity of
the central SMBH. The long-term study suggests a mild bluer-when-brighter
behavior, typical for BL Lacs.
