Optical Intra-Day Variability In 3c 66a: A Decade 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 broad-bands using the 1.2-m telescope of the Mt Abu InfraRed Observatory (MIRO). The source was observed on 160 nights; on 89 of these nights, it was monitored for more than 1 h 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 the highly variable S5 0716+ 71, it showed an IDV duty cycle of about 8 per cent only. Our statistical studies suggest IDV time-scales ranging from similar to 37 min to about 3.12 h and, in one case, the possibility of quasi-periodic variations with a characteristic time-scale of similar to 1.4 h. The IDV amplitudes in the R band were found to vary from 0.03 mag to as much as 0.6 mag, with a larger amplitude of variation when the source was relatively fainter. The typical rate of the flux variation was estimated to be similar to 0.07 mag h(-1) in both the rising and the falling phases. However, rates of brightness variation as high as 1.38 mag h(-1) were also detected. The shortest time-scale of the variation (i.e. 37 min) sets an upper limit of 6.92 x 10(14) cm on the size of the emission region and about 3.7 x 10(8) M-circle dot as an estimate of the mass of the black hole, assuming that the origin of the rapid optical variability is in close vicinity to the central supermassive black hole. The long-term study suggests a mild bluer-when-brighter behaviour, typical for BL Lacertae objects.