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 ∼37 min to about 3.12 h and, in one case, the possibility of quasi-periodic variations with a characteristic time-scale of ∼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 ∼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 × 10¹⁴ cm on the size of the emission region and about 3.7 × 10⁸ M 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.