Intra-night variability of the blazar CTA 102 during its 2012 and 2016 giant outbursts

TL;DR

This study presents extensive multicolour optical observations of the blazar CTA 102 during its 2012 and 2016 outbursts, revealing significant intra-night variability and suggesting changing Doppler factors as a key mechanism.

Contribution

It provides detailed analysis of intra-night variability during major outbursts and supports the Doppler factor change hypothesis as the primary cause.

Findings

CTA 102 reached nearly 11th magnitude, possibly its brightest state.

Significant intra-night variability up to 0.2 mag in 30 minutes.

No clear saturation in the rms-flux relation; structure function slope ~0.4.

Abstract

We obtained and analyzed more than 100 hours of multicolour optical time series of the blazar CTA 102 during its 2012 and 2016 outbursts. The object reached almost 11-th mag at the end of 2016, which is perhaps the brightest blazar state ever observed! During both outbursts, CTA 102 showed significant and rapid variability on intra-night time scales, reaching up to 0.2 mag for 30 min on some occasions. The "rms-flux" relation, built for all datasets, shows a large scatter and no apparent saturation on the magnitude scale. The ensemble structure function of the light curves can be fitted well with a straight line of a slope of $\sim$0.4. The time lags between the different optical bands appear to be consistent with zero, taking into account our time resolution. We discuss different variability scenarios and favor the changing Doppler factor of the emitting blobs as the most plausible one to account for the observed intra-night variability.