Characterizing Long-term Optical Variability Properties of $\gamma$-ray Bright Blazars

TL;DR

This study analyzes over a decade of optical light curves from 12 gamma-ray bright blazars, revealing variability patterns similar to gamma-ray observations and suggesting co-spatial emission regions, with some sources showing quasi-periodic oscillations.

Contribution

It provides a comprehensive comparison of optical and gamma-ray variability properties of blazars, highlighting similarities and differences in flux distribution and RMS-flux relations.

Findings

Blazars show log-normal flux distribution in optical band.

Optical and gamma-ray emissions are highly correlated in most sources.

Some blazars exhibit quasi-periodic oscillations in optical emission.

Abstract

Optical observations of a sample of 12 $\gamma$-ray bright blazars from four optical data archives, AAVSO, SMARTS, Catalina, and Steward Observatory, are compiled to create densely sampled light curves spanning more than a decade. As a part of the blazar multi-wavelength studies, several methods of analyses, e. g., flux distribution and RMS-flux relation, are performed on the observations with an aim to compare the results with the similar ones in the \gama-ray band presented in Bhatta & Dhital 2020. It is found that, similar to $\gamma$-ray band, blazars display significant variability in the optical band that can be characterized with log-normal flux distribution and a power-law dependence of RMS on flux. It could be an indication of possible inherent linear RMS-flux relation, yet the scatter in the data does not allow to rule out other possibilities. When comparing variability properties in the two bands, the blazars in the \gama-rays are found to exhibit stronger variability with steeper possible linear RMS-flux relation and the flux distribution that is more skewed towards higher fluxes. The cross-correlation study shows that except for the source 3C 273, the overall optical and the $\gamma$-ray emission in the sources are highly correlated, suggesting a co-spatial existence of the particles responsible for both the optical and $\gamma$-ray emission. Moreover, the sources S5 0716+714, Mrk 421, Mrk 501, PKS 1424-418 and PKS 2155-304 revealed possible evidence for quasi-periodic oscillations in the optical emission with the characteristic timescales, which are comparable to those in the $\gamma$-ray band detected in our previous work.