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

Abstract

Optical observations of a sample of 12 γ-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 γ-ray band presented in Bhatta & Dhital (2020). It is found that, similar to γ-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 γ-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 crosscorrelation study shows that except for the source 3C 273, the overall optical and the γ-ray emission in the sources are highly correlated, suggesting a co-spatial existence of the particles responsible for both the optical and γ-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 γ-ray band detected in our previous work.

Multiband light curves from eccentric accreting supermassive black hole binaries

“…Lastly, the BL Lacertae object PKS 2155-304 has several reports of 1.7-year γ-ray periodicity, but different studies disagree about whether optical data has the same 1.7-year periodicity or roughly half of that (0.87 years) (e.g Zhang et al 2014;Sandrinelli et al 2014Sandrinelli et al , 2016Covino et al 2020;Peñil et al 2020;Bhatta 2021). If optical flares from the disk around a binary are outof-phase with jet flares, one would expect a periodicity in the optical band at roughly twice the jet periodicity.…”