# Gamma-ray and Optical Oscillations of 0716+714, MRK 421, and BL Lac

## TL;DR

This study investigates gamma-ray and optical light curves of three BL Lac objects, finding a ~680-day periodicity in BL Lac and suggesting that quasi-periodicities may be common in blazars, linked to relativistic jets.

## Contribution

First detection of a ~680-day periodicity in BL Lac's gamma-ray and optical emissions using a Bayesian approach, indicating potential commonality of quasi-periodicities in blazars.

## Key findings

- BL Lac shows a ~680-day periodicity in gamma-ray and optical bands.
- No significant periodicity found in 0716+714 and MRK 421.
- Quasi-periodicities may be more frequent in blazars than in quasars.

## Abstract

We examine the 2008-2016 $\gamma$-ray and optical light curves of three bright BL Lac objects, 0716+714, MRK 421, BL Lac, which exhibit large structured variability. We searched for periodicities by using a fully Bayesian approach. For two out of three sources investigated no significant periodic variability was found. In the case of BL Lac we detected a periodicity of ~ 680 days. Although the signal related to this is modest, the coincidence of the periods in both gamma and optical bands is indicative of a physical relevance. Considering previous literature results, possibly related $\gamma$-ray and optical periodicities of about one year time scale are proposed in 4 bright $\gamma$-ray blazars out of the 10 examined in detail. Comparing with results from periodicity search of optical archives of quasars, the presence of quasi-periodicities in blazars might be more frequent by a large factor. This suggests the intriguing possibility that the basic conditions for their observability are related to the relativistic jet in the observer direction, but the overall picture remains uncertain.