# Optical Variability of TeV Blazars on long time-scales

## TL;DR

This study presents long-term optical observations of three TeV blazars, revealing significant flux variability and insights into their emission mechanisms through spectral energy distribution analysis.

## Contribution

It provides extensive multi-year optical monitoring data and interprets long-term variability in blazars with models involving relativistic electron injection and Doppler boosting effects.

## Key findings

- All three blazars showed strong flux variability.
- Color variations are mildly chromatic over long timescales.
- Variability in BL Lac may be due to changes in Doppler boosting.

## Abstract

We present the results of photometric observations of three TeV blazars, 3C 66A, S5 0954+658 and BL Lacertae, during the period 2013--2017. Our extensive observations were performed in a total of 360 nights which produced $\sim$6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy timescales. We also examine the optical Spectral Energy Distributions of these blazars, which are crucial in understanding the emission mechanism of long-term variability in blazars. All three TeV blazars exhibited strong flux variability during our observations. The colour variations are mildly chromatic on long timescales for two of them. The nature of the long-term variability of 3C 66A and S5 0954+658 is consistent with a model of a non-thermal variable component that has a continuous injection of relativistic electrons with power law distributions around 4.3 and 4.6, respectively. However, the long-term flux and colour variability of BL Lac suggests that these can arise from modest changes in velocities or viewing angle toward the emission region, leading to variations in the Doppler boosting of the radiation by a factor ~1.2 over the period of these observations.