Photometric Variability of BL Lacertae and 1ES 1426+428 Blazars in the Optical and Gamma Ranges

The results of photometric observations of two bright blazars of the northern hemisphere, namely, BL Lacertae and 1ES 1426+428, during 2018–2020 through BVRI filters of the Johnson/Bessel system are given. The observations were performed with the two telescopes: AZT-8 of observation station Lisnyky of the Astronomical Observatory of the Taras Shevchenko National University of Kyiv (Kyiv oblast, Ukraine) and Zeiss-600 of high-altitude observatory Peak Terskol of the International Center for Astronomical, Medical, and Ecological Research (IC AMER) of the National Academy of Sciences of Ukraine. In total, more than sixty nights of observations were recorded and processed. The main goals have been in performing a cross-matching analysis of blazar light curves in BVRI bands to detect the short-term variability (STV) and long-term variability (LTV) and to investigate the chromaticity of color parameters. For both objects, fluctuations of brightness up to 1^m in 2018–2020 were recorded in the BVRI bands of the Johnson/Bessel system with a total error of 0.03^m–0.1^m. The intraday variability (IDV) was revealed for BL Lacertae November 17/November 18, 2018. During the calculations of the color indices, the trend of bluish color with an increase in the brightness (bluer-when-brighter, BWB) was found. For BL Lacertae, the LTV was reliably detected by using different pairs of filters. Also, the BWB trend with an average correlation (over 0.5) was recorded for 1ES 1426+428. The presence of such fluctuations in the color of blazars was due to the synchrotron radiation of the jet. The revealed partial correlation of variations in brightness with low time resolution (more than a week) between the photometric optical observations and the data of the Fermi gamma-ray telescope in 2018–2020 require additional research.