Blazars are the jetted-AGNs in which the relativistic jets are aligned at very small ( < 10 degrees) viewing angles. The Doppler-boosted non-thermal emission from the relativistic jets is highly variable at all observed timescales over essentially the entire electromagnetic spectrum. Flux variability on very short (or intraday) timescales has been one of the puzzling issues in the field of blazar astronomy and is helpful in probing the size of the emitting regions near SMBH as well as the underlying emission mechanisms. I am working on the TeV blazars in optical to X-ray energy band. In the 1st project, We have examined 20 XMM-Newton light curves (LCs) of high synchrotron peaked blazar PG1553+113. In some LCs it showed large variability in the energy range 0.3-10 keV. Using the discrete correlation function (DCF) technique, we found positive and zero correlation with zero time lag between soft energy (0.3-2 keV) and hard energy (2.0-10 keV) bands. We found flux variability timescale in the range of 2.3 to 24.3 ks. In the second project, We report the flux and spectral variability of PG 1553+113 on intra-night to short-term timescales using BVRI data collected over 45 nights from March 2019 to September 2019 involving ten optical telescopes around the globe.
Intraday flux and color variations using two powerful tests: the power-enhanced F-test and the nested ANOVA test. In the STV timescale magnitude variation is 0.8 mag in R-band. We also investigated the spectral energy distribution (SED) using B, V, R, and I pass-band data. We found a mean optical spectral index of ∼ 0.94 ± 0.03 with a maximum variation of 0.25 by fitting a power law in the optical (BVRI) spectral energy distribution of PG 1553+113. In the last project we analyzed the optical photometric data from TXS 0506+056, a TeV blazar, collected using five ground-based telescopes from October 2018 to August 2021. In particular, we studied the flux and spectral variability properties of this blazar on IDV to LTV time scales. The weighted mean value of α is 2.069 ± 0.168 over this LTV timescale. During our observations on the IDV timescale, it nearly always follows the (bluer-when-brighter) BWB trend. In this pre-thesis submission talk, we briefly discuss physical mechanisms which may be capable of explaining the observed flux and spectral variability of PG 1553 + 113 and TXS 0506+056 on diverse timescales.