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We report on a multiwavelength study of the high-synchrotron-peaked BL Lac 1ES 1218+304 using near-simultaneous data obtained during the period from 2018 January 1 to 2021 May 31 (MJD 58119–59365) from various instruments, including Fermi–LAT, Swift–XRT, AstroSat , and optical data from Swift-UVOT and the TUBITAK observatory in Turkey. The source was reported to be flaring in the TeV γ-ray band during 2019, but no significant variation was observed with Fermi–LAT. A sub-hour variability is seen in the SXT light curve, suggesting a compact emission region for the variability. However, hour-scale variability is observed in the γ-ray light curve. A 'softer-when-brighter' trend is observed in the γ-ray, and an opposite trend is seen in the X-ray, suggesting that the two emissions are produced through two different processes, as expected from a high-frequency-peaked BL Lac source. We have chosen the two epochs in 2019 January to study and compare their physical parameters. A joint fit of SXT and LAXPC provides a constraint on the synchrotron peak, estimated to be ∼1.6 keV. A clear shift in the synchrotron peak is observed from ∼1 keV to above 10 keV, revealing its extreme nature or behaviour like an extreme blazar-type source. The optical observation provides a colour-index variation as 'blue-when-brighter'. The broad-band spectral energy distribution is fitted with a single-zone synchrotron-self Compton model, and their parameters are discussed in the context of a TeV blazar and the possible mechanism behind the broad-band emission. [ABSTRACT FROM AUTHOR]

Blazars are a class of AGN, one of their jets is pointed towards the earth. Here, we report about the multiwavelength study for blazar S5 1803+78 between MJD 58727 and MJD 59419. We analysed gamma-ray data collected by Fermi-LAT, X-ray data collected by Swift-XRT & NuSTAR, and optical photons detected by Swift-UVOT & TUBITAK observatory in Turkey. Three flaring states are identified by analysing the gamma-ray light curve. A day-scale variability is observed throughout the flares with the similar rise and decay times suggesting a compact emission region located close to the central engine. Cross-correlation studies are carried out between gamma-ray, radio, and X-ray bands, and no significant correlation is detected. The gamma-ray and optical emission are significantly correlated with zero time lag suggesting a co-spatial origin of them. The broad-band spectral energy distribution (SED) modelling was performed for all the flaring episodes as well as for one quiescent state for comparison. SEDs are best fitted with the synchrotron-self Compton (SSC) model under a one-zone leptonic scenario. The SED modelling shows that to explain the high flaring state, strong Doppler boosting is required. [ABSTRACT FROM AUTHOR]