Multiwavelength study of TeV blazar 1ES 1218 + 304 using gamma-ray,X-ray and optical obser v ations

Abstract

We report on a multiwavelength study of the high-synchrotron-peaked BL Lac 1ES 1218 + 304 using near simultaneous dataobtained 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 v ariability. Ho we ver, hour-scale v ariability 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 synchrotronpeak is observed from ∼1 keV to abo v e 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