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

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Diwan R., Prince R., Agarwal A., Bose D., Majumdar P., ÖZDÖNMEZ A., ...More

Monthly Notices of the Royal Astronomical Society, vol.524, no.3, pp.4333-4345, 2023 (SCI-Expanded) identifier identifier

  • Publication Type: Article / Article
  • Volume: 524 Issue: 3
  • Publication Date: 2023
  • Doi Number: 10.1093/mnras/stad2088
  • Journal Name: Monthly Notices of the Royal Astronomical Society
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Aerospace Database, Applied Science & Technology Source, Communication Abstracts, Compendex, INSPEC, Metadex, zbMATH, DIALNET, Civil Engineering Abstracts
  • Page Numbers: pp.4333-4345
  • Keywords: BL Lacertae objects: individual: 1ES 1218+304, galaxies: active, galaxies: jets, gamma-rays: galaxies, radiation mechanisms: non-thermal
  • Istanbul University Affiliated: Yes


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.