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

TL;DR

This multi-wavelength study of the TeV blazar 1ES 1218+304 reveals rapid variability, a shift in the synchrotron peak, and distinct emission processes, enhancing understanding of extreme high-energy blazar behavior.

Contribution

The paper provides the first simultaneous multi-wavelength analysis of 1ES 1218+304, revealing a shift in the synchrotron peak and detailed physical parameters using broadband SED fitting.

Findings

Detected sub-hour variability in X-ray emission.

Observed a shift in the synchrotron peak from 1 keV to above 10 keV.

Identified a 'blue-when-brighter' trend in optical observations.

Abstract

We report the multi-wavelength study for a high-synchrotron-peaked BL Lac 1ES 1218+304 using near-simultaneous data obtained during the period from January 1, 2018, to May 31, 2021 (MJD 58119-59365) from various instruments including Fermi-LAT, Swift-XRT, AstroSat, and optical from Swift-UVOT $\&$ TUBITAK observatory in Turkey. The source was reported to be flaring in TeV $\gamma$-ray band during 2019, but no significant variation is observed with Fermi-LAT. A sub-hour variability is seen in the SXT light curve, suggesting a compact emission region for their variability. However, hour scale variability is observed in the $\gamma$-ray light curve. A "softer-when-brighter" trend is observed in $\gamma$-rays, and an opposite trend is seen in X-rays suggesting both emissions are produced via two different processes as expected from an HBL source. We have chosen the two epochs in January 2019 to study and compare their physical parameters. A joint fit of SXT and LAXPC provides a constraint on the synchrotron peak, roughly estimated to be $\sim$1.6 keV. A clear shift in the synchrotron peak is observed from $\sim$1 keV to above 10 keV revealing its extreme nature or behaving like an EHBL-type source. The optical observation provides color-index variation as "blue-when-brighter". The broadband SED is fitted with a single-zone SSC model, and their parameters are discussed in the context of a TeV blazar and the possible mechanism behind the broadband emission.