Broadband Study of High-Synchrotron-Peaked BL Lac Object 1ES 1218+304

Abstract

The origin of the multiwavelength emission from the high-synchrotron-peaked BL Lac 1ES 1218+304 is studied using the data from {\it Swift UVOT/XRT}, {\it NuSTAR} and {\it Fermi-LAT}. A detailed temporal and spectral analysis of the data observed during 2008-2020 in the $\gamma$-ray ($>100$ MeV), X-ray (0.3-70 keV), and optical/UV bands is performed. The $\gamma$-ray spectrum is hard with a photon index of $1.71\pm0.02$ above $100$ MeV. The {\it Swift UVOT/XRT} data show a flux increase in the UV/optical and X-ray bands; the highest $0.3-3$ keV X-ray flux was $(1.13\pm0.02)\times10^{-10}{\rm erg\:cm^{-2}\:s^{-1}}$. In the 0.3-10 keV range the averaged X-ray photon index is $>2.0$ which softens to $2.56 \pm 0.028$ in the 3-50 keV band. However, in some periods, the X-ray photon index became extremely hard ($<1.8$), indicating that the peak of the synchrotron component was above $1$ keV, and so 1ES 1218+304 behaved like an extreme synchrotron BL Lac. The hardest X-ray photon index of 1ES 1218+304 was $1.60 \pm 0.05 $ on MJD 58489. The time-averaged multiwavelength spectral energy distribution is modeled within a one-zone synchrotron self-Compton leptonic model using a broken power-law and power-law with an exponential cutoff electron energy distributions. The data are well explained when the electron energy distribution is $E_{\rm e}^{-2.1}$ extending up to $\gamma_{\rm br/cut}\simeq(1.7-4.3)\times10^{5}$, and the magnetic field is weak ($B\sim1.5\times10^{-2}$ G). By solving the kinetic equation for electron evolution in the emitting region, the obtained electron energy distributions are discussed considering particle injection, cooling, and escape.