Detection of X-ray Polarization in the High Synchrotron Peaked Blazar 1ES 1959+650

Abstract

We report the measurement of X-ray polarization in the high synchrotron peaked blazar 1ES 1959+650. Of the four epochsof observations from the {\it Imaging X-ray Polarimetry Explorer}, we detected polarization in the 2$-$8 keV band on two epochs. From model-independent analysis of the observations on 28 October 2022, in the 2$-$8 keV band, we found the degree of polarization of $\Pi_X$ = 9.0 $\pm$ 1.6\% and an electric vector position angle of $\psi_X$ = 53 $\pm$ 5 deg. Similarly, from the observations on 14 August 2023, we found $\Pi_X$ and $\psi_X$ values as 12.5 $\pm$ 0.7\% and 20 $\pm$ 2 deg, respectively. These values are also in agreement with the values obtained from spectro-polarimetric analysis of the I, Q, and U spectra. The measured X-ray polarization is larger than the reported values in the optical, that ranges between 2.5$-$9\% , when observed during 2008 to 2018. Broadband spectral energy distribution constructed for the two epochs are well described by the one zone leptonic emission model with the bulk Lorentz factor ($\Gamma$) of the jet larger on 14 August 2023 compared to 28 October 2022. Our results favour shock acceleration of the particles in the jet, with the difference in $\Pi_X$ between the two epochs being influenced by change in the $\Gamma$ of the jet.