X-ray polarization observations of IC 4329A with IXPE: Constraining the geometry of the X-ray corona

Abstract

X-ray polarimetry is a powerful tool to probe the geometry of the hot X-ray corona in active galactic nuclei (AGN). Here, we present our results on the characterisation of the X-ray polarization of the radio-quiet Seyfert-type AGN IC 4329A at a redshift of $z$ = 0.016. This is based on observations carried out by the {\it Imaging X-ray Polarimeter (IXPE)}. {\it IXPE} observed IC 4329A on January 5, 2023, for a total observing time of 458 ks. From the model-independent analysis, we found a polarization degree (${\hat{I}}_X$) of 3.7$\pm$1.5$\%$ and a polarization position angle ($\hat{I}{¨}_X$) of 61${}^{\circ }$$\pm$12${}^{\circ }$ in the 2$-$8 keV energy range (at 1$\ddot{I}ƒ$ confidence level). This is also in agreement with the values of ${\hat{I}}_X$ and $\hat{I}{¨}_X$ of 4.7$\pm$2.2$\%$ and 71${}^{\circ }$ $\pm$14${}^{\circ }$ respectively obtained from spectro-polarimetric analysis of the I, Q and U Stokes spectra in the 2$-$8 keV energy band (at the 90$\%$ confidence). The value of ${\hat{I}}_X$ in the 2-8 keV band obtained from the model-independent analysis is lower than the minimum detectable polarization (MDP) value of 4.5$\%$. However, ${\hat{I}}_X$ obtained from spectro-polarimetric analysis in the 2-8 keV band is larger than the MDP value. In the 3-5 keV band, we found ${\hat{I}}_X$ of 6.5 $\pm$ 1.8, which is larger than the MDP value of 5.5$\%$. The observed moderate value of ${\hat{I}}_X$ obtained from the analysis of the {\it IXPE} data in the 3$-$5 keV band argues against a spherical lamp$-$post geometry for the X-ray corona in IC 4329A; however, considering simulations, the observed polarization measurements tend to favour a conical shape geometry for the corona. This is the first time measurement of X-ray polarization in IC 4329A. Measurements of the X-ray polarization in many such radio-quiet AGN will help in constraining the geometry of the X-ray corona in AGN.

7 pages, 4 figures, Submitted to the Journal of Astronomy and Astrophysics