X-ray polarization observations of IC 4329A with IXPE: Constraining the geometry of 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 characterization 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 Imaging X-ray Polarimeter (IXPE). IXPE observed IC 4329A during 5–15 January 2023, for a total observing time of 458 ks. From the model-independent analysis, we found a polarization degree ($\Pi _{X}$) of $3.7\pm 1.5$% and a polarization position angle ($\Psi _{X}$) of 61$^{\circ }\pm 12^{\circ }$ in the 2–8 keV energy range (at 68% confidence). This is also in agreement with the values of $\Pi _{X}$ and $\Psi _{X}$ 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 90% confidence). The value of $\Pi _X$ 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, $\Pi _X$ 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 $\Pi _X$ of $6.5 \pm 1.8$, which is larger than the MDP value of 5.5%. The observed moderate value of $\Pi _{X}$ obtained from the analysis of the 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 favor 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.