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 Imaging X-ray Polarimeter (IXPE). 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 (Π𝑋) of 3.7±1.5% and a polarization position angle (Ψ𝑋) of 61◦±12◦ in the 2−8 keV energy range (at 1𝜎 confidence level). This is also in agreement with the values of Π𝑋 and Ψ𝑋 of 4.7±2.2% and 71◦ ±14◦ 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 Π𝑋 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, Π𝑋 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 Π𝑋 of 6.5 ± 1.8, which is larger than the MDP value of 5.5%. The observed moderate value of Π𝑋 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 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.