Mapping the circumnuclear regions of the Circinus galaxy with the Imaging X-ray Polarimetry Explorer

TL;DR

This study uses the Imaging X-ray Polarimetry Explorer to detect and analyze X-ray polarization in the Circinus galaxy, revealing the geometry of its circumnuclear regions and confirming aspects of AGN unification models.

Contribution

First X-ray polarization detection in a Seyfert galaxy, providing new insights into the geometry of AGN circumnuclear regions.

Findings

Polarization degree of 28% in the neutral reflector

Polarization angle roughly perpendicular to radio jet

Consistent with an equatorial torus with 45-55° half-opening angle

Abstract

We report on the Imaging X-ray Polarimetry Explorer (IXPE) observation of the closest and X-ray brightest Compton-thick active galactic nucleus (AGN), the Circinus galaxy. We find the source to be significantly polarized in the 2--6 keV band. From previous studies, the X-ray spectrum is known to be dominated by reflection components, both neutral (torus) and ionized (ionization cones). Our analysis indicates that the polarization degree is $28 \pm 7$ per cent (at 68 per cent confidence level) for the neutral reflector, with a polarization angle of $18{\deg} \pm 5{\deg}$, roughly perpendicular to the radio jet. The polarization of the ionized reflection is unconstrained. A comparison with Monte Carlo simulations of the polarization expected from the torus shows that the neutral reflector is consistent with being an equatorial torus with a half-opening angle of 45{\deg}-55{\deg}. This is the first X-ray polarization detection in a Seyfert galaxy, demonstrating the power of X-ray polarimetry in probing the geometry of the circumnuclear regions of AGNs, and confirming the basic predictions of standard Unification Models.