Limits on X-ray Polarization at the Core of Centaurus A as Observed with the Imaging X-ray Polarimetry Explorer

Abstract

We present measurements of the polarization of X-rays in the $2-8\mathrm{keV}$ band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100ks observation from the Imaging X-ray Polarimetry Explorer (IXPE). Nearly simultaneous observations of Cen A were also taken with the Swift, NuSTAR, and INTEGRAL observatories. No statistically significant degree of polarization is detected with IXPE. These observations have a minimum detectable polarization at $99\%$ percent confidence (MDP${}_{99}$) of $6.5\%$ using a weighted, spectral model-independent calculation in the $2-8\mathrm{keV}$ band. The polarization angle $\ddot{I}ˆ$ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy ($0.3-400\mathrm{keV}$) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption ($N_H\sim 10^{23}{\mathrm{cm}}^{-2}$) and a Fe K$\hat{I}\pm$ emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below $2\mathrm{keV}$. This spectrum suggests that the reprocessing material responsible for this emission line is optically thin and distant from the central black hole. Our upper limits on the X-ray polarization are consistent with the predictions of Compton scattering, although the specific seed photon population responsible for production of the X-rays cannot be identified. The low polarization degree, variability in the core emission, and the relative lack of variability in the Fe K$\hat{I}\pm$ emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.

Accepted for Publication in ApJ, 15 pages