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 keV band from the nucleus of the radio galaxy Centaurus A (Cen A), using a 100 ks 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% confidence (MDP99) of 6.5% using a weighted, spectral model-independent calculation in the 2–8 keV band. The polarization angle ψ is consequently unconstrained. Spectral fitting across three orders of magnitude in X-ray energy (0.3–400 keV) demonstrates that the SED of Cen A is well described by a simple power law with moderate intrinsic absorption (N H ∼ 1023 cm−2) and a Fe Kα emission line, although a second unabsorbed power law is required to account for the observed spectrum at energies below 2 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 the 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 Kα emission line support a picture where electrons are accelerated in a region of highly disordered magnetic fields surrounding the innermost jet.

Observations of 4U 1626–67 with the Imaging X-Ray Polarimetry Explorer

“…Instead, we are able to constrain Π X in the 2-8 keV band to be < 13% for 3C 273 and 3C 279 and < 28% for 3C 454.3 and S5 0716+714. The non-detection of X-ray polarization in these sources is consistent with previous IXPE results on BL Lac in both LSP and ISP states Peirson et al 2023) as well as other radio galaxies (Ehlert et al 2022). For 3C 273 and 3C 454.3, the undetected X-ray polarization could point towards inverse-Compton scattering, either external Compton or SSC, however the unpolarized optical/IR emission during the IXPE observations is preventing us from coming to definitive conclusions.…”

“…We also used an unbinned maximum likelihood method (Marshall 2021); the best-fit values were slightly different, with q, u = (0.01, −0.01), but the 1σ error region also included the origin and the 2σ confidence region for the polarization fraction reached up to 3.7%. As in Ehlert et al (2022), an accounting for unpolarized background was included in the likelihood.…”

IXPE and Multiwavelength Observations of Blazar PG 1553+113 Reveal an Orphan Optical Polarization Swing

“…Owing to the three gas pixel detectors (GPDs; Costa et al 2001;Bellazzini et al 2003;Baldini et al 2021) in its focal plane, IXPE provides an unprecedented opportunity to measure the X-ray polarization of cosmic sources. Since its launch on 2021 December 9, IXPE has observed different types of AGNs (e.g., Ehlert et al 2022;Marinucci et al 2022;Gianolli et al 2023;Ingram et al 2023;Tagliacozzo et al 2023;Ursini et al 2023), including blazars (e.g., Liodakis et al 2022;Middei et al 2023). PG 1553+113 is one of several HSP blazars observed by IXPE.…”

Blazars at Very High Energies: Emission Modelling

“…Spectra will also be requisite to complete our knowledge on the still unknown redshifts for many BL Lacs [166]. The Imaging X-ray Polarimetry Explorer (IXPE) launched on 2021 December 9 will observe several AGN and blazars during its first years and already provided spectropolarimetric constraints on the radiogalaxy Cen A, suggesting that the X-rays from its core are due to Compton scattering of low-energy photons by non-thermal electrons accelerated in regions with highly disor-dered magnetic fields within a few parsecs of the black hole, possibly around the innermost jet [167]. In blazars, X-rays explore either the high-frequency part of the synchrotron bump, or the low-frequency part of the (leptonic or hadronic) high-energy bump, and X-ray polarization will be a strong diagnostic of the different radiation and acceleration models.…”