X-ray polarimetric signatures induced by spectral variability in the framework of the receding torus model
1 Observatoire Astronomique de Strasbourg, Université de Strasbourg, CNRS, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France
2 Université Grenoble Alpes, IPAG, 38000 Grenoble, France
3 CNRS, IPAG, 38000 Grenoble, France
Received: 8 March 2016
Accepted: 26 April 2016
Context. Obscuring circumnuclear dust is a well-established constituent of active galactic nuclei (AGN). Traditionally referred to as the receding dusty torus, its inner radius and angular extension should depend on the photo-ionizing luminosity of the central source.
Aims. We quantify the expected time-dependent near-infrared (NIR), optical, ultraviolet (UV) and X-ray polarization of a receding dusty torus as a function of the variable X-ray flux level and spectral shape.
Methods. Using a Monte Carlo approach, we simulate the radiative transfer between the multiple components of an AGN adopting model constraints from the bright Seyfert galaxy NGC 4151. We compare our model results to the observed NIR to UV polarization of the source and predict its X-ray polarization.
Results. We find that the 2−8 keV polarization fraction of a standard AGN model varies from less then a few percent along polar viewing angles up to tens of percent at equatorial inclinations. At viewing angles around the type-1/type-2 transition, there is a different X-ray polarization variability in a static or a receding torus scenario. In the former case, the expected 2−8 keV polarization of NGC 4151 is found to be 1.21% ± 0.34% with a constant polarization position angle, while in the latter scenario it varies from 0.1% to 6% depending on the photon index of the primary radiation. Additionally, an orthogonal rotation of the polarization position angle with photon energy appears for very soft primary spectra.
Conclusions. Future X-ray polarimetry missions will be able to test whether the receding model is valid for Seyfert galaxies seen at a viewing angle close to the torus horizon. The overall stability of the polarization position angle for photon indexes softer than Γ = 1.5 ensures that reliable measurements of X-ray polarization are possible. We derive a long-term observational strategy for NGC 4151 assuming observations with a small to medium-sized X-ray polarimetry satellite.
Key words: galaxies: active / galaxies: Seyfert / galaxies: individual: NGC 4151 / polarization / radiative transfer / X-rays: galaxies
© ESO, 2016