X-ray polarization fluctuations induced by cloud eclipses in active galactic nuclei
Astronomical Institute of the Academy of Sciences,
Bočni II 1401,
Received: 26 August 2014
Accepted: 31 October 2014
Context. A fraction of active galactic nuclei (AGN) show dramatic X-ray spectral changes on the day-to-week time scales associated with variation in the line of sight of the cold absorber.
Aims. We intend to model the polarization fluctuations arising from an obscuration event, thereby offering a method of determining whether flux variations are due to occultation or extreme intrinsic emission variability.
Methods. Undertaking 1−100 keV polarimetric simulations with the Monte Carlo code Stokes, we simulated the journey of a variety of cold gas clouds in front of an extended primary source. We varied the hydrogen column density nH and size of the absorber, as well as the initial polarization state of the emitting source, to cover a wide range of scenarios.
Results. Simulations indicate that different results are expected according to the initial polarization of the extended continuum source. For unpolarized primary fluxes, large (~50°) variations of the polarization position angle ψ are expected before and after an occultation event, which is associated with very low residual polarization degrees (P ≪ 1%). In the case of an emitting disk with intrinsic, position-independent polarization, and for a given range of parameters, X-ray eclipses significantly alter the observed polarization spectra, with most of the variations seen in ψ. Finally, non-uniformly polarized emitting regions produce very distinctive polarization variations due to the successive covering and uncovering of different portions of the disk. Plotted against time, variations in P and ψ form detectable P Cygni type profiles that are distinctive signatures of non-axisymmetric emission.
Conclusions. We find that X-ray polarimetry is particularly adapted to probing X-ray eclipses due to Compton-thin and Compton-thick gas clouds. Polarization measurements would distinguish between intrinsic intensity fluctuations and external eclipsing events, constrain the geometry of the covering medium, and test the hypothesis of non-uniformly emitting disks predicted by general relativity.
Key words: galaxies: Seyfert / polarization / radiative transfer / relativistic processes / scattering / X-rays: general
© ESO, 2014