X-ray polarization fluctuations induced by cloud eclipses in active galactic nuclei

Astronomical Institute of the Academy of Sciences, Bočni II 1401, 14131 Prague, Czech Republic
e-mail: frederic.marin@asu.cas.cz

Received: 26 August 2014
Accepted: 31 October 2014

Abstract

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 n_H 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.