X-ray polarization fluctuations induced by cloud eclipses in active
  galactic nuclei
  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: For unpolarized primary fluxes, large (about 50deg) variations of the
polarization position angle psi are expected before and after an occultation
event, which is associated with very low residual polarization degrees (P lower
than 1 per cent). 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 psi. 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 psi form detectable P~Cygni type profiles that are
distinctive signatures of non-axisymmetric emission. Conclusions: Polarization
measurements are thus particularly adapted to 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.
