Are there reliable methods to estimate the nuclear orientation of Seyfert galaxies?

Orientation, together with accretion and evolution, is one of the three main drivers in the Grand Unification of Active Galactic Nuclei (AGN). Being unresolved, determining the true inclination of those powerful sources is always difficult and indirect, yet it remains a vital clue to apprehend the numerous, panchromatic and complex spectroscopic features we detect. There are only a hundred inclinations derived so far; in this context, can we be sure that we measure the true orientation of AGN? To answer this question, four methods to estimate the nuclear inclination of AGN are investigated and compared to inclination-dependent observables (hydrogen column density, Balmer linewidth, optical polarization, and flux ratios within the IR and relative to X-rays). Among these orientation indicators, the method developed by Fisher, Crenshaw, Kraemer et al., mapping and modeling the radial velocities of the [O iii] emission region in AGN, is the most successful. The [O iii]-mapping technique shows highly statistically significant correlations at >95% confidence level for rejecting null hypothesis for all the test cases. Such results confirm that the Unified Model is correct at a scale ranging from kiloparsec to a fraction of a parsec. However, at a radial distance less than 0.01 pc from the central black hole, warps and misalignments may change this picture.Comment: 33 pages, 11 figures, 11 tables, accepted for publication in MNRA

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.Comment: 8 pages, 5 figures. Accepted for publication in A&