Polarization of Quasars: Electron Scattering in the Broad Absorption Line Region

It is widely accepted that the broad absorption line region (BALR) exists in most (if not all) quasars with a small covering factor. Recent works showed that the BALR is optically thick to soft and even medium energy X-rays, with a typical hydrogen column density of a few 10$^{23}$ to $>$ 10$^{24}$ cm$^{-2}$. The electron scattering in the thick absorber might contribute significantly to the observed continuum polarization for both BAL QSOs and non-BAL QSOs. In this paper, we present a detailed study of the electron scattering in the BALR by assuming an equatorial and axisymmetric outflow model. Monte-Carlo simulations are performed to correct the effect of the radiative transfer. Assuming an average covering factor of 0.2 of the BALR, which is consistent with observations, we find the electron scattering in the BALR with a column density of $\sim$ 4 $\times$ 10$^{23}$ cm$^{-2}$ can successfully produce the observed average continuum polarization for both BAL QSOs and non-BAL QSOs. The observed distribution of the continuum polarization of radio quiet quasars (for both BAL QSOs and non-BAL QSOs) is used to constrain the dispersal distribution of the BALR. We find that, to match the observations, the maximum continuum polarization produced by the BALR (while viewed edge-on) peaks at $P$ = 0.34%, which is much smaller than the average continuum polarization of BAL QSOs ($P$ = 0.93%). The discrepancy can be explained by a selection bias that the BAL with larger covering factor, and thus producing larger continuum polarization, is more likely to be detected. A larger sample of radio quiet quasars with accurate measurement of the continuum polarization will give better constraints to the distribution of the BALR properties.Comment: 17 pages,9 figures, accepted by AP

Superfluidity enhanced by spin-flip tunnelling in the presence of a magnetic field

It is well-known that when the magnetic field is stronger than a critical value, the spin imbalance can break the Cooper pairs of electrons and hence hinder the superconductivity in a spin-singlet channel. In a bilayer system of ultra-cold Fermi gases, however, we demonstrate that the critical value of the magnetic field at zero temperature can be significantly increased by including a spin-flip tunnelling, which opens a gap in the spin-triplet channel near the Fermi surface and hence reduces the influence of the effective magnetic field on the superfluidity. The phase transition also changes from first order to second order when the tunnelling exceeds a critical value. Considering a realistic experiment, this mechanism can be implemented by applying an intralayer Raman coupling between the spin states with a phase difference between the two layers.Comment: 10+4 pages, 8 figure

Super-resolution image transfer by a vortex-like metamaterial

We propose a vortex-like metamaterial device that is capable of transferring image along a spiral route without losing subwavelength information of the image. The super-resolution image can be guided and magnified at the same time with one single design. Our design may provide insights in manipulating super-resolution image in a more flexible manner. Examples are given and illustrated with numerical simulations.Comment: 7 pages, 6 figure