Spectropolarimetry of the changing-look active galactic nucleus Mrk 1018

We have obtained new spectropolarimetric observations at visible wavelengths of the changing-look active galactic nucleus (AGN) Mrk 1018. The AGN direct spectrum shows an extremely weak continuum with faint broad Hβ and Hα emission lines. Both lines can be fit with a single very broad emission line component of full width at half maximum FWHM ≃ 7200 km s−1, with no evidence of the additional 3000 km s−1-wide component that was previously detected. While this is in agreement with line formation in a Keplerian disk, the line profile variability suggests that the broad emission line region is likely more complex. The continuum polarization of Mrk 1018 is low; it is not higher in the current faint state compared to the past bright state, confirming that dust obscuration is not the mechanism at the origin of the change of look. The polarization profile of the Hα line is asymmetric with no rotation of the polarization angle, which possibly reveals line formation in a polar outflow. Alternatively, the polarization profile may be the consequence of a time delay between the direct and the polarized light. Interestingly, the polarization signatures predicted for broad lines emitted around supermassive binary black holes are not observed.

Visualization of Polarization and Two Dimensional Electron Gas Distribution in AlGaN/GaN Heterostructure Using Scanning Nonlinear Dielectric Microscopy

AlGaN/GaN heterostructure was observed using scanning nonlinear dielectric microscopy, which can measure both carrier and polarization profile in AlGaN/GaN heterostructure. As a result, GaN spontaneous polarization and AlGaN polarization which was sum of spontaneous polarization and piezoelectric polarization were clearly distinguished. Moreover, two dimensional electron gas was observed at the AlGaN/GaN interface. These results show that scanning nonlinear dielectric microscopy is useful method for evaluation of two dimensional electron gas profile and polarization profile in AlGaN/GaN heterostructure.