Spectropolarimetry of the tidal disruption event AT 2019qiz: a quasi-spherical reprocessing layer

We present optical spectropolarimetry of the tidal disruption event (TDE) AT 2019qiz on days +0 and +29 relative to maximum brightness. Continuum polarization, which informs the shape of the electron-scattering surface, was found to be consistent with 0 per cent at peak brightness. On day +29, the continuum polarization rose to ~1 per cent, making this the first reported spectropolarimetric evolution of a TDE. These findings are incompatible with a naked eccentric disc that lacks significant mass outflow. Instead, the spectropolarimetry paints a picture wherein, at maximum brightness, high-frequency emission from the accretion disc is reprocessed into the optical band by a nearly spherical, optically thick, electron-scattering photosphere located far away from the black hole. We estimate the radius of the scattering photosphere to be $\sim 100\rm \, au$ at maximum brightness - significantly larger than the tidal radius ($\sim 1\rm \, au$) and the thermalization radius ($\sim 30\rm \, au$) where the optical continuum is formed. A month later, as the fallback rate drops and the scattering photosphere recedes, the continuum polarization increases, revealing a moderately aspherical interior. We also see evidence for smaller scale density variations in the scattering photosphere, inferred from the scatter of the data in the Stokes q-u plane. On day +29, the H α emission-line peak is depolarized to ~0.3 per cent (compared to ~1 per cent continuum polarization), and displays a gradual rise towards the line's redder wavelengths. This observation indicates the H α line formed near the electron-scattering radius.