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

Spectropolarimetry provides a new constraint on the geometry of tidal debris in TDEs, and has the potential to shed light on the much-debated source of optical emission from TDEs. 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 % at peak brightness. On day +29, the continuum polarization rose to ∼ 1 %, making this the first reported spectropolarimetric evolution of a TDE. These findings are incompatible with a naked eccentric disk that lacks significant mass outflow. Instead, the spectropolarimetry paints a picture wherein, at maximum brightness, high-frequency emission from the accretion disk 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 ∼ 100 au at maximum brightness — significantly larger than the tidal radius (∼ 1 au) and the thermalization radius (∼ 30 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. On day +29, the H𝛼 emission-line peak is depolarized to ∼ 0.3 % (compared to ∼ 1 % continuum polarization), and displays a gradual rise toward the line’s redder wavelengths. This observation indicates the H𝛼 line formed near the electron-scattering radius.