Spectropolarimetry of the tidal disruption event AT 2019qiz: a quasispherical reprocessing layer

Abstract

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 $\sim 1$ 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 $\sim 100\mathrm{au}$ at maximum brightness -- significantly larger than the tidal radius ($\sim 1\mathrm{au}$) and the thermalisation radius ($\sim 30\mathrm{au}$) 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$\alpha$ emission-line peak is depolarized to $\sim 0.3$ per cent (compared to $\sim 1$ per cent continuum polarization), and displays a gradual rise toward the line's redder wavelengths. This observation indicates the H$\alpha$ line formed near the electron-scattering radius.

8 pages, 4 figures, accepted for publication in MNRAS

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