An asymmetric electron-scattering photosphere around optical tidal disruption events

TL;DR

This paper uses polarimetric observations to investigate the structure of optical emission in tidal disruption events, revealing an asymmetric, electron-scattering photosphere and providing new insights into the formation of accretion disks.

Contribution

It introduces spectropolarimetry as a novel observational method to probe the geometry and physical conditions of TDEs, supporting models of extended reprocessing envelopes and early disk formation.

Findings

Continuum polarization is wavelength-independent.

Emission lines are partially depolarized.

Predicted polarization levels match observations for extended envelopes.

Abstract

A star crossing the tidal radius of a supermassive black hole will be spectacularly ripped apart with an accompanying burst of radiation. A few tens of such tidal disruption events (TDEs) have now been identified in the optical wavelengths, but the exact origin of the strong optical emission remains inconclusive. Here we report polarimetric observations of three TDEs. The continuum polarization is independent of wavelength, while emission lines are partially depolarized. These signatures are consistent with optical photons being scattered and polarized in an envelope of free electrons. An almost axisymmetric photosphere viewed from different angles is in broad agreement with the data, but there is also evidence for deviations from axial symmetry before the peak of the flare and significant time evolution at early times, compatible with the rapid formation of an accretion disk. By combining a super-Eddington accretion model with a radiative transfer code we generate predictions for the degree of polarization as a function of disk mass and viewing angle, and we show that the predicted levels are compatible with the observations, for extended reprocessing envelopes of $\sim$1000 gravitational radii. Spectropolarimetry therefore constitutes a new observational test for TDE models, and opens an important new line of exploration in the study of TDEs.