Polarisation angle variability in tidal disruption events

Abstract

Tidal disruption events (TDEs) occur when a star is disrupted by the tidal forces of a supermassive black hole, and these events produce bright multi-wavelength flares. Polarimetric measurements of TDEs allow us to disentangle the geometry and the mechanisms characterising the accretion process. We carried out the first systematic study of the time evolution of the optical polarisation angle ($Θ$) in a sample of classified TDEs, combining our own data with all available measurements from the literature, with the goal of testing the currently available models that describe TDE emission. We assembled data from all available observing epochs with significant linear polarisation detections ($Π-3σ_Π>0\%$) for sources with at least two such epochs, and we determined the overall variability trends across the sample in various time frames, such as days from peak time and the fallback time ($t_0$) derived from the different models. Our final sample comprises 12 transients, including three Bowen fluorescence flares (BFFs). The majority of the sources show significant $Θ$ variability. The distribution of $|\mathrm{d}Θ/\mathrm{d}t|$ peaks near ($\sim 2^{\circ}$ d$^{-1}$. BFFs tend to display sustained late-time $Θ$ evolution, likely due in part to their slower fading. No universal trend emerges when time is normalised by $t_0$. Short-timescale $Θ$ variability is common in TDEs and is difficult to reconcile with simple axisymmetric reprocessing models that predict a constant polarisation angle. The observed phenomenology favours scenarios with evolving, non-axisymmetric geometries and/or shocks, possibly coupled with changes in optical depth. Denser polarimetric monitoring, contemporaneous spectroscopy, and X-ray/UV coverage are required to break the remaining degeneracies.

Figures (8)

• Figure 1: Histogram of the rate of change of the polarisation angle per day for the sources in the sample. Non-detections are not considered in this calculation, as $\Theta$ is undefined in such cases. In red, a log-normal distribution defined in base $e$ with the same central value ($\mu$) and dispersion ($\sigma$) of the data is overplotted.
• Figure 2: Maximum $\Theta$ excursion per source in $25^\circ$ bins. Non-detections are omitted, as $\Theta$ is undefined in such cases.
• Figure 3: $\Theta$ as a function of days since optical peak for the TDEs in the sample. Non-detections are omitted, as $\Theta$ is undefined in such cases.
• Figure 4: $\Theta$ as a function of days since optical peak for the BFF subsample. Non-detections are omitted, as $\Theta$ is undefined in such cases.
• Figure 5: $\Theta$ measurements of the sources in the sample, as a function of $t_0$, estimated based on the shock model ryu2020. Non-detections are omitted, as $\Theta$ is undefined in such cases.