Optical polarization of stellar-fed active and quiescent supermassive black holes

TL;DR

This study uses optical polarimetry to investigate the polarization properties of TDEs and AGN flares, revealing insights into their reprocessing geometries, disk formation, and outflow asymmetries.

Contribution

It provides the first detailed polarization measurements of TDEs and Bowen flares, linking polarization behavior to accretion states and outflow geometries.

Findings

Low polarization in thermal TDEs with stable angles

Variable polarization angles in Bowen flares indicating complex geometries

Polarization correlates with Eddington ratio and outflow extension

Abstract

Using the MOPTOP polarimeter on the 2-meter Liverpool Telescope, we conducted a monitoring campaign targeting three optically-discovered TDEs (AT2024bgz, AT2024pvu, and AT2024wsd) and two Bowen flares in AGN (AT2020afhd and AT2019aalc). The three thermal TDEs showed low intrinsic polarization levels ($\Delta P \approx 0$-$6\%$) with stable polarization angles. The Bowen flares also showed variable polarization degree ($ \Delta P \approx 0$-$8\%$), but significant polarization angle variability: AT2020afhd exhibited a $\Delta \theta = 83 \pm 8 \,^\circ$ shift at 150 days post-optical peak, while the AT2019aalc displayed quasi-periodic swings of $\Delta \theta \approx 40 \, ^\circ$ amplitude starting 190 days after peak brightness. The TDEs of this study are well described by models invoking rapid disk formation and reprocessed emission from optically thick outflows, whereas the Bowen flares reveal more complex reprocessing geometries, potentially consistent with TDEs occurring in AGN gas-rich environments. We find that moderate polarization is observed at later times for TDEs with low-Eddington ratios and highly extended photospheres. This implies that, as the accretion level declines, we expect more asymmetric reprocessing layers along a given viewing angle. Since the outflow density and velocity depend sensitively on the inclination angle, we expect TDEs with low-Eddington ratios and highly extended photospheres to exhibit varying levels of polarization. The polarization of AT2019aalc (Seyfert 1) hints at a clumpy, asymmetric outflow and the presence of a tilted, precessing accretion disk, while the polarization of AT2020afhd (AGN type 2) is consistent with the detection of a scattered light echo.