Optical polarization from colliding stellar stream shocks in a tidal disruption event

TL;DR

This study presents optical polarimetry of a tidal disruption event, revealing highly polarized emission likely caused by shocks during accretion disk formation rather than relativistic jets, challenging previous jet-based interpretations.

Contribution

It provides the first optical polarization measurements of a TDE and proposes a new shock-based origin for polarized emission, diverging from jet models.

Findings

Peak polarization of 25% suggests synchrotron radiation.

Lack of radio jet emission indicates shocks, not jets, produce polarization.

Optical polarization linked to stellar stream collisions during accretion.

Abstract

A tidal disruption event (TDE) occurs when a supermassive black hole rips apart a passing star. Part of the stellar material falls toward the black hole, forming an accretion disk that in some cases launches a relativistic jet. We performed optical polarimetry observations of a TDE, AT 2020mot. We find a peak linear polarization degree of $25\pm4$%, consistent with highly polarized synchrotron radiation, as is typically observed from relativistic jets. However, our radio observations, taken up to 8 months after the optical peak, do not detect the corresponding radio emission expected from a relativistic jet. We suggest that the linearly polarized optical emission instead arises from shocks that occur during accretion disk formation, as the stream of stellar material collides with itself.