# Discovery of the subsecond linearly polarized spikes of synchrotron origin in the UV Ceti giant optical flare

## TL;DR

This study reports the first detection of linearly polarized optical spikes during a giant flare on UV Ceti, revealing non-thermal synchrotron emission from ultrarelativistic electrons in stellar magnetic fields.

## Contribution

It provides the first evidence of polarized optical synchrotron emission in UV Ceti stars, demonstrating a non-thermal mechanism in stellar flares previously observed mainly in solar flares.

## Key findings

- Detection of polarized optical spikes with >35% linear polarization.
- Identification of these spikes as synchrotron emission from ultrarelativistic electrons.
- First observation of such emission in UV Ceti-type stars.

## Abstract

During our optical monitoring of UV Ceti, iconic late-type flaring star, with high temporal resolution using the Russian 6-m telescope in 2008 we detected a giant flare with the amplitude of about 3 magnitudes in $U$-band. Near flare maximum more than a dozen of spike bursts have been discovered with triangular shapes and durations from 0.6 to 1.2 s and maximal luminosities in the range $(1.5-8)\times10^{27}$ erg~s$^{-1}$. For the half of these events the linear polarization exceeds 35% with significance better than $5\sigma$. We argue that these events are synchrotron emission of electron streams with the energies of several hundred MeV moving in the magnetic field of about 1.4 kG. Emission from such ultrarelativistic (with energies far exceeding 10 MeV) particles is being routinely observed in solar flares, but has never been detected from UV Ceti type stars. This is the first ever detection of linearly polarized optical light from the UV Ceti-type stars which indicates that at least some fraction of the flaring events on these stars are powered by a non-thermal synchrotron emission mechanism.