Doubling of minute-long Quasi-Periodic Pulsations from super-flares on a low mass star

TL;DR

This study observed two large stellar flares with quasi-periodic pulsations, revealing a doubling of oscillation periods likely caused by magnetohydrodynamic wave resonances in coronal loops, providing evidence of compact plasma loops in a star's corona.

Contribution

First detection of period doubling in stellar QPPs during super-flares, supporting MHD wave resonance models in stellar coronae.

Findings

QPP periods doubled during flares

Oscillations lasted several minutes with no sub-minute oscillations

Estimated loop lengths are 0.2-0.7 R*

Abstract

Using the ULTRASPEC instrument mounted on the 2.4-m Thai National Telescope, we observed two large flares, each with a total energy close to 10^34 erg with sub-second cadence. A combination of a wavelet analysis, a Fourier transform plus an empirical mode decomposition, reveals quasi-period pulsations (QPP) which exhibit an apparent doubling of the oscillation period. Both events showed oscillations of a few minutes over a interval of several minutes, and despite the availability of sub-second cadence, there was no evidence of sub-minute oscillations. The doubling of the QPP periods and shorter lifetime of shorter-period QPP modes strongly favour resonant dynamics of magnetohydrodynamic waves in a coronal loop. We estimate loop lengths to be 0.2-0.7 R*, in agreement with a typical length of solar coronal loops. These observations presents rare and compelling evidence for the presence of compact plasma loops in a stellar corona.