A coronal explosion on the flare star CN Leonis

TL;DR

This paper reports the observation of a rare, rapid thermal soft X-ray burst during a stellar flare on CN Leo, providing insights into coronal explosions and micro-flares that may heat stellar coronae.

Contribution

It presents the first observation of a very short (~2 sec) thermal soft X-ray peak coinciding with optical flare peaks, indicating a coronal explosion on a star.

Findings

Detected a 2-second thermal soft X-ray burst during a stellar flare.

Hydrodynamic modeling suggests an extremely short energy deposition time.

Observations support the existence of micro-flares contributing to coronal heating.

Abstract

We present simultaneous high-temporal and high-spectral resolution observations at optical and soft X-ray wavelengths of the nearby flare star CN Leo. During our observing campaign a major flare occurred, raising the star's instantaneous energy output by almost three orders of magnitude. The flare shows the often observed impulsive behavior, with a rapid rise and slow decay in the optical and a broad soft X-ray maximum about 200 seconds after the optical flare peak. However, in addition to this usually encountered flare phenomenology we find an extremely short (~2 sec) soft X-ray peak, which is very likely of thermal, rather than non-thermal nature and temporally coincides with the optical flare peak. While at hard X-ray energies non-thermal bursts are routinely observed on the Sun at flare onset, thermal soft X-ray bursts on time scales of seconds have never been observed in a solar nor stellar context. Time-dependent, one-dimensional hydrodynamic modeling of this event requires an extremely short energy deposition time scale of a few seconds to reconcile theory with observations, thus suggesting that we are witnessing the results of a coronal explosion on CN Leo. Thus the flare on CN Leo provides the opportunity to observationally study the physics of the long-sought "micro-flares" thought to be responsible for coronal heating.