  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.
