A coronal explosion on the flare star CN Leonis

¹ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: jschmitt@hs.uni-hamburg.de
² Dipartimento di Scienze Fisiche e Astronomiche, Università di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ Inst. f. Astrophysik Universität Göttingen, 37077 Göttingen, Germany

Received: 7 November 2007
Accepted: 24 January 2008

Abstract

We present simultaneous high-temporal and high-spectral resolution observations of the nearby flare star CN Leo at optical and soft X-ray wavelengths. 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. In addition to this usually encountered flare phenomenology we find, however, an extremely short ( s) soft X-ray peak, which is very likely of thermal, rather than nonthermal nature and coincides temporally with the optical flare peak. While at hard X-ray energies nonthermal 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.