[Previous] | [Session 13] | [Next]
V.L. Kashyap, J.J. Drake (SAO), M. Audard, M. Guedel (LA-PSI)
Microflares and nanoflares in the quiet Solar corona are thought to be distributed as power laws (e.g., Hudson 1991, Sol.Phys. 133,357; Parnell & Jupp 2000, ApJ 529,554) of the form dN/dE=k E-\alpha where E is the energy of the flare and k is a constant. If the power-law is steep enough (\alpha>2), then in principle a multitude of small impulsive events would be sufficient to account for the energy output of the entire corona. Determining \alpha in solar and stellar coronae is therefore of considerable interest for constraining flare-based mechanisms of coronal heating.
Measuring the value of \alpha in the case of stars is fraught with difficulty due to the small duty cycles of the observations as well as the low counts in the light curves that make it hard to detect flares. For strong flares, values of \alpha in the range 1.5-1.6 have been measured (Collura et al.\ 1988, A&A 205,197; Osten & Brown 1999, ApJ 515,746). Careful analysis including weaker flares show \alpha in the range of 1.8-2.4 for active solar analogs and other main-sequence stars (Audard et al.\ 1999, ApJ 513,L53; Audard et al.\ 2000, submitted to ApJ).
We have developed a new method to detect and characterize the flare distribution without having to detect all of the flares, or having to correct for ``deadtime'' due to strong flares. Working directly on photon arrival time lists avoids losing sensitivity to the weaker flares by binning the light curve. We achive these advances by comparing the observed distribution of arrival-time differences with arrival-time differences derived from a model with a given \alpha.
We analyzed a deep (139 ks) EUVE observation of the SB2 non-eclipsing binary FK Aqr (HD214479,dM2e/dM3e,8.3 pc) to characterize the distribution of flares and microflares. We find that for this dataset 2.1<\alpha<2.5, and thus the entire emission on FK Aqr may be due to micro- and nano-flaring activity. These results are compared to those of similar analyses applied to other stars.
The author(s) of this abstract have provided an email address for comments about the abstract: vkashyap@cfa.harvard.edu