Study of quiescent state X-ray emission from flare stars

Results on the X-ray emission from flare stars observed with the imaging proportional counter (IPC) on the Einstein Observatory are described. The mean value of the quiescent state X-ray luminosity L_x of the seven detected flare stars is found to be $$\sim2\times10^{28}\enspace\text {erg}\enspace\text{s}^{-1}$$. Analysis of the energy spectra of three of the stars gives the value of the corona temperature as 3×10⁶ K. Results on gradual flux variations by a factor of ∼2 in G 735 and the occurrence of a short duration (≃400 s) flare in G 729 are presented. A study of the distribution of the ratio of the X-ray luminosity to the bolometric luminosity (L_x/L_Bol) for all the X-ray-observed flaring and non-flaring dwarf M stars shows that the L_x/L_Bol distribution of the flare stars is remarkably similar to that of the regular period RS CVn binaries but dissimilar to that of the non-flaring dwarf M stars. A strong correlation is found between L_x and L_Bol for the flare stars and regular period RS CVn binaries described by $$L_\text x=10^{-3.23\pm0.22}L_\text{Bol}$$. The correlation between L_x and the luminosities of chromospheric and transition region lines in flare stars is also discussed. No compelling evidence is found for a dependence of L_x/L_Bol (or L_x) on the rotation velocity or rotation period for the limited sample of flare stars with available data. Finally similarities between the X-ray spectra of the flare stars and those of the RS CVn binaries are discussed.