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 Lx of the seven detected flare stars is found to be |~2×10²⁸⁻¹|⁠. 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 (Lx/LBol) for all the X-ray-observed flaring and non-flaring dwarf M stars shows that the Lx/LBol 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 Lx and LBol for the flare stars and regular period RS CVn binaries described by |L_ x=10-3.23±0.22LBol|⁠. The correlation between Lx and the luminosities of chromospheric and transition region lines in flare stars is also discussed. No compelling evidence is found for a dependence of Lx/LBol (or Lx) 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.