Astronomy and Astrophysics, volume 483, 887-890 (2008/6-1)
Line intensity enhancements in stellar coronal X-ray spectra due to opacity effects.
ROSE S.J., MATRANGA M., MATHIOUDAKIS M., KEENAN F.P. and WARK J.S.
Abstract (from CDS):
The I(15.01Å)/I(16.78Å) emission line intensity ratio in Fexvii has been reported to deviate from its theoretical value in solar and stellar X-ray spectra. This is attributed to opacity in the 15.01Å line, leading to a reduction in its intensity, and was interpreted in terms of a geometry in which the emitters and absorbers are spatially distinct. We study the I(15.01Å)/I(16.78Å) intensity ratio for the active cool dwarf EV Lac, in both flare and quiescent spectra. The observations were obtained with the Reflection Grating Spectrometer on the XMM-Newton satellite. The emission measure distribution versus temperature reconstruction technique is used for our analysis. We find that the 15.01Å line exhibits a significant enhancement in intensity over the optically thin value. To our knowledge, this is the first time that such an enhancement has been detected on such a sound statistical basis. We interpret this enhancement in terms of a geometry in which the emitters and absorbers are not spatially distinct, and where the geometry is such that resonant pumping of the upper level has a greater effect on the observed line intensity than resonant absorption in the line-of-sight.