Line intensity enhancements in stellar coronal X-ray spectra due to opacity effects

Context. The I(15.01 Å)/I(16.78 Å) emission line intensity ratio in Fe xvii has been reported to deviate from its theoretical value<br/>in solar and stellar X-ray spectra. This is attributed to opacity in the 15.01 Å line, leading to a reduction in its intensity, and was<br/>interpreted in terms of a geometry in which the emitters and absorbers are spatially distinct.<br/>Aims. We study the I(15.01 Å)/I(16.78 Å) intensity ratio for the active cool dwarf EV Lac, in both flare and quiescent spectra.<br/>Methods. The observations were obtained with the Reflection Grating Spectrometer on the XMM-Newton satellite. The emission<br/>measure distribution versus temperature reconstruction technique is used for our analysis.<br/>Results. We find that the 15.01 Å line exhibits a significant enhancement in intensity over the optically thin value. To our knowledge,<br/>this is the first time that such an enhancement has been detected on such a sound statistical basis. We interpret this enhancement<br/>in terms of a geometry in which the emitters and absorbers are not spatially distinct, and where the geometry is such that resonant<br/>pumping of the upper level has a greater effect on the observed line intensity than resonant absorption in the line-of-sight.