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

TL;DR

This study reports the first statistically significant detection of line intensity enhancement in Fe XVII X-ray spectra of EV Lac, attributed to resonant pumping effects rather than simple opacity, challenging previous assumptions about stellar coronae.

Contribution

It introduces the first observation of a line intensity enhancement due to resonant pumping in stellar X-ray spectra, providing new insights into coronal geometry and radiative transfer effects.

Findings

Detected significant line intensity enhancement in Fe XVII spectra.

Interpreted enhancement as resonant pumping rather than opacity effects.

Challenged previous models assuming spatially distinct emitters and absorbers.

Abstract

Context. The I(15.01 A)/I(16.78 A) emission line intensity ratio in Fe XVII has been reported to deviate from its theoretical value in solar and stellar X-ray spectra. This is attributed to opacity in the 15.01 A 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. Aims. We study the I(15.01 A)/I(16.78 A) intensity ratio for the active cool dwarf EV Lac, in both flare and quiescent spectra. Methods. 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. Results. We find that the 15.01 A 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.