Detection of X-ray Resonance Scattering in Active Stellar Coronae

An analysis of Lyman series lines arising from hydrogen-like oxygen and neon ions in the coronae of the active RS CVn-type binaries II Peg and IM Peg, observed using the {\it Chandra} High Resolution Transmission Grating Spectrograph, shows significant decrements in the Ly$\alpha$/Ly$\beta$ ratios as compared with theoretical predictions and with the same ratios observed in similar active binaries. We interpret these decrements in terms of resonance scattering of line photons out of the line-of-sight; these observations present the first strong evidence for this effect in active stellar coronae. The net line photon loss implies a non-uniform and asymmetric surface distribution of emitting structures on these stars. Escape probability arguments, together with the observed line ratios and estimates of the emitting plasma density, imply typical line-of-sight sizes of the coronal structures that dominate the X-ray emission of $10^{10}$ cm at temperatures of $3\times 10^6$ K and of $10^8$ cm at $10^7$ K. These sizes are an order of magnitude larger than predicted by simple quasi-static coronal loops models, but are still very small compared to the several $10^{11}$ cm radii of the underlying stars.