Δ[ITAL]N[/ITAL] = 0 Dielectronic Recombination as a Line Formation Process in Photoionized Plasmas

Abstract

In photoionized plasmas, X-ray line emission is dominated by radiative cascades following recombination and photoexcitation, with a negligible contribution from collisional excitation due to the relatively low electron temperatures. Most existing photoionized plasma emission codes include only radiative recombination (RR) when calculating the level populations. Such models have been shown to be inadequate to explain the X-ray emission from H-like and He-like ions in the spectra of some Seyfert 2 active galactic nuclei and X-ray binaries obtained by the grating spectrometers on board Chandra and XMM-Newton. It has been shown that photoexcitation must be included to obtain theoretical models consistent with observations. In this Letter we show that for X-ray emission from iron L-shell ions, one must also include dielectronic recombination (DR) as a level population mechanism. As an example, the well-known 3-->2 lines of Fe XVII have been analyzed in detail. The 3F line at 16.78 Å is most affected by DR of Fe XVIII at low temperatures, while it is negligibly weak in purely RR models.