On the contribution of charge-exchange induced X-ray emission in the ISM and ICM

Service d'Aéronomie du CNRS, 91371 Verrières-le-Buisson, France e-mail: rosine.lallement@aerov.jussieu.fr

Received: 4 November 2003
Accepted: 11 May 2004

Abstract

X-ray emission can be generated by charge-exchange (CXE) between highly charged ions of a hot plasma and neutral species of an interacting cool/warm gas, a phenomenon recently observed in the case of the solar wind interaction with cometary, interstellar, and geocoronal neutrals (Lisse et al. [CITE]; Cox [CITE]; Cravens [CITE], [CITE]).
Charge-exchange processes are included in most theoretical models of hot interstellar plasmas interacting with partially neutral gas, resulting in a modification of the physical states of the gases in the interaction region. However, the contribution of the charge-exchange induced X-ray emission produced at these interfaces has hitherto not been considered to be significant. The detailed calculations performed by Wise & Sarazin ([CITE]), motivated by the observations of X-ray emission following charge-exchange in laboratory fusion devices, have shown that the emission is negligible in the case of an SNR fast shock. Our goal here is to investigate its relative importance in different astrophysical cases. We simplistically consider interfaces between partially neutral and hot gas in the following cases: (a) a supernova blast wave propagating in a neutral (or partially ionized) ISM (b) a galactic wind engulfing a halo dense cloud; (c) a high velocity cloud (HVC) moving through the halo, and (c) a dense cloud moving in intra-cluster gas. Although the phenomenon is restricted to the very narrow envelopes defined by the mean free path of the neutrals through the hot plasma, it is easy to show that its efficiency is such that the volume emissivity from these interfaces can be orders of magnitude above the emissivity of the hot gas itself, and, more important, that its relative contribution increases with decreasing hot gas density. As a consequence it should be at maximum in mixing layers in very low density hot gas.
Our preliminary results suggest that the charge-exchange X-ray emission from the interface does not contribute significantly in case (a), in agreement with Wise & Sarazin, except marginally for lines of sight tangent to the interfaces, but that it can be a non-negligible fraction of the hot gas emission in cases (b–d). Detailed self-consistent models of plasma and neutral atom distributions in conduction fronts or contact discontinuities are needed for better estimates. 
In the easiest test case of HVCs, there is a good agreement between our predicted equivalent emission measure  cm^-6 pc for complex C-type clouds and their soft X-ray emission measured by ROSAT (e.g. Kerp et al. [CITE]). 
If confirmed by more realistic calculations, a contribution of the CXE emission may play a role in a number of astrophysical cases: (i) tight correlations between Hα and X-ray patterns; (ii) enhanced X-ray limb brightening at interfaces; (iii) spectral and abundance “anomalies”, because charge-exchange spectra differ from thermal spectra and contain only emission lines, resulting in some biases when they are interpreted with classical models. 
Interestingly, interaction between cold IS clouds and the intra-cluster gas finally represents the most favorable case for the CXE emission, with the following potential consequences. At low spectral resolution, CXE emission can mimic global hot gas cooling. However, it is not in fact a global cooling, but instead accelerated cooling restricted to small physical areas, associated with the relative motions of the neutral clouds.
In galaxy clusters, if CXE emission is a non-negligible contributor to the total X-ray diffuse emission, it may help reduce the need for cooling flows.