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

Abstract

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. 1996;Cox 1998;Cravens 1997Cravens , 2002. 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 (1989), motivated by the observations of X-ray emission following chargeexchange 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 EM ≈ 0.01 cm −6 pc for complex C-type clouds and their soft X-ray emission measured by ROSAT (e.g. Kerp et al. 1996). 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 col...

Charge Transfer Reactions

“…They also noted that charge exchange occurs mainly to highly excited states of the ion and can thus produce spectral features characteristic of a recombining plasma in what would otherwise be described as an ionizing shock. A more recent investigation by Lallement (2004a) also came to the conclusion that CXE from the interface regions is negligible, although charge exchange could slightly increase the limb brightening if they happen to be oriented along our line of sight.…”

“…Thus, it may be worthwhile to consider which astrophysical settings are favorable for the generation of CXE. According to Lallement (2004a), these are in particular the interfaces between hot gas and cool clouds, which may be identified by the following criteria:…”

“…According to Lallement (2004a), the relative importance of CXE compared to thermal emission of a hot plasma, is proportional to the product vn c n e −2 , where v is the relative velocity between the hot and cold gas, n c the cold gas density, and n e the hot gas density at the interface. Thus, CXE is particularly important when a tenuous hot plasma encounters a dense cold cloud at a high velocity.…”

“…However, this is not global cooling, but instead accelerated cooling restricted to small physical areas, associated with the relative motions of the neutral clouds. Lallement (2004a) concluded that, if CXE is a non negligible contributor to the total X-ray diffuse emission in galaxy clusters, then the cooling strength would be overestimated, and thus the deposited mass. In this way, charge exchange may even be linked to dark matter.…”

Large-scale distributions of mid- and far-infrared emission from the center to the halo of M 82 revealed with AKARI

“…The masses of Si and Fe in the hot plasma phase are 1.4 × 10 3 × f 0.5 M and 1.8 × 10 3 × f 0.5 M , respectively (Tsuru et al 2007). The charge-exchange process could be important in X-ray emission from the Cap (Lallement 2004), where the ionized superwind from M 82 can be assumed to collide with cool ambient gas located at the Cap. From the destruction of the PAHs at the Cap, we expect that the hot plasma is somewhat enriched with carbon there, which might be related to the marginal detection of the C vi emission line at 0.459 keV due to the charge-exchange process by Tsuru et al (2007).…”