Charge-exchange X-ray emission

Soft X-ray emission due to charge exchange (CX) between solar wind highly charged ions and atoms and molecules has recently been recognized to occur in a number of solar system objects. Here I present measurements and modelling of CX X-ray emission around comets, planets, and within the heliosphere (the cavity carved by the solar wind in the interstellar medium). This latter onini-directionnel emission is due to the interaction between solar wind ions and the flow of interstellar neutrals. It has now been found to be responsible for a large fraction of (if not all) the emission previously attributed to the Local Interstellar Hot Bubble (a 100 parsecs cavity around the Sun supposed to be filled with one million K gas). I also argue for the existence of CX X-ray emission in galaxies. Because new X-ray satellites have sufficient sensitivity and spectral resolution to detect the signatures of the CX lines, there is a strong need for theoretical work on CX state-selective population cross-sections and ion radiative transition probabilities, their dependence on the interacting species and the collisional energy.