Charge Exchange in X-ray Astronomy

The observation of charge exchange emission potentially provides a new observational window on on the structure of boundaries between hot and cold regions, their relative velocities, and the actual mixing that occurs. Interpreting such data, and even identifying that the process exists, requires accurate knowledge of the velocity-dependent partial cross sections for the production of X-ray lines by charge exchange. I will point out the computational and laboratory challenges to obtaining these atomic parameters, suggesting that a combination of theoretical and experimental efforts will be required. As an example of astrophysical charge exchange, I will discuss current work on determining the contribution of interplanetary charge exchange to the 1/4-keV diffuse X-ray background. This was once thought to be almost entirely due to thermal emission from nearby interstellar gas at 1x10^6 K -- the "Local Hot Bubble" -- but the realization that charge exchange between interstellar neutrals passing through the Solar system and heavy ions in the Solar wind would produce X rays inspired calculations of the importance of this effect. The best models showed that charge exchange in the Solar system could readily produce *all* of the observed 1/4-keV radiation, but were based on very crude and uncertain partial cross sections for the production of the many X-ray lines involved. I will report preliminary results from laboratory measurements of some of these cross sections, and from a sounding rocket flight intended to make a direct determination of the importance of charge exchange in producing the 1/4-keV background radiation.