Comet Hyakutake x‐ray source: Charge transfer of solar wind heavy ions

Recently, Lisse et al. (1996) reported on exciting observations by the Röntgen X‐ray satellite (ROSAT) of x‐ray and extreme ultraviolet emissions from comet C/Hyakutake 1996 B2. The spatial distribution of the emissions was displaced sunward of the nucleus and the spatial extent was about 10 5 km. Lisse et al. (1996) suggested that the emission could be explained by thermal bremsstrahlung associated with hot electrons, possibly due to solar wind interaction effects. In the present paper, an alternate emission mechanism is proposed. The solar wind contains a large number of minor/heavy ion species with a range of charge states, such as O 6+ , C 5+ , N 5+ , and Si 10+ . These ions will readily charge transfer with cometary neutrals, producing ions which can be highly excited and consequently emit photons in the extreme ultraviolet and x‐ray part of the spectrum. The photon emission rate is proportional to the solar wind heavy ion flux and hence to the solar wind flux and, with some assumptions concerning the solar wind velocity, to the solar wind number density. The emission rate should be greatest downstream of the bow shock along the sun‐comet axis in agreement with the observed spatial distribution. The x‐ray images are really images of the line of sight integration of the solar wind density convoluted with the cometary neutral density. A total EUV/x‐ray luminosity for comet Hyakutake from this charge transfer mechanism agrees with the observed luminosity of 4 × 10 15 ergs s −1 within a factor of two.