X-Rays in Hale-Bopp and Other Comets: Theory and Observations

X-ray emission discovered in comet Hyakutake and then observed in Hale-Bopp and seven other comets with the ROSAT, EUVE, and BeppoSAX orbiting observatories is a general phenomenon. This emission exceeds that expected for scattering of the solar X-rays in comets by three orders of magnitude. Four processes have been suggested as basic excitation mechanisms: (1) charge transfer of solar wind heavy ions, (2) scattering of the solar X-rays by very small (attogram) dust particles, (3) line radiation from the cometary plasma (Bingham et al. 1997, Science, 275, 49), and (4) electron bremsstrahlung (Northrop et al. 1997, Icarus, 127, 246). We show that the formula for impurity radiation in the fusion plasma used by Bingham et al. is not applicable to the cometary plasma. Dielectronic, radiative, and three-body recombinations cannot compete with dissociative recombination in comets. Electron impacts produce ultraviolet photons and only two X-ray emissions, O 525 eV and C 277 eV, at a level <= 2% of the observed emission in comets. Corrections for two errors found in Northrop et al. reduce electron bremsstrahlung to 0.5% of the observed emission. Therefore, only processes (1) and (2) may currently be considered. Correlations of X-ray emissions observed with EUVE in four comets with gas and dust production rates as well as the brightness maxima and their offsets from the nuclei, favor charge transfer as the dominant process. However, this does not preclude a significant contribution of attogram dust scattering in very dusty comets like Hale-Bopp. A dust-related increase in X-rays from Hale-Bopp observed with BeppoSAX also favors this process.