Production of cometary X rays by electron impact and Bremsstrahlung processes

Bremsstrahlung and electron impact excitation are suggested in a few papers to be the main processes for generation of X rays in comets. To check this hypothesis, we determined a peak X‐ray volume emission rate in comet Hyakutake during the Roentgen satellite (ROSAT) observation. We found that scaling of the electron fluxes observed in comet Halley from the Vega probe to the known properties of the solar wind electrons agrees with theory of electron acceleration by lower hybrid waves in comets [ Shapiro et al. , 1999]. Using those scaled electron fluxes, we calculated Bremsstrahlung at the peak brightness in comet Hyakutake at a level of 0.2–0.3% of the observed peak emission. Bremsstrahlung in comet Hale‐Bopp constitutes 4% of the X‐ray emission observed with the EUVE, and the Bremsstrahlung spectrum, calculated for that comet by Shapiro et al. [1999], needs corrections. Emissions of the K‐lines of O 525 eV and C 277 eV, excited by energetic electron impact in comet Hyakutake, are at a level of 1.4–2.2% of the observed X‐ray emission. The calculation of energetic electron impact excitation in cometary oxygen by Shapiro et al. [1999] does not result in purely X‐ray emission but mostly results in visible and ultraviolet emission with just a small contribution at 525 eV which is equal to 1.7% of the total emission. With the Giotto electron fluxes the X‐ray emissions from Bremsstrahlung and electron impact would be smaller by orders of magnitude than the above values. The X‐ray luminosity associated with the energetic electron mechanism depends on the gas production rate and on the heliocentric distance in a different way than does the luminosity associated with the charge transfer mechanism. A test for these dependences using our EUVE observations rules out the energetic electron excitation mechanism on this basis. There are no papers that have adequately justified energetic electrons as the main source of X rays in comets.