X‐Ray Spectra of Comets

X-rays from a comet have been discovered for the first time in C/Hyakutake by Lisse et al. We discuss the excitation mechanism and the energy source of these cometary X-rays. We compute the intensity and the spectrum of bremsstrahlung X-rays and characteristic X-rays induced by nonthermal energetic electrons in the coma. Our results are compared with the X-ray data of C/Hyakutake, and the number density and kinetic energy of the electrons are evaluated. The electron density is found to be consistent with the in situ observation of P/Halley. The energy flux required to generate the observed X-ray luminosity is well explained by the solar-wind power falling on the coma. Hence, we suggest that the nonthermal electrons accelerated by the interaction of the comet with the solar wind are responsible for the X-ray emission.